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Project Specific Discussions => Dynamic Processors => Topic started by: Scenaria on November 07, 2004, 10:25:31 AM

Title: All things LA2a related
Post by: Scenaria on November 07, 2004, 10:25:31 AM
Hey guys,

if anyone has questions in regards to the infamous LA2a, feel free to post your questions here... theres bits and pieces in thread fragments in this forum and I felt it would be better to try and keep the collection in one place :)

this is for ALL la2a questions....

peace!

 :guinness:
Title: All things LA2a related
Post by: fum on November 08, 2004, 11:50:44 PM
Ok,

For my LA2A's, I'm using Allied part #6K88VG for the power transformer.  Let me run down the wiring  to make sure I've got it clear ( and everyone else has it too =) ):

Primary:

2 Black wires.  Hooked up to the feed from fused, switched mains

Secondaries:

250V
2 Red wires:  hooked up to the 1N4007 diodes ( one each)
Red/Yellow ( center tap of two reds):  Goes to ground?

6.3V
2 Green wires:  Heater supply and to VU Lamp
Green/yellow (center tap of two greens):  Goes to ground?

Grey: shield, goes to ground

Regards

ju
Title: All things LA2a related
Post by: Cainester on November 09, 2004, 01:39:51 PM
Quote from: "fum"
Ok,

For my LA2A's, I'm using Allied part #6K88VG for the power transformer.  Let me run down the wiring  to make sure I've got it clear ( and everyone else has it too =) ):

Primary:

2 Black wires.  Hooked up to the feed from fused, switched mains

Secondaries:

250V
2 Red wires:  hooked up to the 1N4007 diodes ( one each)
Red/Yellow ( center tap of two reds):  Goes to ground?

6.3V
2 Green wires:  Heater supply and to VU Lamp
Green/yellow (center tap of two greens):  Goes to ground?

Grey: shield, goes to ground

Regards

ju


My question too, and in addition to that:  
I have looked for links that tell exactly how to hook up the heaters and I have found nothing.  I did find one breakdown of the elements in a tube and noticed that a 12ax7 has 3 connections to the heaters.  Pins 4 and 5 seem to connect to either side of the element, and pin 9 connects to the middle.  So, which pins do I connect the 6.3v to?

The 6aq5's heater is on pins 3,4, so this is a simple 1 to 1 connection, right?

Thanks,
Caine
Title: All things LA2a related
Post by: NewYorkDave on November 09, 2004, 01:41:44 PM
For a 12AX7 on a 6.3V heater supply, connect 4 and 5 together. Connect one side of the supply to 4&5 and the other to 9.
Title: All things LA2a related
Post by: PRR on November 09, 2004, 04:48:05 PM
> how to hook up the heaters and I have found nothing.

http://www.lynx.bc.ca/~jc/hum.html

At first glance, heaters are just heaters, you just have to give them power.

The 12A?7 family gives you two heaters that can be fed 12V series or 6V parallel.

BUT: the cheapest source of heater power is raw AC from the transformer. And that is equivalent to a 6V 60Hz tone leaking all over the audio. So heater wiring layout is critical. The link above gives good tips and a photo of a bad job.

Most guitar amps put the audio components either very low on the socket or on a tag-strip, and then the best layout is to bring the twisted heater wires straight-up from the socket. Most pro audio tube layout favors setting the heater wiring in first, pressed hard against the chassis, and wiring the audio circuits high. For a row of tubes, you bring the twisted heater wiring out perpendicular to the row, then bend and run parallel to the row, then bend into the next socket, to keep the heater wiring away from the audio.

Good well-twisted well laid-out AC heaters don't hum.... decades of very clean gear was built this way and very little of it resorted to DC heater power. Bad AC heat will hum bad.
Title: All things LA2a related
Post by: cayocosta on November 09, 2004, 05:20:13 PM
Here's a layout for the La2a heaters:

http://www.vacuumbrain.com/The_Lab/TA/LA2/la2a_heater_layout.jpg

Thanks to CJ for hosting.
Title: All things LA2a related
Post by: Cainester on November 09, 2004, 06:15:34 PM
Thanks for the great info: Dave, PRR, and Ron,


One more question I have for now is, what is NE2?

Caine
Title: All things LA2a related
Post by: Cainester on November 09, 2004, 06:22:19 PM
Nevermind,
I found it here:
http://www.vacuumbrain.com/docs/t5.html

bottom of the page...
Great info.
Thanks,
Caine
Title: All things LA2a related
Post by: Scenaria on November 09, 2004, 07:01:00 PM
NE2 is the neon lamp (used for regulation)

check one of the bags... look for the piece made of glass.. bag is labeled "neon lamp"
Title: All things LA2a related
Post by: Cainester on November 10, 2004, 11:51:10 AM
I thougt the neon lamp was to light the meter.
Can it be moved over there to perform that function, or should the meter be lit with a separate light?
Title: All things LA2a related
Post by: Scenaria on November 10, 2004, 11:53:52 AM
nice thought but I dont think its going to be bright enough....

I simply strap a bayonet style leaded socket to the meter bracket and tie it to the power switch..

on the UA version theres a small hole in the front upper right hand side above the meter to hold a lamp bracket... I ommitted this hole as I thought that screw looked tacky...
Title: All things LA2a related
Post by: NewYorkDave on November 10, 2004, 11:54:26 AM
Keep the neon away from all the audio circuits.

If you want to light your meter, just use a 6V lamp on the heater supply.
Title: All things LA2a related
Post by: CJ on November 10, 2004, 02:49:44 PM
The bulb is used as a regulator. If you remove it, you have to add some zeners.
Title: All things LA2a related
Post by: gyraf on November 10, 2004, 03:30:16 PM
Or do like the re-issues - keep the bulb there for authenthic looks, but use some zeners hidden in heatshrink to do the real job.. :grin:

Jakob E.
Title: All things LA2a related
Post by: Scenaria on November 10, 2004, 03:32:23 PM
bah

be a real man...

stick with the neon  :razz:
Title: All things LA2a related
Post by: CJ on November 10, 2004, 04:18:52 PM
I have heard no noise from the neon.
Title: All things LA2a related
Post by: gyraf on November 10, 2004, 04:20:24 PM
the neon is just there for stabilizing the reference for the meter in GR-mode. Not chritical, to say the least..
Title: All things LA2a related
Post by: Ethan on November 10, 2004, 04:54:38 PM
I've had 2 zeners burn up and die there, replaced them with neons and no problems since.  Also "upgrading" the 2W resistors in the PS to 4W seems to keep them cool and doing so will probably help them live a longer life.
Title: LA2a problems
Post by: Ad0lescnts on November 21, 2004, 05:32:58 PM
Hi guys,
I'm new here, I usually am on the Homerecording.com bbs and recently found this one, there's a lot of good information here.

Ok so for my question,
I know it's kind of open ended but:
I just got finished with my DIY LA2a using Dave's book, but I have A LOT of buzzing.  Whenever I turn the gain knob I get a bunch of different buzzing noises, and once i pass the 6 o'clock point it sends my meter to the top.  I tried grounding everything all over the place and to the centertap of the 250V wires/AC plug ground.  This just gave me some different buzzing sounds.

Could my problem be that my Hammond transformer doesn't have a centertap for the heaters?  Where should I start on fixing the problem?

Thank you very much,
T
Title: All things LA2a related
Post by: NewYorkDave on November 21, 2004, 06:31:30 PM
Your heaters are just floating with no reference to ground? If that's the case, that could certainly be part of the problem! Try two 100-ohm 1-watt resistors, one from each "leg" of the heater supply to circuit ground.
Title: All things LA2a related
Post by: Ad0lescnts on November 21, 2004, 08:59:42 PM
ok I just did this and the buzzing is significantly less.  However, there is still some hum and the VU meter still goes nuts when I turn the gain above 6.

what next?

Thank you!
T
Title: All things LA2a related
Post by: CJ on November 22, 2004, 06:00:18 PM
I have seen this on my LA2 also. Probably ultrasonic oscillation. You might need to tweak your wiring a little. Get the meter to peg then start moving your wiring around with a chop stick. And make sure your front panel is grounded to the chassis if you yhave one of the hinged chassis.
I subbed in an 12AY7 for the AX7a first stage amplifier. Less gain which helped the oscillation, plus you are  likely to find a 12AY7 that sounds better than your typical AX7a. More of a vintage sounding tube.
Title: All things LA2a related
Post by: Ad0lescnts on November 23, 2004, 03:34:12 AM
AWESOME!
you were right, it was oscillating frequencies.  I found that the problem was the two wires for the limit/compress switch.  It got rid of my squeals at higher gains.

The only problem is that there is sitll some hum, and i found when i put my finger close (not touching) the two posts for the limit/compress switch, the humming goes away.  How do i fix this?

Thanks so much for all the help,
T
Title: All things LA2a related
Post by: Ethan on November 23, 2004, 01:55:50 PM
Quote from: "Ad0lescnts"

The only problem is that there is sitll some hum, and i found when i put my finger close (not touching) the two posts for the limit/compress switch, the humming goes away.  How do i fix this?


Make sure the metal of the switch is making good contact with your chassis.  If the chassis is painted or anodized you may have to scape off a bit around the hole for the switch to do this.

Yeah CJ's awesome.  I bugged the crap outta him when I was building my LA2.  I owe the man a few dozen rounds of beer! :sam:  :guinness:
Title: All things LA2a related
Post by: CJ on November 23, 2004, 02:47:14 PM
So you were the guy!
 :mad:
 :guinness: :razz:
FYI: keep your grid wires low, next to the chassis, and keep your B+ lines high. Also, try to make the B+ wires cross any grid wires at a 90 degree angle. jeeeez, giving away all my top secret stuff. oh well, getting old. time to download the hard drive!
 :oops:
Title: All things LA2a related
Post by: Ad0lescnts on November 24, 2004, 01:48:36 AM
awesome!
I looked into all those things and i got rid of all the buzzing.  My only concern is that it does get pretty noisy when i turn the gain up all the way, but I was playing with it today and it seems i only really need to have the gain set at about 2 to get the volume where i need it running back into my interface.  At this level there are no noise problems.  Is this standard practice for these things?

Man this thing is awesome, it's amazing how many different sounds you can get just out of the one peak reduction knob!

thanks,
T
Title: All things LA2a related
Post by: Cainester on November 24, 2004, 11:02:19 AM
Should the Sowter trannies be mounted inside or outside the chassis?
Thanks,
Caine
Title: All things LA2a related
Post by: Scenaria on November 24, 2004, 11:34:48 AM
you can mount em on the outside... I dont think theres room on the inside for the output one anyhow
Title: All things LA2a related
Post by: Cainester on November 26, 2004, 12:10:04 PM
When the circuit goes to ground, does it matter where the ground is tied?
i.e.  T2 ground is much closer to the xlr ground than the ground near T4B.       :oops:
Title: All things LA2a related
Post by: CJ on November 29, 2004, 06:09:42 PM
I would stick to the stock ground layout. Jahnsens book or see
http://www.vacuumbrain.com/docs/la2.html
Title: All things LA2a related
Post by: Cainester on December 09, 2004, 05:10:41 PM
Just a couple of solders before completion.. :grin:
Question:
Is C13 required if there will be no stereo couple for now?
I ask because I musta not ordered it..
Caine
Title: All things LA2a related
Post by: CJ on December 09, 2004, 06:13:21 PM
if thats the cap that goes to the barrier strip, forget about it.
Title: All things LA2a related
Post by: Scenaria on December 09, 2004, 06:27:27 PM
is that the one going from the stereo trim to the 6aq5a?

I dont have the schem in front of me..

its a picofarad value...
Title: All things LA2a related
Post by: Cainester on December 09, 2004, 06:29:41 PM
Yeah,
It goes to pin 6 of the 6aq5 and its value is 510pf.
Thats my very last solder point.
Title: All things LA2a related
Post by: Scenaria on December 09, 2004, 06:43:05 PM
You should put that value in there...

the earlier one used a single 30uf

then they changed it to a 30uf in series with a 1k  and strapped the 510pf across em...

I know keef has a theory as to that change and im sure others in here do too...

I think you will be ok powering up to check for proper voltage levels etc.. but dont take my word for it :)

you should be able to pick that cap value up down at radio shack :)
Title: All things LA2a related
Post by: Cainester on December 10, 2004, 04:34:16 PM
I powered the unit without any tubes or T4 and am getting some pretty high voltages.
275 unrectified, and 405 after.  
Is this ok?
Title: All things LA2a related
Post by: Scenaria on December 10, 2004, 04:45:41 PM
a few things for you to check out...

the two turrets on the rear right hand side... be sure you have approx 6.3 volts AC there..



to ground R29 should be around 360V DC (measure the front turret)
to ground R34 should be around 275V DC (measure the front turret)


check across your neon lamp... it should be around 54-60V

you shouldnt be getting 405V anywhere  :shock:
Title: All things LA2a related
Post by: Cainester on December 10, 2004, 04:56:06 PM
I have 405vdc in a lot of spots.  

heaters = 7.28
R29 and R34 = 405
Neon = 25.6

The rectifiers both go the same direction right?
Title: All things LA2a related
Post by: Scenaria on December 10, 2004, 04:59:49 PM
ok power it off...

something is not correct

:)

and yes... the cathodes (the part with the line on it) should be on the turret closest to you... the anode should be in the rear...
Title: All things LA2a related
Post by: cayocosta on December 10, 2004, 05:09:43 PM
Without tubes installed the voltage will be very high - there's no load.
Title: All things LA2a related
Post by: Cainester on December 10, 2004, 05:11:17 PM
Should I put all the tubes in and measure again?
Title: All things LA2a related
Post by: Scenaria on December 10, 2004, 05:11:50 PM
just dont put the T4b in yet :)

if you put the tubes in.. go back and check across the neon again.. if everything else looks about right
Title: All things LA2a related
Post by: SSLtech on December 10, 2004, 05:20:20 PM
yes indeed, the readings are expected to be high without tubes fitted.

I think I typed in some values to check in the notes I sent to Scenaria... They varied by about 5volts or so. When you put the tupes in you should have lower readings on the plate end of the plate resistors. The 6.3V should read about 7-point-something volts off-load also.

The neon should light (takes a long time to decay without warm tubes in!) and should have 55-65 Volts on it.

Keith
Title: All things LA2a related
Post by: Cainester on December 10, 2004, 05:22:52 PM
ok..
R29 368v
R34 271.1
neon 17.23
heaters 6.8

BTW  I had to put a 560pf cap for c13
Title: All things LA2a related
Post by: SSLtech on December 10, 2004, 05:30:58 PM
that neon... is it lighting up?
Title: All things LA2a related
Post by: Cainester on December 10, 2004, 05:33:23 PM
negative.
Title: All things LA2a related
Post by: Cainester on December 10, 2004, 05:35:01 PM
I just gave it some juice from R4 zero adjust and if fired up..
Title: All things LA2a related
Post by: Scenaria on December 10, 2004, 05:40:40 PM
whats the value across your neon?
Title: All things LA2a related
Post by: Cainester on December 10, 2004, 05:43:48 PM
17.1 to 52.5 when zero adjust goes from nothing to all.
About mid sweep the voltage goes to 63, but the neon doesn't light.  It only lights when the R is about 65%
Title: All things LA2a related
Post by: Scenaria on December 10, 2004, 05:49:10 PM
set your switch to GR and see if you can dial your VU in at O V.U. with the zero adjust...


might be about time to pop that T4 in there :)
Title: All things LA2a related
Post by: Cainester on December 10, 2004, 05:51:32 PM
Yes it will and when it does the voltage across the neon is 52.7 and T4 pin 5 is 8.5
Title: All things LA2a related
Post by: Scenaria on December 10, 2004, 05:54:03 PM
Rock-N-Roll

your numbers look good

drop that T4 in there  :green:

 :guinness:  :guinness:  :guinness:  :guinness:

BTW your neon value is a little low.. but so is mine.. still havent figured out why theres such a discrepency from one unit to another...

once the box heats up you will need to readjust the zero setting...

whats your email address?
Title: All things LA2a related
Post by: Cainester on December 10, 2004, 06:18:20 PM
Scenaria, SSLtech, cayocosta, cjenrick, PRR, and NewYorkDave, you guys ROCK.  I have one fully functional LA2a.  It is hum free, and is sweet as molasses!! :guinness:.
I will post pictures soon.
Thanks to all
Caine
[cainester @@ aol}
Title: All things LA2a related
Post by: Scenaria on December 10, 2004, 06:46:29 PM
congrats!

though a little smoke and fire might have been fun  :razz:
Title: All things LA2a related
Post by: Cainester on December 13, 2004, 03:45:31 PM
Ok, I used my LA2 this weekend and WOW! What tone..  What balls..  I only have one problem.  It will not limit.  If I connect a pot and vary it, there is no differenct in the slope of reduction.  I can use a standard 5k lin pot right?  I also notice that when I swithc to limit, the resistance at the pot falls to approx 0. Should that happen?
Title: All things LA2a related
Post by: cayocosta on December 13, 2004, 03:57:07 PM
Waiter - check please.
Title: All things LA2a related
Post by: gyraf on December 13, 2004, 04:02:14 PM
There is very, very little difference between the comp and limit modes of the LA2A.

Calling it limiting is a huge exaggeration..

Jakob E.
Title: All things LA2a related
Post by: cayocosta on December 13, 2004, 04:09:15 PM
With R7 at 1k I didn't notice any difference between limit and compress but with R7 at 2.7K it appears to just compress a little more in limit mode.
Title: All things LA2a related
Post by: Cainester on December 13, 2004, 04:09:31 PM
Quote from: "cayocosta"
Waiter - check please.

Awww Maaannn :sad:
Ok,
So maybe I didn't think through the 0 ohms too long! :oops:
But, I thought the limit was brick wall.  If it's not, then it functions correctly.
Title: All things LA2a related
Post by: Cainester on December 13, 2004, 04:30:23 PM
One last question.....
For today :wink:

The limit / compress switch...
Does closed = compress and open = limit?
Title: All things LA2a related
Post by: SSLtech on December 13, 2004, 04:39:52 PM
Quote from: "Cainester"
Does closed = compress and open = limit?

Correct.  :cool:

Keith
Title: All things LA2a related
Post by: SSLtech on December 13, 2004, 04:44:41 PM
Okay... couple of observations here:

The GR display doesn't change in compress or limit modes... just ggives an approximation. (check it out on the schematic: the limit switch changes only the effectivienss of the GR, not the amount of light in the EL panel, therefore the seperate metering LDR is unaffected by the switch... just the signal)

The test for the value of the limit resistance is to feed in a tone via a level control BEFORE the compressor, switch to compress and note that the signal rises slightly as the input is increased.

Repeat the test switched to limit, now note that the output signal should more or less stop a little above the threshold... if the signal actually starts to turn down above the threshold, reduce the value of the resistor. If it still climbs slightly, increase the resistor.

Use a pot to gauge the setting, then replace the pot with the nearest value fixed resistance.

Keith
Title: All things LA2a related
Post by: cayocosta on December 13, 2004, 04:46:07 PM
Actually Caine, I thought you meant you had no compression or limiting - sorry about that. Otherwise, you did great - congratulations! R7 is bypassed in compress mode - with the switch closed.
Title: All things LA2a related
Post by: SSLtech on December 13, 2004, 04:52:59 PM
Quote from: "cayocosta"
Actually Caine, I thought you meant you had no compression or limiting - sorry about that. Otherwise, you did great - congratulations! R7 is bypassed in compress mode - with the switch closed.


And to answer Cainester's earlier question, yes it is because of this bypassing that the resistance falls to 0 ohms at the pot.. (think about it! :wink: )

You don;t say how you're observing the gain reduction... if you're looking in "meter-gain-reduction" mode, you won;t see anything. -If you're looking in "Meter-output+4" mode, you'll see it.

Keith
Title: All things LA2a related
Post by: Cainester on December 13, 2004, 05:08:15 PM
Yeah, I'm looking at +4.  In limit, I've not found the happy medium yet.  The output voltage gradually increases as more gain is applied  to a point and then the out voltage starts to decrease.  It seems it will always do this to some extent, so I must find where it does it the least and stick that value in there..
Title: All things LA2a related
Post by: cayocosta on December 13, 2004, 05:10:36 PM
Keith, thanks for the heads-up on the limit mode test - I've never heard of that before. In fact, that might be the first new bit of la2a info since CJ's T4 hack! How about some sweeps?

http://www.vacuumbrain.com/The_Lab/TA/Rons_LA2a/la2a_sweeps.jpg
Title: All things LA2a related
Post by: Cainester on December 21, 2004, 06:03:34 PM
Ok,
The unit sounds awesome, but..
When I run a tone and scope the output, the single wave has turned into 4.
What could cause that?
Caine
Title: All things LA2a related
Post by: jensenmann on January 09, 2005, 07:07:18 AM
Hi folks
I´m planning to built a Bloo LA but could not yet find out which transformers to use for in/out. Can the UTCs be found anywhere? What - if not - are good replacements? How do they fit in the Bloo kit/enclosure?
thanks
Jens
Title: All things LA2a related
Post by: Scenaria on January 09, 2005, 07:50:42 AM
ebay dude...

theres been a ton of A10 and A24's on ebay the past few days..

do a search for UTC...
Title: All things LA2a related
Post by: Scenaria on January 09, 2005, 07:53:40 AM
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=67815&item=5742525285&rd=1

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=67815&item=5742340383&rd=1


http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=67815&item=5741006007&rd=1
Title: All things LA2a related
Post by: Zee1usa on January 13, 2005, 01:02:39 AM
Hello,

 I am wiring up my input and output XLR's on my L.a.2.a.

 (Warning: Lack of sleep and extreme solder inhalation!!!)

If I read on page 2 of the "L.a.2.a. re-issue" manual it lists the barrier strip pinout. (1) is input + and (8) is output + . (5) is input - and (10) is output -

Now, look at the 1968 schematic. barrier strip (1) input  goes to pin 6 on UTC input. (8) goes to pin 6 on UTC output. (5) goes to pin 1 UTC input. (10) goes to pin 1 UTC output.

 I have 3 sources of info on this wiring that are all conflicting. I'm wondering if all 3 ways are correct?

sources:

1) bloo info
2) cayocosta diagram
3) info above I found: (old schematic and reissue manual).


Thanks,

    Z
 :?
Title: All things LA2a related
Post by: cayocosta on January 13, 2005, 09:12:18 AM
It appears that the 1968 schematic illustrates the A-10 (input tranny) primary labeling inverted - with tranny pin 6 hot. Older schematics as well as photos of reissue units illustrate tranny pin 1 hot.

If I'm correct, then this is the way to wire the input for XLR pin 2 hot:

XLR input pin 2 --> barrier strip terminal 1 (optional) --> HA100X/A10 tranny pin 1 (primary) HOT input to tranny
XLR input pin 3 --> barrier strip terminal 5 (optional) --> HA100X/A10 tranny pin 6 (primary)

(My layout reflects this, and this supports the reissue manual's instructions to use barrier strip terminal 1 for hot)

Now, the output side is always illustrated the same way in all the schematics (without the XLR) - barrier strip terminal 10 to tranny output pin 1 hot. However, photos of the reissues (with XLR's) indicate that XLR pin 2 is wired to barrier strip 8 (pin 6 of the output transformer), which would reverse the polarity from the output tranny.

This is how (in photos) the reissue unit's output is wired for XLR pin 2 hot:

XLR output pin 2 --> barrier strip terminal 8 (optional) --> A24 tranny pin 6 (secondary)
XLR output pin 3 --> barrier strip terminal 0 (optional) --> A24 tranny pin 1 (secondary) HOT output from tranny

(My layout reflects this too, and this also supports the reissue manual's instructions to use barrier strip terminal 8 for hot)

But, the trouble here is that pin 6 of the output tranny secondary is not the hot side.

Has the signal's polarity been flipped by the time it gets to the output transformer primary, so that reversing the XLR connections puts the signal back in phase on the way out?

If so, then it makes sense.
Title: All things LA2a related
Post by: cayocosta on January 13, 2005, 09:20:39 AM
Reissue:

(http://home.earthlink.net/~cayocosta/diy_files/la2a/LA-2A_tranny_wiring.jpg)
Title: All things LA2a related
Post by: cayocosta on January 13, 2005, 09:29:55 AM
I don't know much, but I believe a triode inverts phase at the anode, and a cathode-follower output is also inverted. So, the signal is ultimately out of phase (2 triodes and a CF) going into the output tranny primary - and that's the reason for the switch at the XLR output.
Title: All things LA2a related
Post by: Zee1usa on January 14, 2005, 12:33:59 AM
Thank you for your input cayocosta, I appreciate it!  I shall wire mine the way you have discussed unless proven otherwise.

 Z
Title: All things LA2a related
Post by: fum on January 14, 2005, 01:02:09 AM
Looks like the Seattle contingent is all working on LA2A ( I'm back to workin on mine again) =)

ju
Title: All things LA2a related
Post by: electronaut on January 14, 2005, 06:50:30 PM
Quote from: "cayocosta"
...a cathode-follower output is also inverted.


Polarity is only inverted when the signal is taken from the plate, not the cathode.  A cathode follower's output is in phase with the input.

But, we're not looking at a cathode follower in the LA2a, are we?  (I'm not familiar with the reissue.)
Title: All things LA2a related
Post by: electronaut on January 14, 2005, 11:56:28 PM
Oh... maybe we are looking at a cathode follower -- but it's a "white" one, eh?
Title: All things LA2a related
Post by: Zee1usa on January 15, 2005, 01:23:01 AM
Trying to figure out what is causing me a low voltage reading at turret A17 which is  supposed to be 90vdc but it is 5vdc with all my tubes in but T4B is not installed. I double checked all wiring, shorts and components etc.  I was very careful wiring this thing up.  

 Across turret A17 and B17 is a 220K (R33) which is reading 260vdc on the B17 side which appears to be proper.  

A17 goes to V3 (pin 6) which is jumped to V3 (pin 1) which then goes to a .02
capacitor on to pin 3 or bottom contact of R37 (potentiometer) etc....

  :?  :mad:

Hope to figure this damn thing out ...so I can drop the T4B in and fire her up. Time to break out the magnifiying glass.... :roll:

Z
Title: All things LA2a related
Post by: cayocosta on January 15, 2005, 07:31:34 AM
The tube's cathodes (pins 3+8) should be tied together and wired to the 1k resistor and bypass capacitor(s) that go to ground. Check to see that your ground there is good.

Be careful with the voltages in there.
Title: All things LA2a related
Post by: Zee1usa on January 15, 2005, 12:58:56 PM
Thanks Cayocosta,

 On V3. pin 8 is wired to pin 3. Pin 3 goes to R31 (1K) . R31 is in paralell
with the .o1 and .02 orange drops which are tied to ground (R36) . Ground is good.

:thumb:

One thing I wonder about is that the "center" pin of V3 is not tied to anything.

FYI:
On V3 pin 4 and pin 5 are tied where on of the heater wires are iconnected, and the other heater wire goes to pin 9.

On V3 pin 2 and pin 7 are tied and go to R2 (peak reduction)
Potentiometer pin 2. Shield is connected on R2 but not at the
other end.


Your responce was much appreciated.

 Z
Title: All things LA2a related
Post by: Scenaria on January 15, 2005, 01:26:42 PM
two things to check...

R12 and R14

be sure that R14 is going to pin 7 (470k)
and R12 is going to pin 8 (2.7k)

also check R36...

If your using a 10k/2w for R34 then be sure that R36 is 470 ohms

if your using a 22k/2w for R34 then be sure that R36 is a 1k resistor..


also... make sure that R3 and R37 are turned at midpoint...

and as ron said.. becareful in there with the high voltages... allow time for everything to discharge and measure with your DMM BEFORE putting your hands in the box
Title: All things LA2a related
Post by: cayocosta on January 15, 2005, 02:36:48 PM
The 260VDC sounds about right on the B+ side of R33, so your B+ is there.

Before you go any further, do all your heaters light up?

If not, stop.

If the heaters do light up, swap out the tube for another 12ax7 and see if that gives you the 90VDC reading to chassis ground at pin 6 of the tube socket.

Also, measure the voltage drop across R33 itself, you should normally see around 170VDC.

Check to make sure none of the pins on the tube socket are touching anything else.
Title: All things LA2a related
Post by: Zee1usa on January 15, 2005, 02:46:32 PM
Thank you Scenaria ! All those look good.  :cool:

Thank you Cayocosta ! Yes, all of my heaters light up. I swapped  the 2  12AX7 for kicks and it did not make any difference.

I will check all the pins on the tube sockets for continuity to make sure none of them are broken for some reason.

I realize it is quite common to get bum tubes, so i'll keep that in mind.

Thanks Gentlemen! Going forward I appreciate your helpful comments,

 Z
Title: All things LA2a related
Post by: Zee1usa on January 15, 2005, 03:01:53 PM
and as ron said.. becareful in there with the high voltages... allow time for everything to discharge and measure with your DMM BEFORE putting your hands in the box

Oh Yeah Baby!!  




Quote from: "cayocosta"

Also, measure the voltage drop across R33 itself, you should normally see around 170VDC.

Check to make sure none of the pins on the tube socket are touching anything else.



R33 B+ is 250 vdc and the other side of R33 that should be reading about 90vdc is reading 5vdc.

Voltage drop across R33 (220K) is 245vdc

None of the pins are shorted, I checked that very carefully.

Thanks Guys,

 Z
Title: All things LA2a related
Post by: cayocosta on January 15, 2005, 03:02:21 PM
This socket is pretty basic - not much going on to cause trouble.

Check continuity from the ground side of R31 to a spot on the chassis that's not painted - this will ensure it is indeed grounded. Make sure that pins 1, 2, 6 and 7 on the socket do not show continuity to chassis ground.

Also, you should read 1k ohms from either pin 3 or 8 to the chassis.

I keep editing this post!
Title: All things LA2a related
Post by: Zee1usa on January 15, 2005, 03:11:05 PM
Quote from: "cayocosta"
This socket is pretty basic - not much going on to cause trouble.

Check continuity from the ground side of R31 to a spot on the chassis that's not painted - this will ensure it is indeed grounded. Make sure that pins 1, 2, 6 and 7 on the socket do not show continuity to chassis ground.

Also, you should read 1k ohms from either pin 3 or 8 to the chassis.

I would replace R33 at this point.


R33 (220K) is good. pulled it and re-measured it.

V3 pins 3 and 8 are tied then go to the 1k which is then grounded, but pin 3 and 8 are not directly going to ground. They goto the 1K then on the other side of that 1K goes to ground.

Is this the issue?

Thank you,

 Z
Title: All things LA2a related
Post by: cayocosta on January 15, 2005, 03:17:27 PM
Yes Z, I was wrong about pins 3 and 8 to ground, I edited my post to check for ground to chassis from the ground side of R31.

I also edited out the suggestion to replace R33 after your voltage drop was posted.
Title: All things LA2a related
Post by: Zee1usa on January 15, 2005, 03:29:34 PM
Quote from: "cayocosta"
This socket is pretty basic - not much going on to cause trouble.

Check continuity from the ground side of R31 to a spot on the chassis that's not painted - this will ensure it is indeed grounded. Make sure that pins 1, 2, 6 and 7 on the socket do not show continuity to chassis ground.

Also, you should read 1k ohms from either pin 3 or 8 to the chassis.

I keep editing this post!



All this checks out good. I am getting 1.081 K ohms across R31 and from pin 3/8 V3 to chassis. Damn those Carbon comps are sloppy!

thats like 8% tolerance out.  Probably does not matter.
oh ya and re soldered it about 5 min ago but it feels cool.
I know those are heat sensitive.

Thanks  :guinness:  :wink:

   Z
Title: All things LA2a related
Post by: cayocosta on January 15, 2005, 03:30:39 PM
Try disconnecting the .02 cap from pin 1 and see if that makes a difference with the voltage.
Title: All things LA2a related
Post by: Zee1usa on January 15, 2005, 03:39:44 PM
Quote from: "cayocosta"
Try disconnecting the .02 cap from pin 1 and see if that makes a difference with the voltage.



Hmmm... Interesting.. it is still 5vdc when i disconnect the .02 (C9)
from pin 1 of V3. Voltage did not change at all.

 So this should shed some light on the matter.! ?


 Thanks  :thumb:
Title: All things LA2a related
Post by: cayocosta on January 15, 2005, 03:45:38 PM
Sorry Z, I wish I could pinpoint your problem a little quicker for you.

By the way, are your other voltages in line?

Back to the heaters - from pin 4 or 5 to chassis ground should read 3.15 VAC - that's AC. Same with pin 9.

Running out of options, I would replace R33 anyway now.
Title: All things LA2a related
Post by: Zee1usa on January 15, 2005, 04:06:00 PM
Quote from: "cayocosta"
Sorry Z, I wish I could pinpoint your problem a little quicker for you.

By the way, are your other voltages in line?

Back to the heaters - from pin 4 or 5 to chassis ground should read 3.15 VAC - that's AC. Same with pin 9.

Running out of options, I would replace R33 anyway now.



Thank you!!

3.29 VAC  V3 pins 4/5 to chassis. 3.29 VAC pin 9 to chassis.

Turret Voltage readings:

A25 to A25 (heaters) 6.6VAC

B21 (R29, 4.7K 2w) = 350vdc

B20 (R34,10K 2w) = B+ 250vdc

A1 NE2 (glowing neon bulb) = 53.7

A20 = 110vdc

A14 = 207vdc

A17 = 5vdc !#$%!%&!!!!     should be 90vdc

A19 = 97.5vdc


Thats what I have. They all look good except the R33 to V3 pin 6
reading that is 5vdc . From the info I have should be 90vdc there or
somthing closer to that.
 :?


Back to the grindstone...

Thanks!!

 Z
Title: All things LA2a related
Post by: Scenaria on January 15, 2005, 04:08:00 PM
you do have the 1k between pins 2 and 7 on V3 right?

lift one leg of that 220k (r33) and measure its resistance.. make sure its a 220k (dont assume the color bands are correct)

oh and make sure you have the jumper strapped across A11 and A14...it wont cause the problem your having but it is something I missed in the docs
Title: All things LA2a related
Post by: Zee1usa on January 15, 2005, 04:14:52 PM
Quote from: "Scenaria"
you do have the 1k between pins 2 and 7 on V3 right?

lift one leg of that 220k (r33) and measure its resistance.. make sure its a 220k (dont assume the color bands are correct)



 Thanks Scenaria !

 These 2 things have ben verified.

 I do have a 1k between V3 pin2/7..  R33 was measured at 220K with my fluke meter. I took it out of the circuit and remeasured it.

I will make sure I have A11 tied to A14. Thanks !

 Z
Title: All things LA2a related
Post by: cayocosta on January 15, 2005, 04:15:47 PM
Quote
On V3 pin 2 and pin 7 are tied and go to R2 (peak reduction)


Yes, tied with the 1K right?
Title: All things LA2a related
Post by: cayocosta on January 15, 2005, 04:16:21 PM
Did you replace that 220k?
Title: All things LA2a related
Post by: Scenaria on January 15, 2005, 04:24:25 PM
one other thing to try....

the shielded wire from #2 (the reduction pot) to A15....

disconnect from A15 to see if theres any difference in your V reading
Title: All things LA2a related
Post by: Zee1usa on January 15, 2005, 04:34:56 PM
Quote from: "cayocosta"
Quote
On V3 pin 2 and pin 7 are tied and go to R2 (peak reduction)


Yes, tied with the 1K right?



Yes, its a 1K..

I changed R33 (220K) no difference.

I checked continuity of the V3 socket for kicks and it is ok.

One thing that i noticed is when I swapped the tubes..
I get 7vdc with one of the 12ax7 tubes.
When I install the other 12ax7 (Ecc83s/jj-tesla)
I get 4.3vdc which is exact. (I was quoting 5vdc to keep it simple).


  Thanks,

 Z
Title: All things LA2a related
Post by: cayocosta on January 15, 2005, 04:41:44 PM
Yes, try Scenaria's suggestion to remove the input to the grid.

Also, what voltage do you read from pin 3 or 8 to chassis ground? Should be less than 1 volt DC.
Title: All things LA2a related
Post by: Zee1usa on January 15, 2005, 04:47:58 PM
Quote from: "Scenaria"
one other thing to try....

the shielded wire from #2 (the reduction pot) to A15....

disconnect from A15 to see if theres any difference in your V reading



Bingo !

I am getting 115vdc...close to 90vdc

 B..I...N..G...O.... bingo was his name O.... sing along.... :sam:  :green:


  Ok, let me scratch my head now.

Why is R2 causing this?? :?
OK, Maybe I just needed to rotate R2 a bit? duh?
I'll hook it back up and rotate.. see if changes.
Uh... maybe thats what that pot controls (that there voltage)?

Hooked it back up . Turned R2 FCC (full counter clock wise) and Im getting 85vdc.  That is close enough to 90vdc for me.

I am a happy camper now, thanks to Cayocosta and Scenaria!!


Thanks Guys for talking me through this !!

ISOLATE ..the key word.

Now, time to plug in the Coveted T4B module and pray to god no smoke pours out.

  Z  :guinness:  :green:  :guinness:
Title: All things LA2a related
Post by: Scenaria on January 15, 2005, 05:17:31 PM
thats still not right..

you need to look over your wiring from R2 over to the T4b area

that voltage shouldnt be changing with the pot

do you have the X on the T4b socket grounded to chassis?
Title: All things LA2a related
Post by: Zee1usa on January 15, 2005, 05:25:35 PM
Quote from: "Scenaria"
thats still not right..

you need to look over your wiring from R2 over to the T4b area

that voltage shouldnt be changing with the pot

do you have the X on the T4b socket grounded to chassis?



I was wondering about that.  I should have asked.  I was a little unsure
about the "X" or "Star" being grounded to the chassis.
instead i grounded the little tab from the T4B socket to ground for some reason thinking it was the "star" point you were talking about , but wondering if I was the "X" that should have been tied to ground.
Thats what I get for not aksing.

I corrected the grounding at the T4B. I still have the  same 5vdc at R33, but I am headed in the right direction (from R2 over to the T4b area) . :thumb:

Thanks Guys,

 Z
Title: All things LA2a related
Post by: wilebee on January 15, 2005, 08:14:08 PM
OK    I just got mine fired up and I'm having voltage problem at v4 at pin 5 (A20) I'm getting 160v and at pin 6 (A19) I'm getting 53v most everythng sees right I don't thave the t4b in yet whats up I'm using a 6005 for the 6aq5 Wil

Wilebee
Title: All things LA2a related
Post by: CJ on January 15, 2005, 08:34:38 PM
Z, make sure one end or R2 is grounded.
Title: All things LA2a related
Post by: cayocosta on January 15, 2005, 08:35:59 PM
What's the voltage on the other side of those resistors - and what value resistors do you have there.
Title: All things LA2a related
Post by: CJ on January 15, 2005, 08:39:33 PM
Willibee, those voltages don't look too bad. Which one bothers you?
They changed the screen schematic on that tube over the years, which schematic are you using?
cj
Title: All things LA2a related
Post by: Zee1usa on January 15, 2005, 08:52:03 PM
Quote from: "CJ"
Z, make sure one end or R2 is grounded.


My wiring is correct on R2 pin 3 goes to turret stud by T4B. R2 pin 1 is the shield which goes to  the ground tag which is at the bottom isolated from the turret stud itself. Also R2 is grounded to the chassis.

 T4B socket appears to be wired correctly.

 :?

Thanks CJ !!

  Z
Title: All things LA2a related
Post by: wilebee on January 15, 2005, 09:03:47 PM
cayocosta: the voltage is 276  
cj the schematic is s a good question I've got 2 of them here one sezs r29  is 47k the other sez 4.7k my voltage divider is r29 4.7k r34 10k and I am using r38 22k off the grid of v4 to the positive terminal of c7 d . I don't really understand why the 22k is there other than a voltage divider except its not referencing ground. Wil

sorry zee1usa I'm not trying to interupt you help  Wilebee
Title: All things LA2a related
Post by: CJ on January 15, 2005, 09:05:14 PM
Ohm out your R2 ground to be sure. Also, grounding the pot does not ground Pin 1.

Also, check for a short between pin 7 and 6 with ohm meter.
Title: All things LA2a related
Post by: Scenaria on January 15, 2005, 09:09:53 PM
R29 should be 4.7k

Use a 22k for R34

and a 1k for R36

if you insist on using a 10k for R34 then change R36 to a 470 ohm
Title: All things LA2a related
Post by: Zee1usa on January 15, 2005, 09:12:56 PM
Quote from: "CJ"
Ohm out your R2 ground to be sure. Also, grounding the pot does not ground Pin 1.

Also, check for a short between pin 7 and 6 with ohm meter.



 Thanks CJ !!

 I should have known fer christ sakes, *none* of my pots were making a good ground. I need to use a star washer  so it will dig through the paint and make sure each pot gets a solid grounding to the chassis.  I got a VERY BAD habit of using the "beeper" for continuity test.
 :roll:


Thanks again,

 Z
Title: All things LA2a related
Post by: wilebee on January 15, 2005, 09:20:16 PM
Steve Thats what I had to begin with minus the 1k for R36  so 4.7 then 22k then 1k for r36 will do  Wil
Title: All things LA2a related
Post by: wilebee on January 15, 2005, 10:50:47 PM
ok r29 is 4.7k r34 is 22k and r36 is 1k ive got 87v at A20 and 57v at A19 the manual says 130v 90v that doesn't seem close ? Wil
Title: All things LA2a related
Post by: Scenaria on January 15, 2005, 11:07:09 PM
turn the pots on the rear of the chassis at the halfway point... then measure them again


im thinking that your diff in voltage might be because your not using a 6aq5?

oh and check to make sure you have the 2.7k on pin 8 of valve 1 and the 470k on pin 7 on valve 1
Title: All things LA2a related
Post by: CJ on January 15, 2005, 11:36:15 PM
Willibee, measure your bias voltage. This will tell you whats going on. This is the voltage from cathod to ground.  Turn your two front pots down all the way. If you have oscillation, this might stop it. You guys passing signal without gain reduction?

Measure your B+ voltage going to the 6AQ5 and 12 AX7 you guys.
Title: All things LA2a related
Post by: Zee1usa on January 16, 2005, 12:09:24 AM
OK, Thanks to my trusty Harbor Frieght Rotory Tool ..AKA..Dremmel. $14.99     (gotta get the diamond 1 inch bits...they work best)

I got a good Chassis contact to all my pots. Your gonna Laugh, I dremmelled out some paint around the trimmer pot on front too and forgot there is no knob on there... :shock:  hahah....oh well , I got a cool Radio Shack knob that looks perfect there. :wink:

Usually with a pot you get the star washer, but the cheap Alpha pot you just get 2 flat washers. I need to go to the hardware store to pick up some star washers to have on hand. (shopping list)

Now I have 114vdc  (24/7) regardless where i rotate the "Peak Reduction" knob. So I think I am in buisiness. Lesson Learned big time. I already knew about proper grounding of the pots to the chassis, so I kick myself in the ass.   :mad:


So far so good....Moving Forward.

 :guinness:  :thumb:

Thank you Guys !

   Z
Title: All things LA2a related
Post by: Zee1usa on January 16, 2005, 12:15:00 AM
Oh ya WILEBEE ...I am using a RCA NOS 6005 also which I ordered from Triode Electronic on the net. I ordered the  6AQ5 and thats what they sent me.

""sorry zee1usa I'm not trying to interupt you help Wilebee""

No problem at all WIL !  I think it is funny that you, fum and I are all workin on  our La2a's this weekend.  

 Z :grin:
Title: All things LA2a related
Post by: Scenaria on January 16, 2005, 12:16:12 PM
so is it working correctly now?

oh and for future knowledge... a little scrape of the paint is all thats needed... the dremel was way overkill.. and only needed on the backside. :\

whats odd is ive built a half dozen of em without ever having to scrape off paint..
Title: All things LA2a related
Post by: Zee1usa on January 16, 2005, 02:36:49 PM
Quote from: "Scenaria"
so is it working correctly now?

oh and for future knowledge... a little scrape of the paint is all thats needed... the dremel was way overkill.. and only needed on the backside. :\

whats odd is ive built a half dozen of em without ever having to scrape off paint..


 Ya, its because my alpha pots  :oops:  did not come with star washers.
only 2 flat washers. There was no way in hell the 2 flat washers were going to  bite through that thick bloo paint. I am sure you were using star washers on yours and thats why you did'nt have an issue. If I would of just used star washers I probably would of been fine. The problem is I was wanting to fire this thing up...impatient....I had not had a chance to get to the hardware store (35 min drive and snowing) last night to get some starwahsers.  I figured if i just cleaned off some paint around the potentiometer holes I would be good to go.


The unit is making alot of hiss, static etc. which i can move my wiring around a bit and take care of that, but At the moment it appears to not be passing audio.  I need to hook it up to my scope and signal generator
and try to see whats up.  :mad:

   Z
Title: All things LA2a related
Post by: Scenaria on January 16, 2005, 02:48:46 PM
not that this has anything to do with the work you have put into it.. but I can say the the biggest problem I have been seeing with the la2 lately is not taking the time to be neat with the point to point wiring... being sure to twist the heater wires tightly and pay attention to the way they are routed in the box along with using shielded cable at the appropriate places...if you are not passing audio... then send some signal in the box and start with the scope at the front end (tranny) make sure you have signal at that point then work forwards from there..I'd checl the "gain pot" to see if any signal is present at that point... check to see if the signal goes up and down with that pot.. so on and so on
Title: All things LA2a related
Post by: Zee1usa on January 16, 2005, 10:09:42 PM
:sad:  I have 5 volts at R33 again.

 Last night when I was trying to do a audio test  I noticed the connection at R2 pin 2 was broken. It was probably broken when I *thought* I fixed it.

So it is 114vdc with R2 pin 2 disconnected and 5vdc when connected.
When I adjust R2 ithe voltage varies between 85vdc and 5vdc.

This is probably the reason why I am not passing audio etc.

Cayocosta:

"Also, what voltage do you read from pin 3 or 8 to chassis ground? Should be less than 1 volt DC."

I have 2.7vdc measured here.

 My wiring looks good. I'll let you know if I make any progress.
 Z

  :oops:
Title: All things LA2a related
Post by: Zee1usa on January 17, 2005, 12:57:52 AM
Ok, I found the moronic error I made fer christ sakes.

 Let me give you a hint.... I had a left over  insulated-turret in my bag.

  :shock:

 I was wondering why I had that extra turret. Now I know what it is for.
I accidently skipped that sentance in my instructions because I got the input transformer plate for a HA-100X and I had a A10 instead. So I skipped that part of the build until scenaria sent me my new pate for my A10. Well I missed that part of the paragraph. I installed my A10 plate and forgot to mount the input trafo corner turret. And when ever the instruction said to go to the insulated turret, I just tied it to the 1 and only insulated turret that was mounted by the T4B.

 I have no excuse.  It is obvious on the diagram. I wasnt even loaded
or enhanced while wiring, I was sober. That may be why?

 Hope my T4B is ok. :?:

I am off to fix it, I'll let you know.

 Humble,

 Z
Title: All things LA2a related
Post by: Scenaria on January 17, 2005, 01:13:13 AM
no need to feel bad...

its how we all learn :)

im sure your t4 is ok..


thos el's can handle some serious voltage..

lesson learned though... never rush to put the t4 in until you have verrified that everything checks out ok..
Title: All things LA2a related
Post by: Zee1usa on January 17, 2005, 03:13:30 AM
Update :


 Ok, passing audio.. No hum..

 I do hear a very faint ringing oscillation, which I am sure moving a few wires will cure. The gain works fine.  I gotta check to make sure its compressing/Limiting properly. I'll check it out more tomorrow evening and let ya know.

Thank You, Cayocosta, CJ and Scenaria! Help Appreciated very much.

Here is a pic of the wiring:

http://img.photobucket.com/albums/v359/zee1usa/bloowiring.jpg

   Z
Title: All things LA2a related
Post by: Zee1usa on January 17, 2005, 03:58:41 AM
More Pics:

Face:

http://img.photobucket.com/albums/v359/zee1usa/Blooface.jpg

http://img.photobucket.com/albums/v359/zee1usa/blooface1.jpg

Back:

http://img.photobucket.com/albums/v359/zee1usa/blooback.jpg

Good Company:

http://img.photobucket.com/albums/v359/zee1usa/Bloowithcompany.jpg



Turret board:

http://img.photobucket.com/albums/v359/zee1usa/turretbloo.jpg

Z
Title: All things LA2a related
Post by: Scenaria on January 17, 2005, 08:08:01 AM
looks good..

check to see that the coupling cap isnt hitting the back pf the vu meter..

if it is then you might need to lenghten the leads and tuck it up top.. thats what I had to do to mine
Title: All things LA2a related
Post by: wilebee on January 17, 2005, 12:36:50 PM
ok heres what I've got on v4 the bias voltage is 8.78vdc and 90v on the plate and 57 on the grid  Wil

Wilebee
Title: All things LA2a related
Post by: wilebee on January 17, 2005, 03:49:41 PM
bump cj scenaria help ? Wil
Title: All things LA2a related
Post by: Scenaria on January 17, 2005, 04:49:36 PM
dbl check your voltage divider...

R34 is it a 10k2w or 22k2w?

what value do you have for R36?
Title: All things LA2a related
Post by: wilebee on January 17, 2005, 05:00:31 PM
r29 is 4.7k 2w
r34 is 22k 2w
r35 is 1k
Title: All things LA2a related
Post by: wilebee on January 17, 2005, 05:00:57 PM
I mean r36
Title: All things LA2a related
Post by: cayocosta on January 17, 2005, 06:46:02 PM
Wilebee,

The only voltages I have are on the 1968 schematic, so that's all I can reference.

For the 6AQ5, the 68 schematic calls for a 10k plate resistor and a 470R cathode resistor.

In this configuration with 275V B+:

The plate volts should be around 135V.
The cathode around 7V.
The screens around 100V.

Wilebee, in one of your posts I think you said that you had this configuration originally - but with a 1k cathode resistor. If that's so, try going back and using a 470 ohm cathode resistor with the 10k plate resistor.

Ron
Title: All things LA2a related
Post by: Scenaria on January 17, 2005, 07:02:31 PM
Quote from: "wilebee"
ok heres what I've got on v4 the bias voltage is 8.78vdc and 90v on the plate and 57 on the grid  Wil

Wilebee


I just checked one of mine that has a 22k/2w

9.05v

89.6v

and

127v on the grid

seems that your seeing about half that value..

first thought would be R35.
Title: All things LA2a related
Post by: Scenaria on January 17, 2005, 10:51:58 PM
Wil,

have you made any progress yet?
Title: All things LA2a related
Post by: Scenaria on January 17, 2005, 11:03:45 PM
Quote from: "wilebee"
Steve Thats what I had to begin with minus the 1k for R36  so 4.7 then 22k then 1k for r36 will do  Wil


I was going back over your posts...

make sure that R29 is a 47k/2w not 4.7
Title: All things LA2a related
Post by: wilebee on January 18, 2005, 11:31:22 AM
you sent me 4.7 k 2 w and the layout on vaccuumtubebrain says 4.7k so which is it 47k or 4.7 k because my first voltage drop is right on the far side of r29 I have 275vdc Wil
Title: All things LA2a related
Post by: cayocosta on January 18, 2005, 11:55:27 AM
Wilibee, the '68 schematic also shows 4.7k, and all the schematics show 275V B+, therefore it's reasonable to assume the older schematics are simply missing a period with regard to R29. Furthermore, increasing R29 to 47k would knock the B+ down considerably from the universal schematic figure of 275V - that you already have with 4.7k installed.

4.7k is correct.
Title: All things LA2a related
Post by: Scenaria on January 18, 2005, 12:03:27 PM
there is two versions of the schematic...

oops my bad...

the newer schem is missing the period :\

the newer rev used a 22k, 4.7k and 1k config...

the older version used a 10k, 4.7k and 470 config..


I believe the newer one was to salvage some life out of the EL panel..
Title: All things LA2a related
Post by: wilebee on January 18, 2005, 01:15:18 PM
ok so what voltages should I have on my grid , screen and cathode on v4 ? Wil
Title: All things LA2a related
Post by: Scenaria on January 18, 2005, 01:25:09 PM
if you are using the 22k/1k config then this...

I just checked one of mine that has a 22k/2w

9.05v

89.6v

and

127v on the grid


remember its an approximation
Title: All things LA2a related
Post by: wilebee on January 18, 2005, 01:36:19 PM
so I want 127vdc on my grid and 90vdc on the plate ?
Title: All things LA2a related
Post by: Scenaria on January 18, 2005, 01:39:39 PM
yes... at or about those values...

remember most of those resistors are 5%+ tolerance so the numbers will shift a little....not even counting diff tubes
Title: All things LA2a related
Post by: wilebee on January 18, 2005, 02:17:00 PM
I guess my confusion is that I have more voltage on the grids that I do on the plate if I have the 22k feeding my plates and the 220k feeding grids then maybe I have tube wired wrong
Title: All things LA2a related
Post by: cayocosta on January 18, 2005, 04:15:47 PM
Jahnsen's La2a Book references 2 different schematics and shows: 125V/104V, and 100V/70V. Both are plate/screen voltage.

The '68 JBL schematics shows: 133V/100V.

3 different schematics indicating that the plate voltage is above the screen (grid) and the voltage range.

Wilibee, I'd look for 100V - 140V on the plate and 70V - 100V on the screen.

I'd have to remove my La2a from the rack to check the voltage - and that's a real pain so I won't - but my notes indicate that my voltages were:

123V/86V plate/screen.

(That's with the 10k plate and 470R cathode resistor from the 1968 schematic.)
Title: All things LA2a related
Post by: cayocosta on January 18, 2005, 05:12:41 PM
Schemo Info:

The '68 schematic is the latest one I am aware of. (If anyone has a more recent version, please make it available.)

You can spot an older schematic because it will feature the jumper block on the input.

(This seems to indicate that the 10K/470R 6AQ5 plate/cathode combination is the last revision.)

If the new reissue units are different - and if anyone is interested - maybe someone can have a look and document (and post) the B+ rail details as well as any other significant differences with regard to the '68. We'd then have another viable point of reference.
Title: All things LA2a related
Post by: electronaut on January 18, 2005, 05:24:33 PM
Has anyone ever done the Jensen LA2A mods explained here (http://www.jensen-transformers.com/as/as091.pdf)?  Any comments?
Title: All things LA2a related
Post by: wilebee on January 18, 2005, 05:27:09 PM
well if i'm understanding scenaria correctly he is saying that I should have higher voltage on my grid then on my plate
Title: All things LA2a related
Post by: Scenaria on January 18, 2005, 05:59:34 PM
I'll re-check the numbers tonight..
Title: All things LA2a related
Post by: CJ on January 18, 2005, 06:24:45 PM
There were some stability problems with the 6AQ5, that's why all the fidling with the screen circuit, cathode resistor, etc.

That tube was designed to drive a voice coil, so the EL panel is probably quite a mismatch.
Title: All things LA2a related
Post by: fum on January 18, 2005, 07:07:00 PM
Ok, and limping across the finish line last in the Great PNW LA2A Baja, here's my voltages:

R29 to ground: 367 V
R34 to ground:  266 V
Across the neon me gots 52.2 V

This is sans T4B.  These look ok to me, thought I'd check in, as R34 could be considered a little low (275 was stated V)

Regards

ju
Title: All things LA2a related
Post by: cayocosta on January 18, 2005, 07:44:45 PM
Ju, voltages look great. My B+ was around 266 also - it will fluctuate with your AC mains voltage.
Title: All things LA2a related
Post by: fum on January 18, 2005, 08:32:54 PM
Ok, plugged in T4B, powered up.  No joy.

Passes audio, meter zeros out, but no meter activity in GR mode.  Will have a look see.

Should the EL panel be on all the time, or does it light up when a signal is applied.  Presently, we gots no lights in the EL panel when T4B is installed?

Regards

ju
Title: All things LA2a related
Post by: electronaut on January 18, 2005, 08:36:12 PM
Quote from: "fum"
Ok, plugged in T4B, powered up.  No joy.

Passes audio, meter zeros out, but no meter activity in GR mode.  Will have a look see.

Should the EL panel be on all the time, or does it light up when a signal is applied.  Presently, we gots no lights in the EL panel when T4B is installed?

Regards

ju


When you fiddle with the Zero Adjust, does your neon light?
Title: All things LA2a related
Post by: CJ on January 18, 2005, 08:44:50 PM
JU, the EL should flicker with the music during GR. Follow the ac volts from the 6AQ5 coupling cap to the T4 socket. You need about 100 vac to get that thing lit.

Make sure your T4 socket  has a  good ground. I think it's 2-4-6-8 get grounded?
Title: All things LA2a related
Post by: Scenaria on January 18, 2005, 08:53:40 PM
this might sound stupid... and please dont take it the wrong way but you did turn up the peak reduction pot right?

I just had to ask as im known to make this dumbass mistakes on occassion... usualy following a few pints of guiness

oh and make sure your R37 is turned at the midway point..the same with R3
Title: All things LA2a related
Post by: fum on January 18, 2005, 09:54:53 PM
Quote from: "Scenaria"
this might sound stupid... and please dont take it the wrong way but you did turn up the peak reduction pot right?

I just had to ask as im known to make this dumbass mistakes on occassion... usualy following a few pints of guiness

oh and make sure your R37 is turned at the midway point..the same with R3


Oh, no.  Wouldn't take it wrong.  I've been known to do stupid things like that  on occasion too.  But in this case, I did execute it's full range to see if it would effect the VU.  R3 and R37 are at midway.  I'll check some of the above after the evening ritual( pajamas, teeth brushin, story readin.  X3)  :grin:  

Another thought.  I've got a flourescent on above this when I'm lookin at the T4 panel.  Is this a "day glo" kind of light, or should it be more faint?

I know that the T4 socket has a good solid ground, as I was using it to test some of the other grounding (one lead on the T4).

Regards

ju
Title: All things LA2a related
Post by: SSLtech on January 18, 2005, 10:17:15 PM
It's faint-ish, but visible except possibly in mid-day Florida summertime outdoors blazing sunlight...

You gots a scope to go huntin' with?

Keith
Title: All things LA2a related
Post by: fum on January 18, 2005, 10:43:07 PM
Yup to the scope, would need directions to the favorite huntin spot tho =)

Look for audio signal being fed to the T4?

ju
Title: All things LA2a related
Post by: CJ on January 18, 2005, 10:49:02 PM
Yes, and you have to have the cover on to get good GR. Otherwise cells saturtated with light.

Ohm out  pin 7 of T4, should go to the 2.7 k/pot jumction. And pin 8 goes to ground. Did you wire up the T4 yourself?

Also, might wanna use a coupling cap on scope probe to prevent killing scope.
Title: All things LA2a related
Post by: fum on January 18, 2005, 11:25:18 PM
Did wire up the T4 myself.  checked pin 7 and 8 and they are good with continuity to the correct points( ground and 2.7K/pot junction).

Here's the poor mans test.  It appears that the compression circuitry is working from the LDR onward, just not getting the panel to light up.  I took the can off the T4B, and turned off my overhead light.  Adjusted volume ( My LA2A likes Zeppelin).  Switch on light, Jimmy and Co get much quieter, but are not metering quieter.  So looks like two problems.  No ELP light, and no metering..

Should I try my other T4B?

Checking what kind of AC voltage is coming into the T4B next.

Regards

ju
Title: All things LA2a related
Post by: Scenaria on January 18, 2005, 11:29:05 PM
how about this...

im assuming you have another la2 if you have another t4b no?

pop the one you just built in the known working la2 and see if it works... if not then you know its T4 related if it does then atleast you didnt risk frying an EL in your working T4
Title: All things LA2a related
Post by: fum on January 18, 2005, 11:31:22 PM
Correction.  Did the last test with the panel down.  The metering works just fine too.  It's purely a ELP not lighting problem.

And no, I don't have another working LA2A yet(although that is a good idea =) ).  This is the first of two LA2A's (actually four, but two that will look like this one)to get built.  I do have the other T4B put together tho.



Regards

ju
Title: All things LA2a related
Post by: CJ on January 18, 2005, 11:36:10 PM
You can put 120 vac wall current onto the el panel thru a .1 cap to see if it lights. or go series thru the existing cap tie wrapped onto the panel.
be careful.
Title: All things LA2a related
Post by: fum on January 18, 2005, 11:37:08 PM
Ok.  CJ says that it takes roughly 100V AC to light up the ELP, and I'm only getting like 10V AC ( if I crank the input), so looks like I need to do some hunting for why this is.

And man it's nice when you guys are online.  Progress goes quick  :!:  Thanks  :thumb:

ju
Title: All things LA2a related
Post by: fum on January 18, 2005, 11:46:10 PM
Was reading back through the thread, and noticed the grounding stuff that Zee talked about.  R37 and R3  loop back on themselves.  If they aren't grounded to chassis.....??  Checking now....  No joy...

OK, grounding is all fixed.  Still have the same problem.  I'll be  going over the schematic and layout,  unless someone has more specific direction =).  What I'm lookin for is some kind of error in the gain reduction / attenuator drive amp.  It's not puttin out enough AC poop to drive the ELP.  Plenty of DC   poop (137 V DC), but only 0-10V of AC.

Thanks for your help guys

Regards

ju
Title: All things LA2a related
Post by: Zee1usa on January 19, 2005, 03:21:36 AM
It appears that my La2a is working properly !   :guinness:  :wink:


I'll have to do some more testing , but so far it seems like it is working ok , hum free and all.  Gain reduction and output metering seems to be good.

 Happy Camper !  :thumb:

 Z

 Fum,Wilebee...we should hook up and compare numbers/functionality.  La2apollooza date pending!  :grin:
Title: All things LA2a related
Post by: cayocosta on January 19, 2005, 09:16:34 AM
Ju,

Set R3 and R37 to full clockwise.

Jahnsen's book recommends this. (That's how my La2a is set.) I believe if you set R3 midway, you're not going to get the full input signal to the 6AQ5 - so turn it up all the way and see what you get then.

Also, is your input signal hot enough? The gain control in effect only controls output and is independent of gain reduction. In other words, with the gain set to off, you will still see gain reduction on the meter (in GR mode), but have no output. Therefore, should you have a very weak input signal - and the gain set very high - the sidechain input signal will still be weak even though you have plenty of output volume.

In my experience, the gain and gain reduction controls never seem to need setting above 10:00. That is to say, with a 0dB input signal, the gain reduction control only needs to be at about 10:00 to trigger around -4dB reduction, and the gain control set to about 10:00 should pass that 0dB input back out in +4 mode at 0dB.

So, for the sake of testing, I would check that I have enough input signal to drive 0dB output in +4 mode with the gain control set to 10:00 and the gain reduction control off - this would ensure your input signal is plenty hot enough. Then set gain reduction to about 12:00 and you should be seeing some action.
Title: All things LA2a related
Post by: fum on January 19, 2005, 01:41:57 PM
*Ron takes out hammer, hits it on head*

the signal I was using was not hot enough.   This thing cranks out the volume  goin out, so I thought I was hot enough.  I busted out a powered DI, hooked it up, and bingo, we gots a little ELP party goin on.  meters, compresses, appears to be damn happy.

Now to move it into the studio to give it some better listening, and then on to gettin it's twin done.

Thanks!!

ju
Title: All things LA2a related
Post by: fum on January 26, 2005, 08:38:02 PM
Allrighty then,  I finished my second unit, no problems, fires up like a champ.

Here's some pics I put together of the project:

http://www.shinybox.com/product_info.php?products_id=35

Thanks go out to both Ron and Steve.

Ron, your wiring diagram totally rocks, and was invaluable to putting this together.

Steve, you did an amazing job putting these kits together.  I've still got a pair of Shinybox version to do, but it's nice to have these complete, and moved into the studio.

Regards

ju
Title: All things LA2a related
Post by: wilebee on January 26, 2005, 09:27:10 PM
well as sone as my new son and daughter leave for college I should be able to finnish troubleshooting mine Wil

Wilebee
Title: All things LA2a related
Post by: JerryPbury on January 31, 2005, 06:40:30 PM
Log or Linear Pots for LA2A? Or does it matter?

I 'm not sure that I understand or know which is correct for the 1 meg & 100k pots.

Thanks,

Jerry
Title: All things LA2a related
Post by: Scenaria on January 31, 2005, 06:50:42 PM
the 100k are log

the 1meg ones can be linear
Title: All things LA2a related
Post by: JerryPbury on January 31, 2005, 08:45:41 PM
Thanks Steve!

I used the good log ones for the 100k instead of the Alpha's and then I ordered some 1m Linears. Just need to get some Trannies and time to finish them up.
Title: All things LA2a related
Post by: wilebee on February 01, 2005, 11:49:16 AM
does it really matter whether you use log or linear on the 2 pannel mounted controls if the only thing its changing is the resistive taper ? the reason I ask is because I have 2 really nice linear ah sealed pots  ? Wil
Title: All things LA2a related
Post by: Scenaria on February 01, 2005, 12:09:23 PM
nope it doesnt really matter...

wont hurt anything
Title: All things LA2a related
Post by: wilebee on February 06, 2005, 11:27:13 PM
ok I'm frustrated!  all of my voltages are right except for the plate voltage on the 6aq5. I've got 83vdc on the plate, 120vdc on the grid and and 8.7dvc on the cathode. There is nothing between pin 5 and a20 but a piece of wire.   Help Wil

Wilebee
Title: All things LA2a related
Post by: cayocosta on February 07, 2005, 09:34:34 AM
Wilebee,

What value resistors are you now using for r29, r34 and r36?

Ron
Title: All things LA2a related
Post by: wilebee on February 07, 2005, 11:29:07 AM
r29= 4.7k 2 watt
r34=22k 2 watt
r36=1k
Wil
Title: All things LA2a related
Post by: wilebee on February 07, 2005, 11:35:42 AM
also my ac filaments are a little hot at 6.75 vac
Title: All things LA2a related
Post by: Scenaria on February 07, 2005, 11:47:52 AM
have you tried a different 6AQ5 tube?
Title: All things LA2a related
Post by: wilebee on February 07, 2005, 12:28:24 PM
yes a brand new amperex 6AQ5 Wil
Title: All things LA2a related
Post by: cayocosta on February 07, 2005, 02:17:36 PM
If your mains AC is a little high your heater voltage will be too - no big deal.

Disconnect the grid wire to pin 7 and see if that makes a difference in your plate and screen voltages.
Title: All things LA2a related
Post by: wilebee on February 07, 2005, 02:37:58 PM
The one that goes to the wiper of r3 I tried that no difference. the problems I was haveing in the prevous posts a few weeks backI found I didnt have the connection from pin7 the the wiper Wil
Title: All things LA2a related
Post by: cayocosta on February 07, 2005, 02:42:11 PM
Check to see if there is any DC voltage on pin 7.
Title: All things LA2a related
Post by: wilebee on February 07, 2005, 02:48:28 PM
will do when I'm at home tonight
Title: All things LA2a related
Post by: cayocosta on February 07, 2005, 03:25:12 PM
Wilebee, I suggest you return to 10k for R34, and 470R for R36. With these in place the voltage should be around 130 plate and 100 screen with about 7 cathode. Give or take 10% or so.

Also, while checking voltage, make sure that you don't have any input signal. Set R3 and R37 to full clockwise (full on). Gain and Peak Reduction to full counter-clockwise (off).

Ron
Title: All things LA2a related
Post by: wilebee on February 07, 2005, 03:28:30 PM
ok I'l give it a go Wil
Title: All things LA2a related
Post by: wilebee on February 07, 2005, 09:37:23 PM
ok no dc voltage on pin 7 so I guess i'm chaning out the plate resistor for 10k and  the cathode for 470r Wil


Wilebee
Title: All things LA2a related
Post by: wilebee on February 07, 2005, 10:51:30 PM
ok changed out the 22k plate resistor for 10k and the 1k cathode for 470 and I now have 118 on the plate  113 on the grid and 6.6 on the cathode
thats closer  or is that close enough ? Wil

Wilebee
Title: All things LA2a related
Post by: cayocosta on February 07, 2005, 11:05:19 PM
Wilebee, what's your B+ voltage now?
Title: All things LA2a related
Post by: cayocosta on February 07, 2005, 11:56:12 PM
The voltages look better now wilebee. The screen grid is still a little high, but maybe you should move on and see how it operates - providing you have everything else sorted out.
Title: All things LA2a related
Post by: wilebee on February 08, 2005, 12:33:18 PM
ya know i forgot to see what my overall voltage was. I'l check that out and see if i have sparks yet  Wil
Title: All things LA2a related
Post by: wilebee on February 08, 2005, 11:01:57 PM
well its passing a signal but I need to figure out why its not compressing I cant seemt to get the meter to zero out in the GR position Wil
Title: All things LA2a related
Post by: Scenaria on February 08, 2005, 11:05:14 PM
take the cover off the T4 can and shine some light on the LDRs... does the meter show any deflection in GR mode?
Title: All things LA2a related
Post by: wilebee on February 09, 2005, 10:21:40 AM
will do tonight
Title: All things LA2a related
Post by: wilebee on February 09, 2005, 10:23:30 AM
one other odd note I tried the urei t4b I have a and the gain seemed to shoot up but still no deflection on the meter Wil
Title: All things LA2a related
Post by: electronaut on February 09, 2005, 12:18:15 PM
Scenaria,

Where do you get the housings for your T4s?

Thanks,

E.
Title: All things LA2a related
Post by: CJ on February 09, 2005, 01:09:52 PM
Housings? We could tell ya, but we would have to kill ya!  :razz:
You wouldn't believe what we take out of those things before the T4 stuff goes inside.
Title: All things LA2a related
Post by: Scenaria on February 09, 2005, 01:17:32 PM
they are NOS octal cans. no way in heck to find em anywhere.. got em from a surplus store (bought the last of them)

wished I knew of a place... but have yet to find any...
Title: All things LA2a related
Post by: daArry on February 09, 2005, 01:40:07 PM
Hey,

I got some of these ye oldie Altec plug-in modules thingies that I rek could be ideal for T4 housing. Fully enclosed with octal type pinout on base and plenty of room inside (that pcb can be removed):

(http://notapplicable.co.uk/audiolab/altec-ting_1.jpg)
(http://notapplicable.co.uk/audiolab/altec-ting_2.jpg)

Whatcharek?
Title: All things LA2a related
Post by: thedug on February 18, 2005, 10:38:00 AM
I am building an LA2A. actually from the BLOO kit.

Anyway, his manual give instructions based on 1 schematic and I also have another schematic.

Anybody have any explanations why one would be better than the other.

The differences are around the values for..

R34, R36, R38, C10, C11, & C13.

The are all around/connected to the 6AQ5.

This is on version http://www.vacuumbrain.com/The_Lab/TA/LA2/la2a_layout2.jpg

I don't have a web link to the other.

I can try to send a table of the diffs if folks are familar with these revisions.

I am just basically curious if there is a general assumption that 1 is better than the other and why.
Title: All things LA2a related
Post by: thedug on February 18, 2005, 10:43:43 AM
This is another version of the vaccum brain layout.. but in schematic format..

http://www.buzz-audio.com/stuff/jpgs/ureila2schematic.jpg

Bascially the other schematic changes:

R34 to 22k2W
R36 to 1k
 
C10 to 50uF
C11 to  .1
 
and loses C13 & R38
Title: All things LA2a related
Post by: wilebee on February 18, 2005, 11:54:54 AM
I think,because of something that sceneria said, that the variances in that part of the circuit were there to either save or extend the life of the t4b's EL pannel  Wil

wilebee
Title: All things LA2a related
Post by: CJ on February 18, 2005, 01:50:19 PM
I have Jahnsen's book. He has all the different values used over the years in the appendix.
I can post it if you want.
If somebody else has that book, put that page up. My book is at home and it's friday.
Title: All things LA2a related
Post by: jdr on February 18, 2005, 04:09:33 PM
(http://img.photobucket.com/albums/v713/jdr555/la2arevisions.jpg)

HTH :guinness:
Title: All things LA2a related
Post by: CJ on February 18, 2005, 04:58:53 PM
Thanks jdr-HTH!
Title: All things LA2a related
Post by: thedug on February 18, 2005, 06:40:26 PM
Cool, thanks!

I need to decide which one to build. The Bloo manual is a mixture of 65 and 79. Any thoughts?
Title: All things LA2a related
Post by: cayocosta on February 19, 2005, 10:42:08 AM
Changing the bias point of the 6AQ5 - in my estimation - would have no effect on the AC voltage range to the El panel, which is set by the gain reduction control. (No DC voltage is present at the El as the coupling cap (C11) blocks DC.)

So, regarding the effect of the plate (R34) and cathode resistor (R36) of the 6AQ5 in each revision, the only impact would be on the tube itself, and from what I gather looking at the voltages of the various revisions listed in Jahsen's book, the 6AQ5 current draw at idle is actually higher with 10k/470R, thereby eliminating longevity of the tube as the motive for the change. Thus, the move to 10k/470R was most probably made only to facilitate a more linear operating range for the 6AQ5 - at higher gain reduction settings.

Lastly, as the latest revisions (1968 forward) use 10k/470R, it would be reasonable to assume these values represent best practice.
Title: All things LA2a related
Post by: JerryPbury on March 13, 2005, 03:08:20 PM
I'm finally finishing off the first of my two Bloo LA2A's. I noticed that in the BOM C6 & R30 were included. And that C4 & C14 were shipped with the units. Also instructions are given for the Stereo Adjust Pot hookup. However there is nothing in regards to installing the Stereo connect.
Is this just an oversight or was it intentional? Can I just drill a hole and put in a 1/4" TS connector and hook up C6, R30 & C14? If I decide not to use the stereo connect how far do I go back as far as removing components (R3 & C13)?

Also, in the instructions for the small turret board it states to "connect R4 (contact 3) to B2 on the mini-turret board"(page 23). Shouldn't this be to B1?

Thank you.
Title: All things LA2a related
Post by: Scenaria on March 13, 2005, 03:45:31 PM
the extra trimmer cap was a mistake on my part when I shipped yours out.. initially I did not think there would be much desire for anyone to want to run this type of compressor in stereo..

so yea you can ommit the C14 if you like...

I would suggest putting in R3 and C13 anyhow.. this way if in the future you want to use a TRS jumper to link two together it wont be much hastle...

Contact 3 should be tied to either B2 or B3 of the mini turret.
you have the numbering backwards on R4... it should be 1,2 and 3 starting with 1 being closest to the hinge. contact 1 goes to B1
Title: All things LA2a related
Post by: JerryPbury on March 13, 2005, 08:06:37 PM
Thanks Steve,

I have the connection correct on the R4 to the mini turret board. I just quoted the wrong line. The connection I meant was from B3 to 2a on SW3. From the schematic it's 6 of one, half a dozen of the other as to where this connects (B3 or R4 contact 3).

So do I still need to use R30 and C6 if I'm not using the Stereo link option?


Jerry
Title: All things LA2a related
Post by: Scenaria on March 13, 2005, 08:15:51 PM
I couldnt give you a difinitive answer as ive never tried to build one without...

I would put em in


the reason as to the Br, R4 thing.. (and I know eactly what your talking about) is that it keeps the wiring neater :)

doesnt matter if contact 2 is picking off B3 or R4 just keeps things cleaner

IMHO theres already enough wire in the boxes  :roll:
Title: All things LA2a related
Post by: JerryPbury on March 13, 2005, 10:06:57 PM
Plenty of wire!

OK. So TS or TRS? I only see connection & ground on the schematic for the stereo connect.

Need to get a smaller 10uf/450v (Nichicon?). There is just to much stuff going on in the middle of the chassis. I am using the Sowter for the output.

Jerry
Title: All things LA2a related
Post by: thedug on March 13, 2005, 10:23:22 PM
....ignore me.....
Title: All things LA2a related
Post by: Scenaria on March 13, 2005, 10:29:01 PM
B2 and B3 are linked
Title: UTC - Universally Too Collectible!!
Post by: EmoRiot on March 18, 2005, 01:57:15 PM
I'm starting to realize the trap of my perfectionism - needing to have UTC transformers.  I envision a future where I'm digitally fishing... obsessively checking ebay for a bite.

So I'm starting to wonder about options.  If I were to go one step below UTC but still very good, where would I go - Jensen transformers?

If you were to choose between ordering a Jensen input transformer or the HA-100X reissue part from UAudio, what would you go with?

Can I have a UTC A-24 output transformer and something else as my input transformer? - since finding an A-24 seems easier.

Also, has anyone found a great hidden resource for HA100, HA100X, or A-10s?  Something nice and convenient, like www.abundantUTCtrannies.com or something?
Title: All things LA2a related
Post by: fum on March 18, 2005, 02:05:23 PM
Ebay is your best bet.  patience and esnipe are your tools of the trade.

I did manage to salvage an A-10 out of an old broadcast transmitter that wil turned me on to, but i've gotten an a-10 and a quad of A-24's in the last year, all from the bay.


ju
Title: All things LA2a related
Post by: Scenaria on March 18, 2005, 02:06:57 PM
you can mix and match if you want..

the input doesnt know the output :)

I find the jensens to be quite expensive (they also require some mods) thats not to say they sound bad.. though I still think the sowters look pretty darn impressive for the $$$

we ran alot of various sweeps comparing a sowter based la2 verses UTC trannys... and found them to be near identical.. as a matter of fact the sowters showed a bit more HF response overall...
Title: All things LA2a related
Post by: Scenaria on March 18, 2005, 02:08:37 PM
this thread being brought up just reminded me...

wil?

did you have any success yet?
Title: All things LA2a related
Post by: fum on March 18, 2005, 02:14:01 PM
Ya Wil,  put down your twins and pick up that soldering iron :!:   :wink:

ju
Title: All things LA2a related
Post by: EmoRiot on March 18, 2005, 02:42:32 PM
What about Lundahl?  Do they require a mod and how do they stack up?
Title: All things LA2a related
Post by: EmoRiot on March 18, 2005, 02:53:27 PM
Also, I see in google there was a thread on recording.org where a lot of you guys were comparing the A10 with the new reissued HA100X.

Unfortunately, only the first page is cached with Google and it appears to be off of recording.org.  Does anyone know the results of this comparative analysis.

Is the reissue worthy of its name?

It just seems to me conceptually that the closest thing to the HA100X would be a new HA100X...

or did UA just label a second-rate unit an HA100X to fool people like me?   :wink:
Title: All things LA2a related
Post by: Scenaria on March 18, 2005, 04:03:47 PM
there were some quirky things with the re-issue that CJ came up with .. IMHO the A10 would be closer to the original thing (it actually IS) than the re-issue...

lundahls are very nice trannys but two things to consider... they seem a bit more transparent (less color) and are a little more tedious to mount...

seriously... go with sowters.. they really are good value for the dollar... (I dont own stock in sowter)  :green:

 :guinness:  :guinness:  :guinness:  :guinness:  :guinness:  :guinness:
Title: All things LA2a related
Post by: CJ on March 18, 2005, 07:22:11 PM
The A-10 and the HA-100X are the exact same transformer. Only the cases have been changed to protect the innocent.

A-10= 1 can
HA-100X = three wanna be mu metal cans and 1  steel can.

As mentioned before, 1:10 ratio is a little hot for the input.
A 10k:10k bridging would be better, which is what Jensen recomends.

Any plate to line output could be used. Triad HS-50/52 would be an upgrade.
Peerless S-217-D would be a real upgrade.

Here is the high end plots of un-terminated transformers:

(http://vacuumbrain.com/The_Lab/TA/UTC/ha100_f1.jpg)

Here is some other info:



(http://vacuumbrain.com/The_Lab/TA/UTC/utc_input_turns.jpg)
Title: All things LA2a related
Post by: thedug on March 19, 2005, 09:20:33 PM
I have a question about LA2A gain.

Should the gain affect the gain reduction at all.

Mine appears to be working, however the gain seems to act like makeup gain and not input gain. I had to crank my mic pre to really see the gain reduction meter start moving.

Maybe I just need to adjust my GR meter.

Doug
Title: All things LA2a related
Post by: Scenaria on March 19, 2005, 11:21:16 PM
use the peak reduction control to set the amount of compression... then use the gain control to set the amount of gain you want coming out of the box..

the gain control does not affect compression

basically the peak reduction sets the amount of compression/threshold


its not like the 1176 where you use the input level..
Title: Totally Tubular
Post by: EmoRiot on March 21, 2005, 12:56:42 PM
I'm getting ahead of myself, as my Bloo kit hasn't even arrived yet, but I thought I'd pass the time by asking questions that will ultimately come up anyway.

Today's Question:  What are the best Tubes to use?  Of course, Telefunken is an obvious choice but I've heard about Mullard tubes being as good, if not better?  What's the best choice here?
Title: All things LA2a related
Post by: Scenaria on March 21, 2005, 02:19:07 PM
dont get too carried away with tubes..

I have tried telefunken smooth plates... RCA's, sovteks, raytheon, CEI and JJ's

to be honest I couldnt hear a difference that could justifiy a $80 12AX7 from a $15 one...

just go with a respectable name brand... dont get caught up in the telefunken stuff with this compressor...

the sound is in the iron :)
Title: Sowter, Lundhal or Jensen Xfr for LA2A?
Post by: Bluzzi on March 22, 2005, 04:08:16 PM
I am looking at starting to buy the parts for 2 LA2A and will be starting with the transformers.

The UTC seem to be getting rarer and more expensive these days so I am contemplating others.

I've heard the Jensen and Lundhal ones result in a cleaner more transparent sound (oxymoron?) and so I have included the Sowter ones as well as possible subs. I don't mind a "clean" sound. By that I mean I am not stuck on the clone being exactly part for part as the original. Of course I don't want to stray too far either and build an altogther different beast.

I would just like your opinions as to the best ones and what the differences are when it comes to the sound. What think you all?

Thanks
Title: All things LA2a related
Post by: EmoRiot on March 22, 2005, 04:15:23 PM
If you look at the previous page on this thread, you'll see our discussion about this exact issue.

I raised the same question and the answer basically boiled down to:

Jensen requires a mod and is expensive
Lundahl is very transparent, almost too much so
Sowter is nearly identical to the UTC and very affordable

Sounds like Sowter's the winner to me.

Like I said you can check the previous page for more in depth discussion and diagrams - the works.



I haven't given up on finding some UTC transformers, though.  When you mention that they're getting more rare and more expensive, have you found a place that has them and they're just more than you'd like to spend?

I'd still be interested in them if you could point me in the direction.  That is, again, if you have found them anywhere.
Title: All things LA2a related
Post by: Bluzzi on March 22, 2005, 04:26:43 PM
The Sowters it is then! I was just on their site and it comes to just under $400.00 Canadian for 2 sets (4 Xfrs). The Lundhal and Jensens would be more so the Sowters wins in all cases.

As to finding UTCs, no my only source was eBay and I don't see that many these days. When they are there they go for too much in my opinion and you never have a 100 percent guarantee that they are working!

The T4B is easy to find so now I must look for the VU meter. Should I buy new or used? Hmmmmm. Time to hunt on eBay again.

Thanks for your comments, it made me make up my mind.

Jim
Title: All things LA2a related
Post by: CJ on March 22, 2005, 05:11:04 PM
Sometimes you can get away cheap if you use your noggin and do a little diggin.

Like this auction. Vintage Triad in and out, blows the socks off UTC. Now sitting at 11 dollars for both. Plus, you get a vintage 12AX7a and other parts:

Triad (http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=67815&item=5762015123&rd=1&ssPageName=WDVW)
Title: All things LA2a related
Post by: Bluzzi on March 22, 2005, 05:19:06 PM
Thanks CJ. But I don't have a noggin for Transformers. I wouldn't know what would be an acceptable substitute for the LA2A.

So are you saying the Triad is good for the input? Without changing any part values?

I'm doing this for fun of building but I am not an electronics technician. I've made kits before and some other projects but from schematics and/or wiring diagrams. So to redesign for me is scary.

Thanks for your tip.

Jim
Title: All things LA2a related
Post by: JerryPbury on March 22, 2005, 05:23:36 PM
I just picked up an A-10 and an A-24 for less than the Sowters on EBAY just this last week. I also bought a Thordarson 25A58 (A10) from the Black market. For the output try doing a search for a Thordarson 25S12 (A-24).

Jerry
Title: All things LA2a related
Post by: Bluzzi on March 22, 2005, 05:30:37 PM
Quote from: "JerryPbury"
I just picked up an A-10 and an A-24 for less than the Sowters on EBAY just this last week. I also bought a Thordarson 25A58 (A10) from the Black market. For the output try doing a search for a Thordarson 25S12 (A-24).

Jerry


I must be looking in the wrong places because I just looked a week ago and found nothing (unless I just missed it). encouraging though.

I'll check for Thordarson part. Thanks.

Jim
Title: All things LA2a related
Post by: CJ on March 22, 2005, 05:57:00 PM
Jim, the Triad's are line to grid (in) and plate to line (out). Just make sure what you bid on has that description along with about a 15db rating and you will be alright.
Title: All things LA2a related
Post by: Bluzzi on March 22, 2005, 07:43:30 PM
Quote from: "CJ"
Jim, the Triad's are line to grid (in) and plate to line (out). Just make sure what you bid on has that description along with about a 15db rating and you will be alright.


OK I'll verify. Let me try and understand what you are saying (don't laugh). Basically the transformers for line input for a tube are almost all the same? The 15 db rating is that the gain?

Jim
Title: All things LA2a related
Post by: CJ on March 22, 2005, 08:04:19 PM
Yes, the line to grid x-formers usually have impedances that are within the ballpark of each other. One thing you do have to watch out for is if you are going into a push-pull tube stage-then you must have a dual secondary or center tapped secondary. Every UTC I have seen is built with dual secondaries. Most Jensen's I have seen only have one secondary.

The db rating is how much voltagepower the transformer can handle before saturation/distortion.

Line level stuff requires about 15 db, where as if used as a mic input, the db level can be lower, like the UTC ouncers.
Title: All things LA2a related
Post by: Bluzzi on March 22, 2005, 08:55:17 PM
Ok, thanks I get it. Hence the larger size due to handling higher level.

I'll keep my out for other Trannies but for my first try at teh La2A I am too timid to take a chance and will be sticking with the usual suspects.
If I can't get the UTC ones I will go for the Sowter. After successfully completing at least one La2A I'll feel a lot more confident. Plus I'll have it to experiment with other transformers!

Thanks guys for not making me feel like a total dummy. Its also very encouraging seeing all of you happy with your finished projects.

Oh one more thing. The chassis will be hand made by a friend that has a machine shop. It will be aluminum however and as we all know soldering to chassis is not effective. Do you think there will be a problem with grounding? I'll be making sure that all grounding is done to solid screwed in terminal strips or screws. My experience is that this is OK as long as the screws do not loosen up in any way.

Aluminum is just easily available and easily machined.

Jim
Title: All things LA2a related
Post by: Scenaria on March 22, 2005, 09:02:15 PM
just use turrets and for grounding use grounding lugs screwed to chassis...

no soldering is done directly to the chassis...

oh and the size really doesnt have to do with the amount of singal the tranny can handle..

the HA-XXX series simply has more shielding...

the A10 I believe is about as decent as the HA series and is in the smaller series cann

U all have to be patient when looking for old UTC's :) just take your time... you will find em.. in the past year ive probably nailed a dozen A-24's and two dozen HA100X's and A10's

they are out there  :green:

but.. the sowters do sound pretty decent and are readily available.

if this is your first DIY.. I would suggest sitting down and itemizing your expenses... figure out your budget... I think many are surprised when they realize the la2's start climbing over $500-$600
Title: All things LA2a related
Post by: EmoRiot on March 24, 2005, 10:55:43 AM
On ebay now there's an HA100X.  It's got three days of bidding left and it's already sitting at 300.00!!!

Am I correct to assume that this thing has already surpassed its value and we should all be waiting for either an A-10 to come up on ebay, or for a HA100X auction involving more rational bidders - or at least more impoverished bidders?  I mean, this thing might get near 500 dollars by the time bidding and sniping is done! :shock:

So much for my eSnipe that was set up.    :cry:


Also, Scenaria, when you mention the HAXXXs having more shielding than the A-10, what effect does that have on sound?  Thanks for any info.
Title: All things LA2a related
Post by: Bluzzi on March 24, 2005, 12:59:58 PM
See, thats what I meant about prices being too high. The occasional one goes for reasonable amount.

The Sowter is always there and will do the job quite well I'm sure.

There is also an A-24 at $131.00 already ! Nope I'll pass, thanks.

Jim
Title: All things LA2a related
Post by: EmoRiot on March 24, 2005, 01:10:56 PM
When Scenaria said "I think many are surprised when they realize the la2's start climbing over $500-$600", he might as well have said "I think many are surprised when they realize the TRANSFORMERS for the la2's start climbing over $500-$600!"   :shock:
Title: All things LA2a related
Post by: CJ on March 24, 2005, 01:32:55 PM
Remember, the A-10 is a better transformer than the 100X. The extra shielding of the X reduces noise, but takes off more high end due to capacitance to ground. There is a trade off for everything. Improve one thing, and you sacrifice another. Electrically, they are the same transformer.

Consider the A-26 as a sub for the A-24 if you spot one. I got two for 50 bucks one time. Same x-former as the 24 except higher pri. impedance, which might be a good thing.
Title: All things LA2a related
Post by: Bluzzi on March 24, 2005, 02:05:07 PM
CJ - If you look on eBay for A-26 you will see a whole bunch of "wiring" machines for sale. It looks to me like they really are "winding" machines.

If anyone is into winding transformers (I know CJ is) maybe these are a good bargain. They even have a turns counter.

Jim
Title: All things LA2a related
Post by: on March 24, 2005, 10:19:55 PM
does any one know what are exceptable voltage diffrences,

im using alen bradly 1/2 watt carbons ,  most all my voltages seem
fine give or take 1-10 on a few.

and i know carbons can be +/- 10-20%


 how critical is this?

i have a ua reissue , but the voltages differ from the schematic

give or take 20-30 volts on some.

and then also there are a few different voltages in jahnsens book too.

 the UA is wired similar to cayocostas layout except the strange little

zener next to the ne-2 , and the 500 pf near the arco vari cap.

(bastards)

thanks
Title: All things LA2a related
Post by: Scenaria on March 24, 2005, 10:57:03 PM
if the voltages are in the ball park.. I would say your fine for testing...

ive built about 8 of em and they are all different

the zener was added later as to stabalize the meter reference as the voltage wouldnt drift as much with heat


you will notice that the neon on its own will regulate a little different as heat build up.. this is why you have the zero adjust on the front panel..

you could put a zener in there if you want but its really not that important :) I just trim my meter with the front panel control.. no biggie
Title: All things LA2a related
Post by: Bluzzi on March 24, 2005, 11:41:23 PM
Another question. Can any standard VU meter be used or adapted? I imagine the answer is yes but just checking before I start looking to buy.

Thanks.

Jim
Title: All things LA2a related
Post by: DIYDan on March 25, 2005, 12:11:05 AM
A good question.  One I'd like to know the answer to as well.  My VU meter is a Sifam.

It has a note on the back for its lamp that says 12V 40ma.  Is it tough to incorporate this into the schematic?
Title: All things LA2a related
Post by: on March 25, 2005, 10:32:57 AM
thanks so much , i appreciate the feed back  :thumb:

driller45
Title: All things LA2a related
Post by: EmoRiot on March 28, 2005, 09:44:17 AM
If a description of an A-10 reads like this:

"Tested OK for continuity.  Half of one of the U shaped terminals are gone on pin 2. Pins 8 & 9 are connected together."

Does that sound like it's useable or destined for a repair?
Title: All things LA2a related
Post by: CJ on March 28, 2005, 03:29:47 PM
Probably usable.
Title: All things LA2a related
Post by: Bluzzi on March 28, 2005, 07:39:51 PM
I'll be keeping my sight out for UTC trannies but the Sowters seem to be readily available and guaranteed to work (you never know from eBay).
Also CJ pointed out some equivalents UTC, Thordarson etc. Right now everyone seems to be going for the A-10 and the A-24. Prices lately have been ridiculous on those.

What about meters though? Is it OK to get any VU meter? I got no response before. Anyone?

Again here I would like to eBay them but I am also looking at the Sifam and teh Modutec. The Modutec looks cheaply made as opposed to the Sifam but is less costly of course.

Any of you make the La2A with the Modutec? If not what meter did you use?


Jim
Title: All things LA2a related
Post by: EmoRiot on March 28, 2005, 08:44:49 PM
Firstly, I don't want to get off the topic of what Bluzzi is asking, so I'll second the motion for a clarification of meter selection.  :wink:

But...

I just picked up a pair of HA100Xs on ebay.  They just posted and I used "buy it now" to save me the emotional roller-coaster of bidding, sniping, and ultimately losing the auction!

Yesterday a HA100X went for 370 in a regular auction.  These two were 375, so all-in-all... they're way more than they should have been but if eBay's got them going 370 for a single, I figure I can always sell one of them (since I only need 1) and possibly make all the cash back... like getting a free HA100X in the long run.  Or, if I build a second LA2A, I've already got the 100X and avoid the hunt.  Either way, win/win.  :cool:

Here's my question:

The pair I just got look like they're missing some screws on the top, but I'm not sure.  I'm only basing my assessment on the picture of the HA100X that sold on ebay yesterday for 370.  Take a look at that one (I've added arrows pointing to what looks like screws):

(http://www.emoriotproductions.com/tranny_screws.jpg)

Now look at a shot of the pair I just picked up (top view):

(http://www.emoriotproductions.com/tranny_top.jpg)


Is it missing something?  Are those screws hard to find or they a simple trip to the hardware store?

ps.  Yeah, what about those meters?
Title: All things LA2a related
Post by: Bluzzi on March 28, 2005, 09:04:03 PM
EmoRiot - congrats on your buy. II saw them but didn't want to buy at those prices. Prices seem to be on the rise as of late. If you look back a bit they were reasobly priced.

But...I can sympathise with just wanting to get it over with and getting them. I would sell the other one and try the Sowter as they are the same thing. I was being stickly in the beginning of my search but when I see guys like CJ and Scenaria mentioning other equivalent parts, hey, they know better.

I am building 2 so I need a pair of everything.

I can't be certain about the screws but I wouldn't worry about them. Probably for mounting.

You know JBL-Pro has the best price for the T4B (unless you want to DIY one).

The only I need to research now is the meter. I can go for the Sifam which I know will work but I am not sure about others.

Please keep this thread going if you are making one I would like to keep in touch with you as we both progress. If it is too much for a thread you can PM me.

After wanting to build first a Green Pre then I switched to a Tube Mike then to a tube preamp I am finally committed to an LA2A. I need compressors the most and wanted to start with a tube design.
Once I get near completion I will post my experiences (sourcing etc) if it helps anyone.

I am very excited about making an LA2A. Feel like a kid!

Jim
Title: All things LA2a related
Post by: EmoRiot on March 28, 2005, 10:25:05 PM
Bluzzi:  "I am very excited about making an LA2A. Feel like a kid!"

lol...  yeah, this is legos on steroids!   :green:
Title: All things LA2a related
Post by: EmoRiot on March 29, 2005, 12:39:34 PM
Bluzzi,

I hope that someone with more expertise than you and I way in on this meter thing, but I'll tell you the info I've got from Jahnsen's book.

"The Selco AL39WF is close to the same size meter as the Beede used in the LA2a.  (Beede is making electronic parts for Ford now and is no longer making meters.  For a great vintage look, check out the Selco "Vintage" Meter types 20A SQ and 25A SQ."

Now, this information is old.  Selco was, i believe, the US distributor for Sifam.  So Sifam is the new name... but the product numbers are the same - at least for the vintage ones (that I can confirm).  

You can check out the vintage meters HERE (http://www.meterdistributor.com/SIFAM/SIFAM%20Pg3.htm).  The regular Sifam meters are also HERE (http://www.meterdistributor.com/SIFAM/SIFAM%20Pg1.htm)

If you need a place to order from, I can recommend IMS.  They're in Los Angeles, CA, which probably means mail order for you but you don't come across a store just for meters very often and the guy there knows his stuff.  You could probably even ask him your meter questions and get a fast answer.  They do mail order.  The number is 818-243-0416.

Another source of meters is SURPLUS SALES (http://www.surplussales.com/Meters/MtrVU.html).  They've got some of the Anthony De Maria Labs' meters, among others.

Someone else asked something about the power needed for the meter.  It makes me wonder also... do different meters draw different amounts of power and is it tough to make that work?  For example, backlit meters vs. non-backlit.  There's got to be a power discrepency here, right?
Title: All things LA2a related
Post by: cannikin on April 01, 2005, 11:12:00 AM
Hey Guys;

I'm putting my Bloo together,  I have Sprague Atoms for C7a/b/c/d
They are HUGE!  I was wondering if I could attach wire to the Leads of the caps and mount them a little (not much) but farther away from the tag strips, so I have some room to work.

Anything I should consider before doing this? I have Belden Hi temp. 800V hook up  wire.  

I think I'm going to have to mount the Power trannie now too, so I can get to the screws.... Ugh!
Title: All things LA2a related
Post by: Scenaria on April 01, 2005, 12:05:34 PM
you dont really need atoms for the power supply... my advice would be to get smaller caps... there isnt much space in there to begin with..
Title: All things LA2a related
Post by: CJ on April 01, 2005, 03:45:27 PM
The HA-100X has four metal tubes that are welded inside the can. They are partially treaded for # 4 screws ata each end to allow for different mounting options.
Title: All things LA2a related
Post by: Bluzzi on April 01, 2005, 04:09:21 PM
EmoRiot - I know about the Sifam but the Surplus Sales site is handy. Maybe some good meters there for the SSL compressor as well. Thanks.


But I still have no clue on  VU meters ?

Besides the Sifam ones can we get one of those Weston vintage ones that pop up on eBay from time to time?

If they are not exactly the same as Sifam will they function as replacement or will we have to adjust something like a resistor value?

I know how to select resistors, capacitors and most components but am totally ignorant when it comes to selecting the correct meter (besides the Sifam one)

Jim
Title: All things LA2a related
Post by: EmoRiot on April 01, 2005, 05:50:40 PM
Bluzzi,

I'm going to try a new strategy in getting someone who actually knows the answer to respond.  I'll give you completely wrong information and hopefully someone will correct us.  :wink:

:wink:  :wink:  :wink:  Uh... yeah, I think you can put any meter in there!  In fact, a little known trick is that if you wire the meter directly to the incoming AC current, you'll get a better read on the meter.  There's 3 resistors that need to go sequentially after the meter, but they can be absolutely any value of resistor made!  If you can't afford a meter, a classic trick that was even used in the revision 3 of the LA2a was to attach a rubber band to a paper clip and wrap it around a deck of cards.  That will meter correctly too!  But you have to find the right tension of rubber band.  That's the only trick with that one.  :wink:  :wink:  :wink:

What do you say guys?  Anyone want to step in and correct us before we're hooking rubber bands up to our AC current?

Bluzzi just wants to know if he can use any vanilla variety of VU meter?  Help!   :shock:  :?: :?:
Title: All things LA2a related
Post by: Bluzzi on April 01, 2005, 07:55:30 PM
EmoRiot - Don't worry about it. The guys here are very helpful and generous with their time and knowledge. Its a big forum and they don't necessarily see or have time to answer all questions. Sometimes I think they want guys like me (more assemblers than electronics engineers or gurus) to learn some things on their own. Its not a bad idea to seek some info by ourselves in books etc. I know a little about electronics but not enough to make decisions on design (especially on a classic design such as the LA2A).

I'll ask my tech resident electronics genius. I just thought some that have built the LA2A clone using an alternate meter would know.

I'll call him tomorrow and get back here with an answer hopefully.

Jim
Title: All things LA2a related
Post by: EmoRiot on April 01, 2005, 09:16:47 PM
Definitely,

The guys on this board are the coolest and most selfless/giving people I've seen in the audio biz.  :thumb: The amount of hours spent by some to make sure that everyone's coming along well in their projects and that the DIY community, as a whole, is progressing in collective knowledge is really mind boggling.  My humor definitely wasn't driven by an ungrateful expectation to get free help from geniuses.

I think you're right.  We'll take this as a challenge to learn on our own.  I've bought  a couple of books on electronics so that I can finally start learning not just the "how" of building stuff... but the "why."

If you figure it out, let us know what you come up with.
Title: All things LA2a related
Post by: PRR on April 01, 2005, 09:58:47 PM
> Anyone want to step in and correct us before we're hooking rubber bands up to our AC current?

It will be a lot more exciting to stick the paper-clip in the wall output.

Kids, don't try this at home!!! Go to the neighbors house and try it.

> no clue on VU meters?

All true-VU meters are interchangable. It is a Standard, as far as electric and dynamic response goes. Mundane non-audio details, like mounting and lighting, are at the maker's discretion, because they don't matter to the audio function.

But there are a lot of fake VU meters. From 1938 through the 1970s, anything that looks like a VU probably is. But a lot of cheap recorders appeared in the 1970s: VU-like meters from old Akai and Sony consumer recorders won't be true-VU, and may be very un-VU, needing extra drive power and having odd dynamics.

> can we get one of those Weston vintage ones that pop up on eBay from time to time?

Weston made a lot of true VU; I think the Standard is based on a Weston design.

But before 1938 there were other Standards. I have a lovely pair of pre-1938 Weston audio meters with internal rectifiers and scales like a VU, but much slower and calibrated to different level. I was in the right place at the right time: it is unlikely you will find anything like that.

> will they function as replacement or will we have to adjust something like a resistor value?

When you hang a VU meter across an audio circuit, it really needs to be very much like a VU meter. Lesser meters will load and distort the signal. The rise of fake-VU in the 1970s is because buffer amps became cheaper than good meters: they could use a lame meter and buffer it off the audio path.

Also a true-VU always has an internal rectifier. When you hook it to an audio output, it gives a reading related to signal. But a basic meter is DC-only: on audio it just waggles a little on bass-hits, does not read level. The true-VU rectifier is specialized: in the 1970s it became cheaper to use generic rectifiers in the buffer-amp on the PCB, and use a common DC meter.

The Sifam is almost certainly fake-VU, but apparently close enough that it can be used in place of a true-VU without recalibration. The dynamics would not fool an old VU-watcher like me, but you are not using it as your main system level indicator.
Title: All things LA2a related
Post by: Bluzzi on April 02, 2005, 12:06:49 AM
PRR - Thanks for the clear explanation on VU meters. I am expecting a few from eBay soon. Is there a way to test if they are good?Can I just use a line level to drive thm directly or could that damage the coil? From what you said it would seem that they can take line level.

Jim  (next I  get two T4B)
Title: All things LA2a related
Post by: CJ on April 02, 2005, 03:52:50 PM
The old vu's are like a swiss watch. Took one apart the other day. Copper oxide diodes are spring loaded in a tube. Little round pellets. Pretty weird.
Title: All things LA2a related
Post by: Bluzzi on April 02, 2005, 06:34:43 PM
Thanks for the Weston tip. I've got 6 Weston's coming from eBay! I think I have enough for now!

I'll be ordering 2 T4B next.

On another note.....
I just bumped into a friend of mine that makes signs etc. Well I asked her about engraving and although she doesn't do that what she does do that may be something to look into is this.

She has a computerized plotter/Cutter that will do up to 24 inch wide and any length precise cutting of adhesive backed vinyl. So the letters get cutout and anything underneath then shows through. This vinyl comes in any colour.

So I thought I could do 2 things.
One is to do a sheet with all the lettering cutout so that the metal colour comes through for the letters. Of course one could also paint those areas.
Juat stick the sheet to the faceplate.

The other is what she suggested. Stick the sheet to a plexiglass sheet that is then held to the faceplate by the switches and pots and mounting screws ( the exact same cutout for the faceplate but in plexiglass. This would protect the vinyl and faceplate from eventual scratches.

If I decide to do this I'll advise if its worth it. Up to today I wasn't going to  do anything but anodyse the faceplate (and only because its free).

Jim
Title: What pots to buy?
Post by: QNote on April 04, 2005, 11:16:03 AM
Hi all!

[In the middle of the Bloo kit project...]

Which pots are ya'll using for this project (Mouser/DigiKey part numbers would be nice).  The log types in the needed values appear to be rare.  I rather not purchase "cheap" ones, unless that is what everyone is going with.
I have some of the older Allen-Bradley pots, but they appear to act a little "dirty"--rather not use them.
Title: All things LA2a related
Post by: EmoRiot on April 04, 2005, 11:38:38 AM
::Which pots are ya'll using for this project::

I was wondering about the Precision Electronic pots used in the UA reissue.  They're 10% tolerance, instead of the Allen-Bradley's 20%, but I don't know if this makes a difference.
Title: All things LA2a related
Post by: QNote on April 04, 2005, 03:19:14 PM
I don't think the tolerance is such a big deal.  I would be looking for a "quality" pot at a reasonable cost.  Longevity and reliability are the most important factors for my choice potentiometers.  Pots from "the shack" are not going to give many years of trouble-free operation.

Which pots are going to be the best value given this criteria?
Title: All things LA2a related
Post by: cannikin on April 05, 2005, 11:30:45 AM
Quick Question:

After linking pins 1,2,4,6,8, on the T4B.  

Do connect them to ground?
Title: All things LA2a related
Post by: QNote on April 05, 2005, 11:40:07 AM
Quote from: "EmoRiot"
  ... I was wondering about the Precision Electronic pots used in the UA reissue.  They're 10% tolerance, instead of the Allen-Bradley's 20%, but I don't know if this makes a difference.


I checked these out.  Digikey carries them for a very reasonable price.  I will be purchasing these, myself.  I already have the linear taper pots (Clarostat) for the rear panel.

I have also ran across a ton of NOS resistors that have never been opened (it appears) until now.  The labels on the box say they were purchased in 1965.  These are a rare find.  There is Ohmite, AB, and many others, in MANY different values from 1/4 watt to 10 watt--all with the MIL part numbers!  I'm hoping everything is here that I will need, as far as resistors.  There were also some "goodies" in the box!  You'll see them in the pictures once I near completion of the unit.

I will also be using "vintage" wire, as well (mainly for the vintage look).  All of the wire is NOS, including twisted pair and some small shielded cabling, as well.  Some of the wiring has the older "cloth" type sheathing--very neat looking.  It appears to be in great shape, too.

Things are getting exciting. Now off to place a tube order...
Title: All things LA2a related
Post by: Scenaria on April 05, 2005, 11:53:52 AM
Quote from: "cannikin"
Quick Question:

After linking pins 1,2,4,6,8, on the T4B.  

Do connect them to ground?



yes
Title: All things LA2a related
Post by: on April 06, 2005, 04:45:04 AM
are there any sugestions on setting the stereo adjust /limiter response

c4/c14 ? (once the compressor is built)

 is using a spectrum anilyzer the best ?

 what are good values to replace c4 with , since that beotch just sits
there once it is set.

 could the stereo adjust /limiter response be replaced with

 fixed resistors , or do units vari too much , to use a standard
value , (i.e testing the factory setting on ua reissue pot and using that.)

  wouldn't that just be a range between 1-1meg @ 1/2 watt ?
(not sure the actual wattage of those pots)

   anyways i don't like those pots , their mean , and i promise there
gonna get it.
Title: High voltages
Post by: JerryPbury on April 06, 2005, 03:35:22 PM
Finished up my first kit. AC is exactly 120V.
AC transformer is putting out 550VAC (+50). All my test points are +25vdc to +60vdc except for the neon (53.5) and T4B which is at spec. I have not run a signal through it. Come to think of it all of my tube projects over the years have always run higher voltages than spec. Any thoughts?

Thanks,

Jerry
Title: All things LA2a related
Post by: Bluzzi on April 06, 2005, 03:41:11 PM
For now I will be buying 2 T4Bs but I would like to experiment with making my own.

The LDRs are no problem but  Iam wondering about the EL panels. They are expensive and some have made T4Bs using LEDs or other EL material available much cheaper.

Can anyone give their opinions if EL other than the LSI ones can result in quality units. By that I mean "sound" as good or better.

Just wondering if its worth it to experiment.

Thanks.

Jim
Title: All things LA2a related
Post by: Scenaria on April 06, 2005, 03:42:06 PM
are your tubes in?

the neon is correct voltage
Title: All things LA2a related
Post by: Scenaria on April 06, 2005, 03:44:08 PM
the EL thing has been done over and over..

simple fact... though the EL's seem expensive ($30 does some rediculous) it is what works and has been tried and tested.

how much could you really save? $20 $25? if your only making one or two then wouldnt that amount be worth the satisfaction of knowing your using what was originally spec'd?
Title: All things LA2a related
Post by: Bluzzi on April 06, 2005, 03:57:02 PM
Yes I know its been done and I can create a "clone" as well, but I was wondering if using another material would alter the qualities.

Besides the money ($2.00 for the case, $6.00 for LDRs aprox.) I would suspect that other EL material may work. Wether it works or not is worth the effort to me in order to "see for myself". Isn't that why we are here in this forum, to have fun and end up with something to be proud of?

I think if you count the time and money put into a DIY there is no saving. For me its more for self satisfaction and curiosity. I figure for under $10.00 I can experiment. I will be eventually getting the EL from LSI as well.

I wanted to see if others had built some without the original material and gotten usefull results.

Jim
Title: All things LA2a related
Post by: JerryPbury on April 06, 2005, 05:02:50 PM
Tubes are in so is T4B. Nothing is hot or smoking or popping. I'm going to pass a signal through it tonight. See what happen's.
Title: All things LA2a related
Post by: Scenaria on April 06, 2005, 05:44:43 PM
my mistake...

if you want to experiment with other light sources then by all means have fun.. I thought by reading your question you were asking if there was a cheaper/better aternative..


jerry... be sure to turn the two pots on the back fully counter clockwise :)
Title: All things LA2a related
Post by: rafafredd on April 06, 2005, 07:02:35 PM
Well, CJ talked about a new T4 he built some time ago from all Radio-Shack parts. He said it was the best sounding he had...
Title: All things LA2a related
Post by: JerryPbury on April 06, 2005, 07:12:53 PM
It seems all high voltages are turrets connected to the 6AQ5A. I used Cayocosta's 1968 layout. There is no connection on pin 1 shown (and no reference in the instructions as to a link between pin # 1 & 7). However all of the other schematics that I have (at least 3) show a link between pin # 1 & 7 on V4. So it looks like I need to link these. More later.

Jerry
Title: All things LA2a related
Post by: Scenaria on April 06, 2005, 07:52:50 PM
linking 1 & 7 isnt necessary.. but you can if you want..

if your voltages are off make sure that R34 R29 and R36 are from the same schematic...

either 470 and 10k/2w or 1k and a 22k/2w

if you get these mixed up your voltages will be off..
Title: All things LA2a related
Post by: Bluzzi on April 06, 2005, 07:57:16 PM
Yes I think I read that somewhere about CJ's T4B. II think he used an alarm clock's EL material. I have some EL material to try from a night light but I welcome any other sources of alternatives.

The whole thing is that ithe T4B also changes the LA2A's attack and release so some T4B designs may be good for drums while others maybe better for vocals etc.

I am speaking not from personal experience but from readings. After I buid 2 LA2A's I'll start experimenting with T4B's and interchanging them to compare. Same with transformers. I'll be using the Sowters but I would like to hear the Jensen and Lundhal ones as well.

Jim
Title: All things LA2a related
Post by: JerryPbury on April 06, 2005, 08:54:03 PM
Steve,

This is what I have.

R29 4.7K / 2 Watt
R34 10k / 2 Watt
R36 470 ohm

It's just the voltages that are connecting to V4 @ pin 2, 5 & 6 (and affected connections).

Actual:           Should be:
B21 370v        360v   +10v
A21 330v        275v   +55v
A20 328v        131v   +197v!
A19 323v        105v   +218v!
A16 47v             7v   +40v

Checked all of my connections and component values. All appear OK. I know it's something dumb but I sure haven't been able to track it down. All other voltages are right on the money including heaters. I have switched every tube and still get exactly the same results. I'm baffled.

Thanks,

Jerry
Title: All things LA2a related
Post by: Scenaria on April 06, 2005, 09:00:45 PM
did you turn the two pots on the back fully counter clockwise?
Title: All things LA2a related
Post by: Scenaria on April 06, 2005, 09:05:07 PM
whats your B+ voltage?

and are you sure that you have a 4.7K in there and not a 47k for R29?
Title: All things LA2a related
Post by: JerryPbury on April 06, 2005, 09:25:07 PM
Steve,

B+ 330v

4.7k

CCW (doesn't make any difference in voltage readings).

Thanks,

Jerry
Title: All things LA2a related
Post by: cannikin on April 07, 2005, 03:49:28 PM
steve;

I'm hooking up the Allied Power Trannie and just want to confirm my connections:

Primary:

1 Black to Live on power connector (Wire to B24 then to B25)
1 Black to Neutral  

Secondary:

Center Tap 6.3v (green w/yellow stripe) to ground Bus/TAG 3
Center Tap 250v (red w/yellow stripe) to ground bus/TAG 3

2 - 6.3v (green) to A24 & A25 with heater Wire.
2 - 250v (red) to A22 & A23

Ground:

Grey to Gound on the Power Connector.

Thanks.
David
Title: All things LA2a related
Post by: QNote on April 07, 2005, 03:52:30 PM
For a DIY EL panel, try this vendor...

www.mpja.com

They carry an EL panel that is only $2.25.  It is larger than what is needed for a T4B, though.  But, if you use a larger housing, I think it would work great.  I didn't see any LDR's, though.  Sorry.

Later,
Title: All things LA2a related
Post by: Scenaria on April 07, 2005, 04:44:15 PM
Quote from: "cannikin"
steve;

I'm hooking up the Allied Power Trannie and just want to confirm my connections:

Primary:

1 Black to Live on power connector (Wire to B24 then to B25)
1 Black to Neutral  

Secondary:

Center Tap 6.3v (green w/yellow stripe) to ground Bus/TAG 3
Center Tap 250v (red w/yellow stripe) to ground bus/TAG 3

2 - 6.3v (green) to A24 & A25 with heater Wire.
2 - 250v (red) to A22 & A23

Ground:

Grey to Gound on the Power Connector.

Thanks.
David


yep.. sounds good
Title: All things LA2a related
Post by: Bluzzi on April 07, 2005, 04:57:03 PM
Quote from: "QNote"
For a DIY EL panel, try this vendor...

www.mpja.com

They carry an EL panel that is only $2.25.  It is larger than what is needed for a T4B, though.  But, if you use a larger housing, I think it would work great.  I didn't see any LDR's, though.  Sorry.

Later,


Thanks Prentice,

I'll try one to see if it works (I don't know if 400HZ will cause a problem).
There are nightlites available here locally and I have a few of those so I will be canibalizing those as well. The LDRs I have sources for and are an easier item.

Jim
Title: Question for anyone who used Sowter Trannies
Post by: cannikin on April 13, 2005, 10:50:16 AM
Hey Guys

I'm still a little lost when it comes to wiring the sowter input trannie.  

Input:

just leave the pink one untied (cap it with some heat shrink) - DONE
Black to ground - DONE
brown and green from the input XLR - DONE (Green to XLR 2 Brown to XLR 3)

Here is my point of being lost on the input:
blue and grey are the outputs.  Do I wire a 68K resistor to each wire?  then from there is connects to...?
Also R7 (2.7K)?

My next confusion:

put a couple of 1k resistors on the grids of the 12BH7A this will help stabalize that tube.  Grids?
Title: All things LA2a related
Post by: JerryPbury on April 13, 2005, 10:55:28 AM
Sowter has a schematic on thier web site for hookup of the LA2A.

http://www.sowter.co.uk/

Look under Pro Vintage LA2A.

Jerry
Title: All things LA2a related
Post by: cannikin on April 13, 2005, 11:02:48 AM
Quote from: "JerryPbury"
Sowter has a schematic on thier web site for hookup for the LA2A.


Thanks I've been looking at that thing for a couple of days, and the answer has eluded me.. Oh did I mention I suck at reading schematics?
Title: All things LA2a related
Post by: Scenaria on April 13, 2005, 11:33:31 AM
I'll try and help ya when I get home...

im falling behind as im litteraly dealing with an average of 50-60 emails every couple of days
Title: All things LA2a related
Post by: JerryPbury on April 17, 2005, 10:24:33 PM
Quote from: "JerryPbury"
This is what I have.

R29 4.7K / 2 Watt
R34 10k / 2 Watt
R36 470 ohm

It's just the voltages that are connecting to V4 @ pin 2, 5 & 6 (and affected connections).

Actual:---------Should be:
B21 370v------360v___+10v
A21 330v------275v___+55v
A20 328v------131v___+197v!
A19 323v------105v___+218v!
A16 47v--------7v_____+40v

Checked all of my connections and component values. All appear OK. I know it's something dumb but I sure haven't been able to track it down. All other voltages are right on the money including heaters. I have switched every tube and still get exactly the same results. I'm baffled.

Thanks,

Jerry


Thought I would bring this up again as I have not found a cause/solution for the high voltages on V4. Any suggestions? I have checked every component, connection and layout several times over with no change.
Title: All things LA2a related
Post by: Bluzzi on April 18, 2005, 01:50:18 AM
I just bought some A-24 transformers and should be getting in a week or 2. (I was going for Sowter but managed to get UTC ones). Got my meters too and its getting exciting! A-10 and T4Bs next and then I'll be almost there.

Is continuety test good enough to test transformers? I'd just like to have an idea as it will be months before I gather all my parts and start wiring.

Thanks.

Jim
Title: All things LA2a related
Post by: CJ on April 18, 2005, 12:57:24 PM
Yes.
Title: All things LA2a related
Post by: Bluzzi on April 18, 2005, 01:43:42 PM
Thanks CJ! Continuety I can handle...whew!

Here's another question in my mind.

I bought 4 A-24s! I am building 2 LA2A so maybe I have all my audio transformers already. Do I have to mod anything in the LA2A circuit if I use an A-24 reversed as imput?

I'll be trying to get A-10s or Sowter but maybe I can get going sooner instead of waiting to get the other trannies.

Jim
Title: All things LA2a related
Post by: CJ on April 18, 2005, 02:32:57 PM
A-24 reversed would be perfect for the LA2.Better than the A-10.

Less step up.

1:5 as opposed to 1:10.

No shields required at line level, especially with the humbucking structure of the UTC products. (two coils on one core, just like a guitar pick-up.

I wonder if Seth Lover stole his humbucker pickup design from UTC?
Title: All things LA2a related
Post by: Bluzzi on April 18, 2005, 05:12:00 PM
Better! That is good. So I will go ahead and keep my eyes open for A-10 (only because I would like to hear the difference). But I will build the 2 LA2As with A-24s to start.

Thanks again for sharing your knowledge CJ.

Jim
Title: All things LA2a related
Post by: CJ on April 18, 2005, 05:42:05 PM
No problem. Let me know how the A-24 as input sounds.
You should have better high end which will help, as the high end gets squashed durring compression.

 :guinness:
Title: All things LA2a related
Post by: Bluzzi on April 18, 2005, 09:06:23 PM
I will CJ - If you are willing to wait a few months! I still have many parts to get....and I'm buying way too many meters etc...can't help it...must stop...
Title: All things LA2a related
Post by: EmoRiot on April 18, 2005, 09:31:53 PM
jim,

congrats on those a-24s!  I saw them and considered getting them just to have around in case I build another LA2a, or four.  :wink:

I've noticed that the bulk discount on ebay is strong.  Each A-24 would have sold for just over 100 bucks each.  But listing all four together comes up cheaper.  Go figure.

What meters have you gone with?
Title: All things LA2a related
Post by: Bluzzi on April 18, 2005, 10:22:14 PM
Quote from: "EmoRiot"
jim,

congrats on those a-24s!  I saw them and considered getting them just to have around in case I build another LA2a, or four.  :wink:

I've noticed that the bulk discount on ebay is strong.  Each A-24 would have sold for just over 100 bucks each.  But listing all four together comes up cheaper.  Go figure.

What meters have you gone with?


Thanks. I was giving up on getting any and I feel I paid too much but I just want to get going on getting the main and rarest parts such as trannies and T4B (a T4B just sold for $200.00 on eBay! You can buy them new for $88.00!). So now I have 4 and as CJ says they can be used as input as well so I am not that desperate to get A-10 or Sowter.

The meters I got are Weston. Big black ones with 2 bulbs in them. They are cool and I prefer to Sifam.

(http://i21.ebayimg.com/03/i/03/b5/71/c4_1_b.JPG)


I also got 4 other Westons but more modern look but also gigantic scale!

(http://i21.ebayimg.com/01/i/03/b4/d3/4b_3.JPG)

The Daven pots are neat and seem to still work (tried only with a VOM so maybe they are noisy).

I am building 2. One for me and one for my friend that has a recording studio and ......a MACHINE SHOP ! Ha! Amazing luck.

If they turn out great (just being cautious) I hope to make 2 more as I would like a pair.

I hope they turn out half as good as everyone else here. For me DIY is not to save money but to have the pride of having built something (even if its not my design) as well as learn a little along the way. I think there is something inside all DIYers that is a little of the mini scientist (mini for me not for the rest of you). We love to discover and chalenge ourselves.
In the end as I told my friend I will pay less for my LA2A in dollars but in time it will be 3 times over. I don't mind. I am finishing my studio for the same crazy reason. D. I. Y its crazy but its passionate.

(I don't know where that came from but there I said it and I'm sticking to my story).

Jim
Title: All things LA2a related
Post by: JerryPbury on April 20, 2005, 11:38:40 PM
I'm shelving my first LA2A until I have completed my 2nd one.

I have checked every component and every connection (3rd time) and can find nothing different from the schematic. I layed it out according to the bloo instructions / Cayacosta layout and the only variance that I could come up with was at the R12 & R14 connections. Bloo instructions are flipped and Cayocosta layout is correct but labels are flipped. I also used a 10K/2W for R34 (22K is not consistant with schematic when using a 4.7K @ R28). I still have B+ (~330vdc) @ A19,20,21 & B20. I am at a total loss as to what is the cause. C7B? I don't know. I checked everything as suggested and it was all in line with what it should be (excepting the voltages) It passes signal and the gain stage works. Voltages at the T4B and neon are correct.

Thanks,

Jerry
Title: All things LA2a related
Post by: EmoRiot on April 20, 2005, 11:57:34 PM
Quote from: "JerryPbury"
Bloo instructions are flipped and Cayocosta layout is correct but labels are flipped.


Could Jerry or someone else expand on what this means precisely.  I'm about to start my first Bloo and I'm not sure if this is an instruction error I could correct preemptively.

thanks!
Title: All things LA2a related
Post by: Scenaria on April 20, 2005, 11:57:42 PM
so if its passing audio.. and t4 looks good is it working or whats the symptoms of it not working?


realize that most of the voltages are approximate.... they will and can vary quite a bit depending on resitors etc.. I know on mine im using carbon comps that are 5% tolerance...
Title: All things LA2a related
Post by: Scenaria on April 21, 2005, 12:19:22 AM
Quote from: "EmoRiot"
Quote from: "JerryPbury"
Bloo instructions are flipped and Cayocosta layout is correct but labels are flipped.


Could Jerry or someone else expand on what this means precisely.  I'm about to start my first Bloo and I'm not sure if this is an instruction error I could correct preemptively.

thanks!


what he is referring to is the versions of the schematic...
Title: BYPASS
Post by: chriss on April 21, 2005, 05:04:37 AM
Hi there,

I'd like to know where to best place a bypass-relais in the LA2A-circuit. Best would be inbetween the in- and out-transformers, so the chain is still transformer-coupled but you can compare before-after. But then: I guess the voltage at C5 is rather high. Can a normal relais handle this?
Any ideas upon this? I haven't found this discussed anywhere on the forum, so I try here.

thanks

Chris
Title: All things LA2a related
Post by: JerryPbury on April 21, 2005, 08:00:36 AM
Quote from: "JerryPbury"
This is what I have.

R29 4.7K / 2 Watt
R34 10k / 2 Watt
R36 470 ohm

It's just the voltages that are connecting to V4 @ pin 2, 5 & 6 (and affected connections).

Actual:---------Should be:
B21 370v------360v___+10v
A21 330v------275v___+55v
A20 328v------131v___+197v!
A19 323v------105v___+218v!
A16 47v--------7v_____+40v


I tried it with an SM57->pre->La2a->amp-> 4" speaker. Sounded pretty much the same as when LA2A was bypassed. I couldn't reall tell if the gain reduction was working. I was just looking to see if it would pass signal and if it had gain. I haven't attempted to set it up yet (other than zero the VU) as I was concerned with the +200 volts @ A19 & A20! I am familiar with tube circuits using carbon comps (I have built 6 tube amps) and service all (10) of my tube amps. None of them have exhibited that much variance. The +55v doesn't concern me as my trannie is putting out +50v over.

I will be out of town this weekend but will attempt to set it up when I return.

Thanks,

Jerry
Title: All things LA2a related
Post by: Scenaria on April 21, 2005, 08:16:01 AM
did you see deflection on your meter in GR mode? put a scope across the output to see if you can see the level reduce in GR mode...
Title: All things LA2a related
Post by: JerryPbury on April 21, 2005, 08:27:49 AM
Quote from: "Scenaria"
did you see deflection on your meter in GR mode? put a scope across the output to see if you can see the level reduce in GR mode...


No deflection, but I hadn't adjusted the zero yet and it was near the bottom of the scale. Had plenty of gain. I could drive the amp into distortion and feedback quite easily. I will put it on a scope when I return from NC.

Thanks,

Jerry
Title: All things LA2a related
Post by: Scenaria on April 21, 2005, 08:32:54 AM
hmmm

make sure the front panel trim can set that meter to zero first...  your right if its sitting way at the bottom its unlikely you would see any movement
Title: All things LA2a related
Post by: JerryPbury on April 21, 2005, 09:00:41 AM
Quote from: "Scenaria"
hmmm

make sure the front panel trim can set that meter to zero first...  your right if its sitting way at the bottom its unlikely you would see any movement


The VU would raise up from the left at start up in GR, just not much. It zeroed from the trim easily. I haven't tried it out since adjusting the zero other than to check voltages again. It sure takes a while for the Electrolytics to drain, you can see it in the neon and the VU after shutoff.

Jerry
Title: All things LA2a related
Post by: Scenaria on April 21, 2005, 09:44:29 AM
hmmm....

mine dont take that long to drain (when the tubes are in) I would say under 10 seconds...


so when you power it up with GR selected where is the needle sitting? at zero or a lot lower?
Title: All things LA2a related
Post by: JerryPbury on April 21, 2005, 10:01:27 AM
Quote from: "Scenaria"
hmmm....

mine dont take that long to drain (when the tubes are in) I would say under 10 seconds...


so when you power it up with GR selected where is the needle sitting? at zero or a lot lower?


Takes several minutes to drain. Checked all of my grounds etc. for continuity and everything checks OK. Tubes are in. I thought that it was a long time. I'm used to them draining much faster.

After adjusting my VU zero, it moves to zero in GR at startup.
Title: All things LA2a related
Post by: Scenaria on April 21, 2005, 10:20:09 AM
then the tubes must not be loading the p/s down...

this is a first ive seen of this one..

wanna mail one down for me to take a look at?
Title: All things LA2a related
Post by: JerryPbury on April 21, 2005, 11:12:43 AM
Quote from: "Scenaria"
then the tubes must not be loading the p/s down...

this is a first ive seen of this one..

wanna mail one down for me to take a look at?


6AQ5A? This is where I have all of the high voltages at the connections to this tube.

I have swapped all tubes with the same results. Both 6AQ5A's, etc.

If that's the one I'll send it out when I get back.

Thanks,

Jerry
Title: All things LA2a related
Post by: cayocosta on April 21, 2005, 11:43:21 AM
The 6AQ5 is not biased correctly. It's not pulling any current, that's why your B+ is high and you're not dropping any voltage across the plate resistor - R34.

This could be caused by:

DC on the grid - pin 7. (That could occur with a leaky capacitor - C9)

A bad cathode ground.

A reversed electrolytic capacitor on the cathode - C10.

The heaters are not on.
Title: All things LA2a related
Post by: JerryPbury on April 21, 2005, 01:02:58 PM
Quote from: "cayocosta"
The 6AQ5 is not biased correctly. It's not pulling any current, that's why your B+ is high and you're not dropping any voltage across the plate resistor - R34.

This could be caused by:

DC on the grid - pin 7. (That could occur with a leaky capacitor - C9)

A bad cathode ground.

A reversed electrolytic capacitor on the cathode - C10.

The heaters are not on.


DC on grid. Will check. C9 did check OK but I did not check for leak.

Verified all grounds.

Verified cap orientation. Electrolytics do get ugly when put in wrong (did this once in a old Dynaco ST70).

All heaters checked OK. Verified voltages and continuity & connections. As well as a visual that all heaters were on.

Thanks! I'll check for leak past the cap. Will check everything else again and if it solves the problem will let you know which one it is.

Jerry
Title: All things LA2a related
Post by: cannikin on April 23, 2005, 01:48:37 PM
Ok I'm finally at the initial testing stage, Here are my measurements without Tubes and T4B:

AC Volts Between A24 & A25 - 7.14 AC
B21 - 379.5 dcV
B20 - 380 dcV


My Neon Isn't illuminating

A1 - 23.68 dcV

Any thoughts??
Title: All things LA2a related
Post by: cayocosta on April 23, 2005, 03:56:47 PM
I say pop in the tubes.
Title: All things LA2a related
Post by: cannikin on April 23, 2005, 04:04:34 PM
Quote from: "cayocosta"
I say pop in the tubes.


Really? pop in the tubes...

but whats up with my neon and the low voltage on A1
Title: All things LA2a related
Post by: cayocosta on April 23, 2005, 04:17:18 PM
Is your B+ voltage at the top of the 470k (R28) the same as the B+ voltage at the top of the 10k (R34)?
Title: All things LA2a related
Post by: cannikin on April 23, 2005, 04:33:38 PM
Quote from: "cayocosta"
Is your B+ voltage at the top of the 470k (R28) the same as the B+ voltage at the top of the 10k (R34)?


They are exactly the same :grin:
Title: All things LA2a related
Post by: cayocosta on April 23, 2005, 04:36:58 PM
I don't know what voltage to expect at the NE-2 with the power supply unloaded. But your voltages appear to be good. Put the tubes in - and be careful.
Title: All things LA2a related
Post by: Scenaria on April 23, 2005, 05:21:54 PM
double check your wiring on the rotary switch...

I just fixed wil's and it had a similar problem... neon didnt light and the VU didnt swing up to zero (it was shorted to ground)... found the GR wiring off the rotary to be incorrect...


the neon should light even if the tubes arent in
Title: All things LA2a related
Post by: cannikin on April 23, 2005, 05:34:56 PM
thanks scenaria;

That was it, I had a little short near SW3.


Anyway the Neon is glowing and my A1 reading is 57 dcV.
Title: All things LA2a related
Post by: Scenaria on April 23, 2005, 06:05:40 PM
rock-n-roll

plug those tubes in :)


and share pix!!! you all put in all of this time but never post pix

 :roll:
Title: All things LA2a related
Post by: Scenaria on April 23, 2005, 06:07:08 PM
oh and be sure to turn the two pots on the back fully clockwise...
Title: All things LA2a related
Post by: cannikin on April 24, 2005, 11:05:56 AM
Hey LA2a Gurus;

Despite my initial (no Tubes) voltage values being correct, I haven't put the tubes in the unit yet, because I have one snag....

The unit is in permanent power "ON" mode.  :shock:

The wiring of the P. trannie:  
Primary:
1 Black to Live on power connector (Wire to B24)
1 Black to Neutral

Secondary:
Center Tap 6.3v (green w/yellow stripe) to ground Bus/TAG 3
Center Tap 250v (red w/yellow stripe) to ground bus/TAG 3

2 - 6.3v (green) to A24 & A25 with heater Wire.
2 - 250v (red) to A22 & A23
Ground:
Grey to Gound on the Power Connector.

The wiring from B24 & B25 to the SW1 are correct.

What is eluding me is the connection of B25 to the Power Trannie.
Right now I have nothing connected to B25 other than SW1

I know thats not right - I'm missing a connection, which is disabling SW1
Title: All things LA2a related
Post by: Scenaria on April 24, 2005, 11:11:44 AM
B24 should come from the hot coming out of the IEC connector... and tied to one side of the power switch

B25 should be tied to the other side of the switch and also tied to the black primary lead of the power transformer...

the neutrl should go right from the IEC to the power transformer
Title: All things LA2a related
Post by: cannikin on April 24, 2005, 01:24:26 PM
Ok The Tubes are in, Here are my #'s:

B21 - 351V      
B20 - 351V**    Should be roughly 275V
A1 - 56.1V      
A14 - 233V**    Should be roughly 216V      
A17 - 7.1V**     Should be roughly 90V
A19 - 146.1V**   Should be roughly 105V
A20 - 156V**     Should be roughly 131V

** Are Suspect.

Whatcha Think?
Title: All things LA2a related
Post by: Scenaria on April 24, 2005, 02:17:38 PM
what values are you using for:

R36
R31
R34
R29

?

do you have BOTH R37 and R3 turned fully clockwise? (looking at them from the back of the chassis)
Title: All things LA2a related
Post by: cannikin on April 24, 2005, 02:33:33 PM
R36 - 470R
R31 - 1K
R34 - 22K 2W
R29 - 4.7K 2W

Both R37 & R3 are 100% Clockwise
Title: All things LA2a related
Post by: on April 24, 2005, 02:59:36 PM
Both R37 & R3 are 100% Clockwise ?

does every body just jeave these two @ clock wise ?

 ive been wondering about all the final settings upon compleation.

 c14(open or closed or fixed) /r37 / and r3 along with c4(open closed or fixed) .

 i am wiring up another la-2 for stereo ,

  does anyone have a method of calibrating two la-2 to stereo along with induvidual calibration?

 any help is appreciated.[/quote]
Title: All things LA2a related
Post by: Scenaria on April 24, 2005, 03:33:30 PM
change out R36 for a 1k and your values should look better

470 would be alright if you swapped the 22k and put a 10k instead
Title: All things LA2a related
Post by: cannikin on April 24, 2005, 04:43:07 PM
swapped R36 to 1k

B21 - 349.2v  :grin:
B20 - 260v     :grin:
A1 - 60.1        :grin:
A20 - 13.37V**  :sad:
A14 - 210v       :grin:
A17 - 10.65V**   :sad:
A19 - 68.3V**   :sad:
Title: All things LA2a related
Post by: cannikin on April 24, 2005, 04:53:50 PM
Quote from: "Scenaria"

oh and make sure you have the jumper strapped across A11 and A14...it wont cause the problem your having but it is something I missed in the docs


Scenaria;

I read this earlier in the thread should I do this also?
Title: All things LA2a related
Post by: Scenaria on April 24, 2005, 04:56:46 PM
yea you can put that jumper in but it wont solve that one voltage

it doesnt make sense that B20 is in the ball park but A20 isnt...

are you sure the heaters are on all the tubes?
Title: All things LA2a related
Post by: cannikin on April 24, 2005, 05:32:39 PM
I traced the heater Wire and everything is correct.

Heres a little explanation of what is going into A20 and B20.
Maybe you can see the issue.

A21(260.1V) to B20 (259.9V) to B19 (259.9V)
B20 (259.9V) to R34 (22K 2W) to A20 (13.27V)
A20 to V4 Pin 5 (13.27V)

I also replaced R34 with another 22k 2W.... no change.
Title: All things LA2a related
Post by: Scenaria on April 24, 2005, 05:39:32 PM
do you have the T4b in?

theres a turret that C11 ties over to coming from the octal socket... this point is coming from pin 5 of V4...just want to make sure your using a turret and not grounding it to chassis or anything?

if the T4b is in .... take it out and check the value of A20

try re-adjusting the two pots on the back... just turn them a little bit... every one I have built I have always noticed I have to turn the pots a few times and then leave them full clockwise...
Title: All things LA2a related
Post by: cannikin on April 24, 2005, 06:01:38 PM
The reading for A20 has been without the T4B
Title: All things LA2a related
Post by: cannikin on April 24, 2005, 06:11:30 PM
Ok the T4B is in...

I turned the R2 "peak reduction" Down all the way and I'm getting 149.9V at A20  and 280.1 at B20
Title: All things LA2a related
Post by: Scenaria on April 24, 2005, 06:19:07 PM
so all of the voltages look to be within range now?

flip the rotary to GR...

is your meter sitting at or around zero?

use the front panel trimmer to set the meter to zero VU


turn the peak reduction down...

throw in some signal... switch your meter to +4

turn up the level and see if the meter is showing level


if it is then switch to GR and turn up the peak reduction to see if theres some compression..

my bet is everything is working now
Title: All things LA2a related
Post by: cannikin on April 24, 2005, 06:25:58 PM
Thanks scenaria for all your help, this has been a huge learning curve.  I'll give it the audio test later tonight, You mentioned to me a while back about strapping 1K resistors across the grids to prevent the unit from becoming an expensive antenna.

Can you elaborate on that for me.

Thanks again.
Title: All things LA2a related
Post by: Scenaria on April 24, 2005, 06:35:30 PM
you might not even need them... so try it without first...

im assuming yout using the sowters?

if you do need them it only need to be done to the 12BH7A

drop a 1k resistor in series on pin 7

and put a 1k in series on pin 2

you already have wires going up to those two pins... so all I did was pull the wires off pins 7 and 2

solder a 1k on pin 7 and the wire to the other side


and a 1k to pin 2 and a wire to the other side

so they fall in series now... get it?

only need to be done if your getting some wierd oscillations on the output
Title: All things LA2a related
Post by: cannikin on April 26, 2005, 12:03:35 PM
Ok I Still have to do the tests Scenaria recommended.  

but...

The Unit is Quiet
It is passing audio! :grin:
The unit is really colored, bordering on... Dirty (although, I have to test it with other Mics and Pres). But any thoughts are welcome.
R1 is working, good gain.
If I turn R2 below 20; the unit makes this Low Freq. clicking sound (actually sounds like its farting).  
Lastly, The VU isn't happening at all, I have to rewire SW3 and all the front panel pots; using better wire- making better contacts (I think the I have a short somewhere).

This is very exciting.

Talk to you soon.
Title: All things LA2a related
Post by: Scenaria on April 26, 2005, 12:24:28 PM
if you havent put the grid stoppers in yet then do so... that clicking sound will go away once you put them in.. same problem I had when I put sowters in..


oh and be sure to make sure your making good ground contact with the pots etc.. scrape a little powder coating off the chassis or do what I do... I run a small piece of wire to each pot and back to the main ground...
Title: slow release time
Post by: Ad0lescnts on May 04, 2005, 08:12:19 PM
Hello,
I finally finished my homebrew LA-2A and it sounds and works well except for one problem.  It seems the release time is verry slow.  While compressing program material the gain reduction meter seems to just move very slowly and stay in the general area according to how much peak reduction is applied.  THen once audio is stopped from being fed into it, the meter gets to about -2 after about a second or two and then sloooowwwly climbs to up to 0.  I'm sure this isn't normal and am wondering what could be the problem?

Thanks!
Tyler
Title: All things LA2a related
Post by: Scenaria on May 04, 2005, 08:21:07 PM
did you make your own T4b or buy one already made?

keep in mind that la2a's by their very nature are rather slow.. more of a leveler than a compressor.. good on things like bass guitar..

not so good on things like drums..
Title: All things LA2a related
Post by: dukasound on May 05, 2005, 09:04:00 AM
Hi
Does anyone discuss about R25 (27K-100K) resistor. I used on mine 33K.
Also, anyone who use Sifam VU, I think, must use 3,6K instead R24-3,9K.
Because I will build another one what mains transformer (220V pri.) is good.
Duka
Title: Re: slow release time
Post by: SSLtech on May 05, 2005, 02:00:32 PM
Quote from: "Ad0lescnts"
While compressing program material the gain reduction meter seems to just move very slowly and stay in the general area according to how much peak reduction is applied.  THen once audio is stopped from being fed into it, the meter gets to about -2 after about a second or two and then sloooowwwly climbs to up to 0

...If you're compressing hard, this sounds like every LA-2a I know... the release is not simple or constant.

Sounds normal to me.

Keith
Title: All things LA2a related
Post by: sleepingtiger on May 10, 2005, 08:35:11 AM
Hello All,

This is my first post. I've been checking it out for a while and I love it here... I'm building the Bloo kit and I'm pretty inexperienced and could use some help. This is confusing to me:

Quote
Posted: Thu Apr 07, 2005 7:49 pm    Post subject:    

--------------------------------------------------------------------------------
 
steve;

I'm hooking up the Allied Power Trannie and just want to confirm my connections:

Primary:

1 Black to Live on power connector (Wire to B24 then to B25)
1 Black to Neutral

Secondary:

Center Tap 6.3v (green w/yellow stripe) to ground Bus/TAG 3
Center Tap 250v (red w/yellow stripe) to ground bus/TAG 3

2 - 6.3v (green) to A24 & A25 with heater Wire.
2 - 250v (red) to A22 & A23

Ground:

Grey to Gound on the Power Connector.  


It's the black wires from the primary that I'm confused about. I've ended up with something that, to me, looks much more like Ron's wiring diagram . What I don't understand is how the switch can work if B24 and B25 are linked together as suggested above. Doesn't that complete the circuit that the switch is there to control? Perhaps I'm having a hard time understanding the wording. The wiring diagram is a lot of help but it doesn't show the connections at the power connector. Can any of you guys get me on the right track?

Much appreciated,
Title: All things LA2a related
Post by: cannikin on May 10, 2005, 07:03:21 PM
I got it to Compress... :grin:

I still need to go over the unit - yet again . :shock:

Now...  trouble spots

1. - my gain is low
2. - I have to put 1K across the grids of the 12BH7A  (because I'm getting a little high frequency tone)
3. - meter is inactive at +10 or +4 - but works for the gain reduction

thoughts?

SleepingTiger - Get Cayacosta's La2a Layout diagram its very helpful in tandem with the bloo manual.
Title: All things LA2a related
Post by: cannikin on May 10, 2005, 08:36:38 PM
I still have some really odd values/readings, some are right on, all others are way off.  I checked my connections and component values at least 8 or 10 times over the coarse of 2 - 3  weeks.  Everything is correct.

I have using reference ground from various places over the unit and the readings are the same.

B21: 361v
B20: 360v :evil:
A1: 57.1v
A20: 288v  :evil:
A14: 241v   :evil:
A17: 100v
A19: 188v  :evil:

How can some values be happening and others wacked?
Could a cold Solder joint cause this?  I didn't see any - but I could have missed it - too much to look at.
Title: All things LA2a related
Post by: Scenaria on May 10, 2005, 09:14:26 PM
just send it down... I will be happy to look at it...

you say you put the 1ks across the grids?

I hope you mean in series with each grid... across to me means bridging between the two
Title: All things LA2a related
Post by: JerryPbury on May 10, 2005, 09:33:56 PM
Quote from: "cannikin"
I still have some really odd values/readings, some are right on, all others are way off.  I checked my connections and component values at least 8 or 10 times over the coarse of 2 - 3  weeks.  Everything is correct.

I have using reference ground from various places over the unit and the readings are the same.

B21: 361v
B20: 360v :evil:
A1: 57.1v
A20: 288v  :evil:
A14: 241v   :evil:
A17: 100v
A19: 188v  :evil:

How can some values be happening and others wacked?
Could a cold Solder joint cause this?  I didn't see any - but I could have missed it - too much to look at.


Welcome to my world. My voltages are still high. I have checked every component, ground, wire, heater connection and touched every solder joint over again, yet my voltages are way high. Cayacosta said it was probably my bias. I checked every related component, ground and heater connection and it didn't change. I even changed all related caps. The voltages still remain high. I'm just dyslexic. I hope I don't do the same thing to the second one.
Title: All things LA2a related
Post by: sleepingtiger on May 11, 2005, 01:29:59 AM
OK,

Finished wiring the Bloo tonight, threw in a fuse, fired it up, and....

nothing... :?

No smoke, no fire, no blown fuse, no voltages anywhere that I checked but at the rear of the power connector and on both sides of the switch (124.6). Obviously, I'm still having a problem at the Allied transformer...

I've got the live connection at the power connector going to B24 which is wired to one side of the switch. Both primary leads are wired to B25 which goes to the other side of the switch. The grey wire (I think that this is the problem) is going to the neutral connection on the power connector. Should this be going to ground? If so, isn't there supposed to be a connection to neutral? I'm afraid to just start moving wires around without getting a better idea of what's wrong. The Bloo instructions just tell me to connect the neutral wire, but I don't know what that is... The Cayacosta diagram doesn't show the connection in detail (or at least I don't understand it if it does...)

I know, I'm an idiot...

Thanks for any help offered...
Title: All things LA2a related
Post by: sleepingtiger on May 11, 2005, 11:32:44 PM
OK,

Sorted it out... (Thanks, Scenaria!) For anyone else who might have the same problem:

Shielded grey to ground. Either black primary can go to neutral while the other goes to one side of the switch at B25. The live wire from the power connector goes to B24 and the other side of the switch... (this info appears earlier in this thread - I had looked over it before)

Turned it on and everything looks promising. I'm going to re-read this thread now before I put the T4B in tomorrow....

Fingers crossed.
Title: All things LA2a related
Post by: drpat on May 12, 2005, 06:24:37 PM
...
Title: All things LA2a related
Post by: Scenaria on May 12, 2005, 06:38:22 PM
tried big ole film caps.. and atoms

10 and 20uf

didnt hear a difference
Title: All things LA2a related
Post by: sleepingtiger on May 13, 2005, 01:25:29 AM
OK,

Here's some voltages:

Location:           should be:         my reading:
B21                  360v                 365v
B20                  275v                 290v
A1                    60v                  53.5v
A20                  131v                107.5v
A14                  216v                238v
A17                  90v                  94v
A19                  105v                129v

The compressor is behaving normally, with peak reduction knob at 9'oclock I'm seeing meter dips of -2 or -3 db when in gain reduction mode. With gain at about the same or a little more it sounds good but the output into my DAW is very low (this is with the gain knob at about 9 or 10o'clock). As I turn the gain knob up the signal begins to distort but even at full CW position I can only get to about -35 db or so into Protools (this is with peak reduction at full CCW - off - position). When I switch the meter mode to +4 or +10 the meter reflects the low output by barely rising from the far left position.

I'm feeding the compressor with a very adequate signal from an A12 pre (I tested it without the comp in the chain to be certain). I'm using the Sowter output tranny. Unfortunately, I don't have a scope. Does anyone have a suggestion for isolating the problem?

Thanks...
Title: All things LA2a related
Post by: cayocosta on May 13, 2005, 07:50:47 AM
Are the pots on the back (R3, R37) full clockwise?
Title: All things LA2a related
Post by: Scenaria on May 13, 2005, 08:11:20 AM
with the pots on the back turn them all the way down then turn them fully clockwise...
Title: All things LA2a related
Post by: sleepingtiger on May 13, 2005, 09:30:58 AM
Yes,

I've done that. At this point I think that I've tried every possible switch / pot position combination and I very specifically turned the ones in the back all the way down then all the way up. I've re-read these posts and tried everything that seems to be applicable...

It seems like the main clue is that the meter shows the low output level in the +4 and +10 modes. Right now I feel like the Sowter output transformer is the most likely culprit but at this point I'm at a loss for what to do. To me, it looks like it's wired in correctly. This is probably something obvious that will bring one of those great forehead-slapping moments later...

Thanks for the responses (Cayacosta - the wiring diagram has been extra helpful). Please keep the ideas coming... :grin:
Title: All things LA2a related
Post by: Scenaria on May 13, 2005, 09:47:23 AM
when you switch to GR on the mter... does it show propper GR?

meaning does the needle reflect reduction as you bring up the Peak reduction knob?

I think I would simply begin tracing the signal forward..

start by scoping the wiper of the input gain knob.. if signal is strong there then move forward...by checking your 12AX7.. if it all looks good then check your 12BH7A

becareful... theres high voltage on those tubes... be sure to set your scope probe to X10
Title: All things LA2a related
Post by: sleepingtiger on May 13, 2005, 10:00:13 AM
Yes,

The meter shows gain reduction normally in GR mode. The further the knob is twisted clockwise the more the needle deflects. It's audible as well. In fact it sounds totally correct and great, just quiet....

I don't have a scope or access to one. Is there any way to figure out what's going on with a meter?

There's also one odd thing that might be a clue. When I unscrewed the left faceplate screw to drop the faceplate, the meter dropped about two db. When I re-tightened it, it went back to zero. There was a point in its travel where the thumbscrew was like a control for the meter. Does that tell you anything?

Thanks again...
Title: All things LA2a related
Post by: Scenaria on May 13, 2005, 11:20:01 AM
your using sowters right?

this wont have anything to do with gain but you should put some grid stoppers on the 12BH7A...

just put em in series with the leads going to each of the grids on that tube

1K is fine

if your seeing the needle changing as the door is opening and closing I would double check my wiring for any loose connections.
Title: All things LA2a related
Post by: sleepingtiger on May 13, 2005, 11:39:23 AM
Yeah, Sowters, and I've already got the grid stoppers in place. I'm going back to the studio in a while and I'm going to re-heat some solder joints in the pertinent areas. Hopefully I'll find the problem that way...
Title: All things LA2a related
Post by: sleepingtiger on May 13, 2005, 03:45:43 PM
OK,

Good news:
The meter no longer moves when the face is loosened or tightened. The negative side was barely touching one of the rear pots. Fixed it.

Bad news:
I'm at a loss as far as the output issue. I've reheated every joint that I can think of and am pretty much just stabbing in the dark. Any help is appreciated, especially from anyone with experience with the Sowter transformers....

Thanks.
Title: All things LA2a related
Post by: Scenaria on May 13, 2005, 03:49:39 PM
ok the good news

no smoke!

just relax... we'll get it working... bear in mind long distance trouble shooting over text can take some time.

I'll see if I can find some time this evening to give you some ideas... no scope really does make it more difficult though...

no worries.. if all else fails you can always send it down to me... though im betting we can get it going in the next few days
Title: All things LA2a related
Post by: cayocosta on May 13, 2005, 04:02:16 PM
Check your voltages for the amp section - the 12ax7 and 12bh7 - against the schematic.
Title: All things LA2a related
Post by: sleepingtiger on May 13, 2005, 06:16:40 PM
Quote
ok the good news

no smoke!


Amen, brother! No shocks or injury either. Definitely a positive perspective...

I'm not worried. It'll all come together. I can already tell that it's the real deal even if the level's low...

Cayocosta, Thanks... Will do. I'm not sure where to find those voltages. I looked earlier but maybe they weren't on the particular schematic that I looked at but I'll figure it out. I'm not at the studio at the moment. I'll get back to it tomorrow...

Thanks again!
Title: All things LA2a related
Post by: cayocosta on May 13, 2005, 06:22:15 PM
http://home.earthlink.net/~cayocosta/diy_files/la2a/la2a_1968.jpg
Title: All things LA2a related
Post by: sleepingtiger on May 13, 2005, 06:32:38 PM
Thanks! You rock hard! I missed this one...
Title: All things LA2a related
Post by: alexc on May 14, 2005, 02:51:51 AM
Hi

here's some pics of my a*d*l 1000. Similar to original
but with pcbs, pcb-mount t4b and not sure what transformers.
Same valves, all in a line.

(http://users.bigpond.net.au/alexcaudio/adl1000_1.jpg)
(http://users.bigpond.net.au/alexcaudio/adl1000_2.jpg)
(http://users.bigpond.net.au/alexcaudio/adl1000_3.jpg)

I like it. Classy sound on guitars and bass.

Never heard an orig, but this things is cool.

Going to build me an la2a and have two! :cool:

Bye
Title: All things LA2a related
Post by: cannikin on May 14, 2005, 05:50:33 PM
Quote from: "JerryPbury"
Quote from: "cannikin"
I still have some really odd values/readings, some are right on, all others are way off.  I checked my connections and component values at least 8 or 10 times over the coarse of 2 - 3  weeks.  Everything is correct.

I have using reference ground from various places over the unit and the readings are the same.

B21: 361v
B20: 360v :evil:
A1: 57.1v
A20: 288v  :evil:
A14: 241v   :evil:
A17: 100v
A19: 188v  :evil:

How can some values be happening and others wacked?
Could a cold Solder joint cause this?  I didn't see any - but I could have missed it - too much to look at.


Welcome to my world. My voltages are still high. I have checked every component, ground, wire, heater connection and touched every solder joint over again, yet my voltages are way high. Cayacosta said it was probably my bias. I checked every related component, ground and heater connection and it didn't change. I even changed all related caps. The voltages still remain high. I'm just dyslexic. I hope I don't do the same thing to the second one.


Well Scenaria & JerryP

I'm Making some headway....

I replaced the crappie ceramic Disks - 470pf (Parallel to the Neon) and 510pf (V4 to R3) with Mica, and my numbers are getting much better.
I also had a metal film 33K at R25, I replaced that with a carbon.

Check this out:

B21 - 356v
B20 - 282v
A1 - 57.1
A20 - 104v
A14 - 229v
A17 - 94.6
A19 - 132v

Though the numbers are much much better ... A20 is a little low and A14/A19 still a little high.

Any thoughts?
Title: All things LA2a related
Post by: sleepingtiger on May 14, 2005, 05:55:24 PM
So I think I've narrowed it down considerably. V2, pin 8 cathode voltage is 120VDC instead of 1.1V and V1, pin 6 plate voltage is about 240VDC instead of 120V. All of the other voltage are a little (consistently) high but close to normal. I'm double checking the wiring and values around the problem area but so far I haven't found the problem...
Title: All things LA2a related
Post by: on May 14, 2005, 09:45:59 PM
if you could speak in actual la-2 (schematic) resistor numbers and not Bloo numbers im sure a lot of other people could help you.

 i have the ciricuit memorized backwards and forwards

 and i have no idea what the heck b21 and a14 are.


perhaps there should be a bloo la-2 thread intstead of a general

 la-2 thread.

 just a thought cause i sure would love to help.
Title: All things LA2a related
Post by: cannikin on May 14, 2005, 10:05:21 PM
Hey driller45;

Silly me - Sorry, didn't think of that...

Anyway I'll try to figure out where they would be on the schematic...

But they are the Turret numbers on the Main Turret.  Going Left to Right  - 1 through  25,  Back row A/Front row B.

Like this

http://www.vacuumbrain.com/The_Lab/TA/LA2/la2a_layout2.jpg
Title: All things LA2a related
Post by: Scenaria on May 14, 2005, 10:16:34 PM
dude!

those numbers look just fine...

is it not working?

dont get caught up in the numbers game.. they are all approximations... remember all of us use diff caps and resistors.. the numbers are just ball park figures...
Title: All things LA2a related
Post by: on May 14, 2005, 10:29:35 PM
cool , now i can try to help :wink:
Title: All things LA2a related
Post by: cannikin on May 14, 2005, 10:35:51 PM
Quote from: "Scenaria"
dude!

those numbers look just fine...

is it not working?

dont get caught up in the numbers game.. they are all approximations... remember all of us use diff caps and resistors.. the numbers are just ball park figures...


Its working great, I like chasing my tail (not :roll: really)  I tend to get compulsive about numbers sometimes.  Honestly all my problems went away when I replaced the 470 and 510pf caps. :razz:
Title: All things LA2a related
Post by: Scenaria on May 14, 2005, 10:54:22 PM
you know what they say

if it aint broke...

CLOSE THE COVER and USE it :)

sheesh  :roll:

congrats!

now show some pictures  :green:
Title: All things LA2a related
Post by: cannikin on May 15, 2005, 12:06:45 AM
Scenaria and everyone thanks for your help and patience with this.
thanks.
Title: All things LA2a related
Post by: Bluzzi on May 15, 2005, 09:57:54 AM
David - Great to hear you are finished. It looks great! Like scenaria said "just use it"!

I am still at the parts gathering stage and just got the A-10s (building 2 LA2A). Next are the T4Bs and then I start on the power xfr, caps and resistors. It will take me months more but as I read all this progress it keeps me going.

Enjoy

Jim
Title: All things LA2a related
Post by: cannikin on May 15, 2005, 10:14:00 AM
Thanks Bluzzi :grin:

Scenaria, Cayacosta, and everyone were so patient & helpful.

Building the LA2a was great fun, from buying the parts to trouble-shooting, and now using it on recordings.  Whenever I finish a project and taste the fruits of my labor, the first thing I want to do is build a second one :cool:

have fun :wink: take your time, and when you don't understand something the answer is here at this knowledgeable and generous forum.
Title: All things LA2a related
Post by: Bluzzi on May 15, 2005, 10:57:32 AM
Yes, the generosity and sincerity here is huge! I can't wait to finish mine and hopefully contribute some minute amount to this community. Thanks for the encouragement (typical of this place).

Jim
Title: All things LA2a related
Post by: sleepingtiger on May 15, 2005, 03:04:43 PM
I've gone back through the output section and I still have the same high voltages at V1 pin6 and V2 pin 8. I've checked and re-checked all of the solder joints and component values and everything appears to be in order. Obviously, something's wrong and I'm not finding it. For now, I've officially run out of ideas... Do any of you have some insight for me?

Congratulations, Cannikin! Looks great!
Title: All things LA2a related
Post by: Scenaria on May 15, 2005, 03:13:05 PM
forgive me if ive already asked you this before..

whats the values you put in for the following...

R29
R34
R36


and double check to see that you have a jumper going from A11 to A14
Title: All things LA2a related
Post by: cayocosta on May 15, 2005, 03:19:12 PM
On the 12bh7 pin 1 you should have around 100V.

Check your ground from pin8 of the 12ax7, you should read 2.7k to chassis ground.

Check the 470k to ground connection from pin 7 too.
Title: All things LA2a related
Post by: sleepingtiger on May 15, 2005, 03:37:17 PM
R29 - 4.7K/2w
R34 - 22K/2W
R36 - 1K

A11 is tied to A14

110VDC at pin 1 of 12bh7a
pin 7 of 12ax7 to ground is 2.7k
pin 8 of 12ax7 to ground is 468k

note that the last 2 values are the opposite of what you just gave me
Title: All things LA2a related
Post by: cannikin on May 15, 2005, 03:37:50 PM
Thanks sleepingtiger,

I did a few things that changed my values, some may have made a difference/some not.   Are you using any ceramic disks?  if you are - replace those! 470pf/510pf.

Check R34 - I changed it to a 10K/2W instead of a 22K/2W.  At that point A20 Value was in line.

The Next thing I did - might be black-magic - but I was using some PCb mount 1/2W resistors (the ones with thin leads) I replaced those and my values adjusted again about 2 to 5v.  

After I did all the above, my voltages were happy, or rather I was Happier with my voltages.

If your VU meter isn't working correctly check how you wired the mini-turret to NEG Pin of VU.  I ended up running a wire from A5 of the mini to the Neg Terminal and the meter behaved properly.

Also Really analyze the Wiring of this layout.

 http://www.vacuumbrain.com/The_Lab/TA/LA2/la2a_layout2.jpg

Compare the your Wiring to the document, I changed some of my Ground Wire locations from R1/R2/R4 to exactly like cayocosta layout, noises went away and helped out.

I'm positive it was a combination of the above that created my extremely high voltages problem.
Title: All things LA2a related
Post by: cayocosta on May 15, 2005, 04:03:38 PM
The 12bh7 is okay.

The 12ax7 is wired backwards - pin 7 and 8.

Pin 8 to 2.7k to ground.

Pin 7 to 470k to ground.
Title: All things LA2a related
Post by: cayocosta on May 15, 2005, 04:14:18 PM
My layout had the labels for those two resistors switched, although the placement and values were correct. Sorry if that caused your problem.

I've revised it and uploaded the new revision to CJ's site. It's also here now:

http://home.earthlink.net/~cayocosta/diy_files/la2a/la2a_layout.jpg
Title: All things LA2a related
Post by: sleepingtiger on May 15, 2005, 04:16:31 PM
Thanks Cayocosta!

Yeah, the instructions are backwards in the Bloo manual for this (page 9) Anyone else doing this kit might want to make a note of this. After I got a little further along in construction I started referring to the schematic more regularly which is what I should have done all along. My bad...

Thanks again Steve, Cayocosta, and everyone else for all of your patient help...
Title: All things LA2a related
Post by: Scenaria on May 15, 2005, 04:26:08 PM
just to make sure...

on page 9 and 10 in the manual.. do you have this table?



Mount R12 (470k) between pin 8 of V1 and the center terminal of V1.

Mount R14 (2.7 k) between pin 7 of V1 and the center terminal of V1.




Component:   Value:   Connections:   Check
R38   22k   V4 (6), V4 (center)   
C13   510p   V4 (6), R3 (top)   
C9   0.02µ   V3 (1), R37 (bottom)   
C12   0.001µ   R37 (top), R37 (bottom)   
Link   Link   R37 (top), R37 (middle)   
R20   1k   V2 (3), V2 (center)   
R12   470k   V1 (8), V1 (center)   
R14   2.7k   V1 (7), V1 (center)   
C1   0.02µ   V1 (7), V1 (1)   

Cause this one has R12 and R14 correct (I think :\)

I think im starting to see whats going on... Out of respect for Ron I never intended on his layout being used in addition to the manual (I know.. his layout rocks!) but I think whats happend a little is that in the bloo manual I used one schematic revision while his layout is using the other... this is causing some confusion for those who are using both the maual and his layout..

I'll re-adjust the manual to follow the layouts schematic if that makes things easier..
Title: All things LA2a related
Post by: sleepingtiger on May 15, 2005, 04:51:59 PM
Yeah,

I'm more than confused at this point...

[/quote]on page 9 and 10 in the manual.. do you have this table?



Mount R12 (470k) between pin 8 of V1 and the center terminal of V1.

Mount R14 (2.7 k) between pin 7 of V1 and the center terminal of V1.




Component: Value: Connections: Check
R38 22k V4 (6), V4 (center)
C13 510p V4 (6), R3 (top)
C9 0.02µ V3 (1), R37 (bottom)
C12 0.001µ R37 (top), R37 (bottom)
Link Link R37 (top), R37 (middle)
R20 1k V2 (3), V2 (center)
R12 470k V1 (8), V1 (center)
R14 2.7k V1 (7), V1 (center)
C1 0.02µ V1 (7), V1 (1)[/quote]


this is exactly what I have but this doesn't agree with the schematic...

Regardless, I changed the values around and the results were not favorable... it increased the output but distorted it
Title: All things LA2a related
Post by: cayocosta on May 15, 2005, 05:02:03 PM
"it increased the output but distorted it"

What was your +4 meter reading as it was distorting.

These things will distort if you push them.
Title: All things LA2a related
Post by: sleepingtiger on May 15, 2005, 05:09:55 PM
It was peaking at about -5 I believe... it wasn't near zero... With the gain knob at about 8o'clock it sounded quiet and awful...

I've still got about 113VDC at pin 8 of the 12bh7a. I figure that's  the biggest clue. The other voltages around the tubes are close in line with what they should be now. I'm looking only at the schemo in the back of the Bloo manual  at this point. The one that you sent is illegible on my mac.
Title: All things LA2a related
Post by: cayocosta on May 15, 2005, 05:34:51 PM
The changes are correct, so don't go back to the way it was.

Yes, you still have a problem with the 12bh7. Should be around 1 volt on pin 8.

Trace your wiring coming into the grid - pin2, from R17, also make sure that R19 is grounded with the other end connected to the grid. Check your pin 3 1k to ground.

If nothing turns up, review all the other 12bh7 connections.

For testing, make sure your input signal is not too hot, or too weak. Gain reduction set to off. Gain set to about 10:00. Pots on the back turned full clockwise. Set this way, a good input signal should output around 0vu, in +4 mode.
Title: All things LA2a related
Post by: Scenaria on May 15, 2005, 05:45:05 PM
hmmm

im looking at the 1968 schematic and it has R12 as 470k and R14 as 2.7k

so does the older one with the jumper plug on the front end...

oh I see.. you mean the pin assignments were reversed

R12 goes to pin 7 and R14 goes to pin 8
Title: All things LA2a related
Post by: cayocosta on May 15, 2005, 05:47:08 PM
Hold on.

Now that I look at the older schematics, I think there's an error on both.

Pin 8 should be around 100 volts, not 1 volt.

There's no DC source for pin 1 to be 100 volts other than the cathode pin 8, which connects to pin 1 through the 1k resistor. Therefore, if pin 8 is 1 volt, pin 1 cannot be 100 volts.

(By the way, the '68 schematic does not show a voltage reading for pin 8.)

Jahnsen's book also confirms the Pin 1 and 8 voltages both at ~100.

So, your 12bh7 is probably okay.

Check out that distortion though.
Title: All things LA2a related
Post by: cayocosta on May 15, 2005, 06:04:12 PM
Yes Steve, the layout was accurate - the resistor values and placement were correct - with the exception of the labels (R12, R14) having been flipped on the two resistors. So, if anyone assembled according to the layout's R12 and R14 labels (rather than the resistance values depicted on the resistors) you will need to swap the resistors - otherwise you're okay.

You might want to give your people a heads-up anyway.

Again, the layout's been corrected and revised as of 05/15/05.
Title: All things LA2a related
Post by: sleepingtiger on May 15, 2005, 06:08:19 PM
This is bizarre...

I'm real confused but I feel like I ought to let you know that I can't figure out any way to keep R17 from smoking at this point...
Title: All things LA2a related
Post by: cayocosta on May 15, 2005, 06:16:18 PM
12BH7 Voltages:

Pin 6: 220V
Pin 1: 100V
Pin 8: 100V
Pin 3: 4V

These are approximate.

What do you have for these?
Title: All things LA2a related
Post by: sleepingtiger on May 15, 2005, 06:20:14 PM
I can't tell you. Whenever I turn it on R17 begins to catch fire. From hat I remember the voltages were close to what you just gave before...
Title: All things LA2a related
Post by: Scenaria on May 15, 2005, 06:22:54 PM
pull out the 12BH7A

does it still smoke?

also double check your values for R15 and R19, R21
Title: All things LA2a related
Post by: sleepingtiger on May 15, 2005, 06:26:59 PM
Yes, it smokes tube or no...
Title: All things LA2a related
Post by: cayocosta on May 15, 2005, 06:34:03 PM
Sounds like you've got a wire or two crossed somewhere.

Replace R17.

Before you power up again, really check your 12bh7 wiring against the layout and schematics.

On the 12bh7, you should be able to trace:

Pin 6 - .1 cap - Pin 2
Pin 2 - 470k - Ground
Pin 3 - 1k - Ground
Pin 7 - 470k - Pin 1
Pin 8 - 1k Pin 1
Title: All things LA2a related
Post by: cayocosta on May 15, 2005, 06:43:35 PM
The 10uf cap coming off pin 1 - is the polarity correct? Plus side goes to pin 1.
Title: All things LA2a related
Post by: Scenaria on May 15, 2005, 06:47:56 PM
umm

your using the sowters correct?

my guess is you have the grid stoppers messed up.. (maybe)


if you used the 1k's for grid stoppers on the 12BH7A they should be put in series between pin 7 and B8

and pin 2 and A9

just tack the 1k resistor to the pin and then solder a lead on the other side of the resistor over to the appropriate turret on the turret board

if you want.. pull the grid stoppers out until you get i working.. then put the grid stoppers back in (all they do is stabalize the 12BH7A) only needed if you are using sowters...
Title: All things LA2a related
Post by: sleepingtiger on May 15, 2005, 06:48:46 PM
Yeah...

R15 - 470k
R19 - 470k
R21 - 100k
Title: All things LA2a related
Post by: sleepingtiger on May 15, 2005, 06:52:28 PM
10uf cap is correct
grid stoppers are correct
Title: All things LA2a related
Post by: sleepingtiger on May 15, 2005, 07:05:22 PM
Now I get it! (I think...)

C3 isn't C5. It's C3. There's no mention of it (C3) that I can find in the Bloo instructions 'til the end of the audio transformer section. There the 10uf electrolytic is called C3 when in reality it's C5. C3 is the .1 that goes between pins 2 and 6. I never placed C3...
Title: All things LA2a related
Post by: Scenaria on May 15, 2005, 07:05:40 PM
what voltage do you measure between ground and the otherside of R19?

what voltage is your B+ rail at?


and your sure your using a 10k for R17? not a 1k... and they are 1/2w right?

correct C5 is the 10uf coupling cap to the o/p tranny

C3 is the .1 uf off of pin 6

ah I see on page 13 it said

Mount C3 (10µF/450V) from V2 pin 1


the wrong designation is in there...

UGH  :oops:

there is a C3 on page 15

Turret Board Components:

Component:   Value:   Turret Locations:   Check
R28   470K   A1, B1   
R16   68k   A2, B2   
R9   220K   A3, B3   
R13   220K   A4, B4   
R11   68K   A5, B5   
R10   1.5K   A5 A6  (left-right)   
R22   33K   B5, A6 (diagonal)   
R21   100K   B6, A7 (diagonal)   
C4   50-380pF   A6, B6   
R18   1K   A7, B7   
R15   470K   A8, B8   
C2   0.1µ/400V   A10, B10   
C3   0.1µ/400V   A11, B11
Title: All things LA2a related
Post by: sleepingtiger on May 15, 2005, 07:45:50 PM
Hey, I really appreciate you guys hanging with me this afternoon. I had to leave (home now). I'll get back to it tonight or tomorrow evening. I've got a good feeling about doing some compressing real soon.... :grin:
Title: All things LA2a related
Post by: cayocosta on May 15, 2005, 07:53:02 PM
"I never placed C3..."

That's your problem.

C3 blocks the DC from the grid - Pin 2 - you've got 120V on your grid.

Put that cap in there.
Title: Success!
Post by: JerryPbury on May 15, 2005, 07:56:11 PM
Hey guys, I finally figured out what I did wrong on mine. R36 was a 470K instead of a 470 ohm. I don't know how many times I checked this area out but I decided to look at it one more time and it just jumped out at me.

Gain reduction works perfect. Output is great. I just tried it out with an SM57 into a Seventh Circle N72 into the LA2A. Wow!

Thanks guys! Now to finish up the 2nd one and put the 2 1176's into thier chassis and finish the SSL and the G9 and install the 2 finished J99's in the Seventh circle chassis, paint the Tele and finish ordering parts for the Harley. Oh yeah and do some recording! Did I mention that I "suffer" from OCD?
 :grin:
Title: All things LA2a related
Post by: Scenaria on May 15, 2005, 08:14:34 PM
thats awsome!

it usualy happens that way... :\

just takes one resitor
Title: All things LA2a related
Post by: AnalogPackrat on May 15, 2005, 09:23:52 PM
Ok, it's a good thing I decided to do some house painting before working on my Bloo today. :wink:  I had stopped at page 9.  Steve, are the text (page 9) and table (on page 10) both swapping pin 7 and pin 8 on V4?  Just want to make sure before I fire up the Weller tonight...  Looking at the three schematics I have it looks that way.

Thanks,
Analog Packrat
Title: All things LA2a related
Post by: Scenaria on May 15, 2005, 09:28:09 PM
yea
Title: All things LA2a related
Post by: sleepingtiger on May 16, 2005, 12:25:02 AM
OK...

That whole thing about C3 was a snipe hunt. It was there the whole time. Sorry about that. I was looking a little too hard for a solution and I suddenly expected to see it jutting out from the tube socket when all along it was right there in the turret board where I soldered it the other night. My turn... :oops:

I replaced R17 and there's no smoke. All of the voltages on the schematic  and that you guys have given me are close. Everything around V2 checks out now...

I've got signal coming through the compressor but something's definitely still not right. Now, in GR mode it distorts horribly. I also get the same distortion in the other two modes but not so easily. The gain knob is low. The distortion is a hard sudden clip instead of a linear thing. A slight rise in the volume of my voice makes the sound go from pleasant to awful in an instant....

That's it for tonight.

Thanks again   :!:
Title: All things LA2a related
Post by: dukasound on May 16, 2005, 01:04:52 AM
Hi
Does anyone used C5 10uF poly instead elco. I cant find elco good quality and I have two 4,7uF.
Duka
Title: All things LA2a related
Post by: SSLtech on May 16, 2005, 09:59:07 AM
Quote from: "sleepingtiger"
Now, in GR mode it distorts horribly. I also get the same distortion in the other two modes but not so easily.

When I've seen this before, it was a bad secondary on the output transformer, which was confirmed by a DC resistance reading. You might want to check yours, just in case. Since the only thing that changes when you switch to "GR" is that the meter sits across the output (a very light load in the order of 7kΩ through some diodes) and if that can have an effect on the output, your output cannot hope to drive 600Ω as it should. It might be a transformer, it might be poor drive from the tube output (a high-resistance cap feeding the POT or similar) -Looks like it's definately in that area.

I assume also that you're feeding it into a very high impoedance destination, or it would balk at this as well.

Keith
Title: All things LA2a related
Post by: cannikin on May 16, 2005, 03:27:12 PM
Hey Guys,

Today I was recording using my LA2a.  After a couple of hours I started getting a major Buzz.  I opened the unit while audio was passing through it and when I put my finger on one of the two C7 30uf 450V caps, the buzz disappears.

Now the same thing happens with no audio passing through it.

strange.. and you thought I wasn't going to bother you anymore :cool:

HAHAHA
Title: All things LA2a related
Post by: sleepingtiger on May 16, 2005, 06:21:13 PM
Keith!

Spot on! You called it... The transformer seems to be fine but apparently I had a cold solder joint. Although I heated a few of them before plugging it in again I believe that it was on the low side of the primary which goes to ground. It's the only one that I hadn't checked before now. Regardless...

It works!  :grin: I don't want to get too excited 'til there's been a little burn-in, especially in light of Cannikin's last post but it sounds and functions like an LA2a. It sounds beautiful!

Thanks again to all of you for your kind and generous help. :thumb:

All the best!
Title: All things LA2a related
Post by: Scenaria on May 16, 2005, 06:44:10 PM
Hey David..

looking at your photos it looks like you tried to squeeze some giant caps in the box for your power supply... my first suggestion is to get some smaller sized ones (they are out there) and put them in on the tag boards...

the thing is that the location of those caps is not in a very good place...

try putting some leads on those caps and move them outside of the box to see if the hum goes away.. if it does then replace them with some that will actually fit on the tags..
Title: All things LA2a related
Post by: cannikin on May 16, 2005, 06:55:57 PM
Thanks Steve; will do.....
Title: All things LA2a related
Post by: cannikin on May 16, 2005, 07:59:02 PM
I'm having a heck of a time finding C7 caps.  where are you guys buying them from?  Every place I know of... has 33uf or 47uf or Sprague Atoms with the correct value.
Title: All things LA2a related
Post by: AnalogPackrat on May 16, 2005, 09:13:11 PM
Cannikin,

I'm in the process of building my Bloo.  I got my big 'lytics at DigiKey.  Part numbers P5950-ND and P5951-ND for 33uf and 47uf 450V.  Note that there are two sizes of the 33uF.  One is taller and a little thinner, the other is shorter and stubby.  I used the short and stubby ones and they fit on the tag strip next to V4 pretty well.  I angled C7d down about 45 degrees to leave some room around V4.

Keep beating on that thing and you'll get it working.  I can't wait to hear mine in action.  

:guinness:

Analog Packrat
Title: All things LA2a related
Post by: SSLtech on May 16, 2005, 09:33:16 PM
Cannikin,

33µF and 47µF work just fine... in fact a little better than 30 and 40. Within reason, the larger, the more ripple they will absorb, so the quieter things will be.

I use 33µ and 47µ in all of mine. I also use 105°C ratings, and everything is groovy.

Keith
Title: All things LA2a related
Post by: cannikin on May 16, 2005, 09:54:57 PM
Thanks so much - AnalogPackrat and SSL tech.

I'm so close to this thing being completed.  hopefully?
this is not the easiest DIY project for the beginner that has only been building gear for 5 months.  I'm starting to feel very self-conscience about posting in this thread.  Thanks for the support.

I just finished reading a little bit from Dave Jahnsen's "how to..." manual, he recommends 4 x 47uf.   But I will order the parts you posted.

thanks
Title: All things LA2a related
Post by: AnalogPackrat on May 16, 2005, 10:50:17 PM
No problem, David.  The parts I listed are Panasonic EB series rated for 105C.  Comparing to the other options at DigiKey they had reasonable leakage specs, too.  The stubby 33uF will fit better than the 47uF, but using the "alternate" placement in the Bloo manual would work if you want all 47uF.  Hang in there!

Analog Packrat
Title: All things LA2a related
Post by: ToroRojo on May 18, 2005, 03:11:32 AM
Finished building my Bloo LA2A last night.  Plugged it in, closed my eyes, turned it on AND...





NOTHING.  The lamp didn't light up either.  After reading through this thread, I learned that the meter zero adjust has the ability to control the lamp voltage.  I tested all voltages after adjusting the meter zero, and everything checked out.  Put in the tubes, measured everything again.  Most of my voltage measurements were about 10V over, but I thought that was acceptable.  Put in the t4b and hooked it up to bass di.  It seems to be working great!  No hum!  The gain and peak reduction controls seem to be a bit hot, but it works and sounds great!

Thanks Scenaria!
Title: All things LA2a related
Post by: Scenaria on May 18, 2005, 08:47:30 AM
no fire... no explosions?

bummer

 :razz:
Title: All things LA2a related
Post by: ToroRojo on May 18, 2005, 02:22:59 PM
Yeah, no explosions.  It reminds me of that time I was stupidly welding next to an oilpan filled with some sort of liquid.  I saw a spark fly off the metal I was working on.  In slow, agonizing motion, it flew towards the pan of liquid.  I saw my life flash before my eyes.  Facing certain death or at least dismemberment, I came to terms with everything in my life in that instant.  This is it.  The end.  Maybe I can meet Elvis a-and Marilyn Monroe too!  The spark hit the  liquid, AND...






















NOTHING.

So I kept welding.  However, I cleaned my garage up after that and moved all explosives away from the welding station.


But regarding the LA2A, should the controls be this hot?  I'm running the gain knob between 10 and 20.  I thought this thing had low headroom!  What kind of settings do you guys typically run?  The LA3As I own are typically set in the 4-6 range for a good healthy output.
Title: All things LA2a related
Post by: CJ on May 18, 2005, 03:16:36 PM
Remember that if you were using the LA2 for vocals that you will not have the same signal strenght as listeninfg to an over compressed 2005 CD relaease which is 20 db too hot. I guess it's better to have the gain and not need it...

Try it with a mic and preamp. I am sure you will be happy.

Steve, they wanted 75 bucks to send you that case of beer priority, so I opted for the 7 to 10 day thing. I hope your not pissed. But you will have to take a big piss in 7 to 10.  :grin:  :guinness:
Title: All things LA2a related
Post by: ToroRojo on May 18, 2005, 04:26:07 PM
The signal chain I was testing with was as follows:

Fender J-Bass -> Phoenix Audio DRS-2 -> Bloo LA2A.
Title: All things LA2a related
Post by: Scenaria on May 18, 2005, 04:30:47 PM
the la2's have TONS of gain... very rare to run he gain knob over the 10 oclock position...

more gain is better than less

:)

yay for beer!

 :guinness:

we'll see how many break between san fran and orlando.. :)
Title: All things LA2a related
Post by: CJ on May 18, 2005, 04:36:14 PM
Well, I wrapped each individual bottle with some faom and put them back into the cartons with plento o bubbles in between everything.

Lady at the PO:

"Are there any explosives, drugs, liquids or alchohol inside this? It feels a bit heavy?"

cj: "Hell no, just ship it, biatch!" :razz:
Title: All things LA2a related
Post by: Scenaria on May 18, 2005, 04:50:20 PM
you should seen when I shipped out an eel once :)

the box had on the side of it "live animal"

took it to a fedex station... the lady was like "umm, what kind of animal is in that box"

my response... was "3 southern pine rattle snakes"

she about sh*t her pants  :green:

I had to open the box to prove it was an eel and not rattle snakes
Title: All things LA2a related
Post by: Butterylicious on May 19, 2005, 12:31:18 AM
I'm seriously contemplating cloning a LA2A from scratch.  I was shocked to discover I have the real McCoy UTC transformers in my "vintage gear collection".  My question:  The T4A seems to be a critical device. I did see the link on making your own T4A, but if it is as critical as I think it is, I don't wanna use a hack job device.  Is it as critical as I think it is?  What's the best route for me to take?  Could someone please give me (a tech school drop out) a brief overview?

Thank you in advance.




Foot note:
"Vintage gear" may be defined by my wife as a "junk collection."
Title: All things LA2a related
Post by: CJ on May 19, 2005, 01:22:11 AM
What do you want to know?
Title: All things LA2a related
Post by: SSLtech on May 19, 2005, 07:01:56 AM
Quote from: "Butterylicious"
The T4A seems to be a critical device.

It IS[/u] the compressor! the rest is just a pair of high-gain amplifiers, a meter, a power supply and a box.

See CJ's page on the T4. Lotsa pics. Also some linked from Kev's page.

Keith
Title: All things LA2a related
Post by: Bluzzi on May 19, 2005, 09:35:24 AM
What I chose to do for my first LA2A is to get all the original or closest to them. Then if I want to try a DIT T4B or a Jensen transformer etc. I have a working clone and a referrence.

If you already have the transformers you are most lucky! Get the T4B from JBL Pro for around $90.00.

Later on you can decide to try alternatives.


Jim

ps: just got my A-10s, Next T4B! Slowly but getting there.
Title: All things LA2a related
Post by: wilebee on May 19, 2005, 12:01:14 PM
I would forgo the jbl t4b and go with sceneria's. as its been saind in this thread before the opto's are matched much better !!!   I have both  Wil

Wilebee
Title: All things LA2a related
Post by: Bluzzi on May 19, 2005, 12:14:02 PM
I wasn't aware that Scenaria sold them separately.

What is your experience with both T4Bs?

Jim
Title: All things LA2a related
Post by: cannikin on May 19, 2005, 01:57:05 PM
Replaced the 40uf/30uf caps, no buzz.  Man.. its got a lot of gain - sounds great.  Anyway its in my rack. thanks everyone.
Title: All things LA2a related
Post by: AnalogPackrat on May 19, 2005, 04:02:50 PM
Way to go, Dave! :guinness:  :guinness:  :guinness:  I spent three hours on mine last night.  I think I'm about half done--can't wait to hear it.

Analog Packrat
Title: All things LA2a related
Post by: Butterylicious on May 19, 2005, 04:08:48 PM
Quote from: "wilebee"
I would forgo the jbl t4b and go with sceneria's. as its been saind in this thread before the opto's are matched much better !!!   I have both  Wil

Wilebee


Well there you have it then.  Hopefully sceneria will either email me or PM me some info on the goods.  No big rush.  This project is still floating around in my head and hasn't reached my fingers yet.  Plus I need to finish at least 1 more restoration B4 I start something new.

Thanks Again!
-Richard
Title: All things LA2a related
Post by: SSLtech on May 19, 2005, 04:14:39 PM
>I wasn't aware that Scenaria sold them separately.
He doesn't.

>What is your experience with both T4Bs?
They are sonically identical to the JBL/Urei/UA's. The only advantage to matching is meter tracking, when set with the correct R25.

It's somehow very ...uplifting... to build your own, -makes it "feel" more valuable than the real thing, available readily -if expensively- to any person willing to spring open his wallet.

If you buy the Bloo kit, you can plug in a real T4B, -I acquired an original CJ T4 for my LA2a, a pair of UA T4s for research and comparison, and I borrowed a NOS box of UREI-era T4B's from a colleague who has several original (VERY original!!!) LA-2a's; also for investigation/comparison. I've built, measured, compared and studied several of Scenaria's Bloo T4s. -Here's the summary:

I sold the UA's to ddt; -not that they were bad in any way, but they were no better, and I'd get a better price for them than the CJ or a home-made one in a plastic relay case. I'm keeping the CJ for my LA-2A and I'm getting a Bloo kit to use as a second LA-2a.

The Bloo is a real LA-2a in every sense that a UA is a real LA-2a.

Keith
Title: All things LA2a related
Post by: Bluzzi on May 19, 2005, 04:49:29 PM
Thanks Kev - I was hoping Scenaria sold them individually so I will still go with JBL Pro. I have the intention of building some but only after I get my LA2As finished.

Jim
Title: All things LA2a related
Post by: wilebee on May 19, 2005, 04:55:45 PM
oops I suppose I should have read back a few more posts sorry for any confusions or headaches I thought he was buying a kit   My bad Wil
Title: All things LA2a related
Post by: Scenaria on May 19, 2005, 07:55:55 PM
Quote from: "CJ"


Steve, they wanted 75 bucks to send you that case of beer priority, so I opted for the 7 to 10 day thing. I hope your not pissed. But you will have to take a big piss in 7 to 10.  :grin:  :guinness:


how could I be pissed... hell beer is on its way! coulda used it today though...

damn fridge died... completely defrosted... lost all my food
Title: All things LA2a related
Post by: CJ on May 19, 2005, 09:02:21 PM
Hopefully the Beer will get there before hurricane season starts.
Predictions indicate 5 landings this year. Sheesh. How do you guys stand it?
 :shock:
Title: All things LA2a related
Post by: Tom on May 20, 2005, 11:07:29 AM
Hello everyone:

I am relatively new to DIY recording gear, and this is my first tube project.  I have started on my Bloo kit, and have a few questions.

Can someone explain to me the function of R38 (22k) in the Bloo kit?
I cannot find this resistor on the LA2A schematic, which is provided at the end of the Bloo instructions.  

I note that CJ? posted an image that listed differences in LA2A versions and two of the versions did not include an R38, the latest version used 220k (compared to 22k).

I am trying to understand the function and purpose of each component as I assemble the kit.  Yea, its slowing me down, but its a lot of fun.

Best regards

Tom
Title: All things LA2a related
Post by: Scenaria on May 20, 2005, 11:37:23 AM
R38 is not needed if you are using the older schematic.

heres the deal

either pick one or the other...

R29 4.7k 2w
R34 10k 2w
R38 22k
C13 510pf

OR

R29 4.7k 2w
R34 22k 2w
NO R38 or C13 is really needed

two revisions of the schematics... one isnt really any better than the other..the onewithout R38 is an older revision.. the one with R38 is a newer revision... pick one and stick with it :)
Title: All things LA2a related
Post by: Tom on May 20, 2005, 11:58:27 AM
Scenaria:

Thanks for that info.  After reading your comments, I managed to track down a link to the newer schematic in another thread.  With new schematic in hand, I will plod on.

Best regards

Tom
Title: All things LA2a related
Post by: Jim50hertz on May 22, 2005, 12:46:25 PM
Hey scenaria

I have what will seem to many as a real dumb question...

I'm currently in the final stages of hookup with my Bloo 2 and am a little confused with the (UK 240V) power transformer wire configuration...

in the bloo manual the blue wire is listed as 240V neutral and the white wire as LIVE.  and in the Link box it says Brown-black.


Does this mean that I link Brown and black to give 240V? or, does it mean I hook up both BLUE and BLACK to the neutral terminal on the power connector, and the BROWN and WHITE wire to turret B25 (BLOO manual page 21) and from there to the live terminal of the power connector?

I'm gonna leave those connections for the moment and hookup the front panel

Jim
Title: All things LA2a related
Post by: Scenaria on May 22, 2005, 01:13:07 PM
if you look on the transformer box you will see the taps.. diagramed

theres two primary windings that need to be linked together for 240V opperation

tie the brn and blk leads together

use the white as your neutral and the blue as your hot lead..
you should be fine


or the other way around.. it really doesnt matter which one you tie the neutral too.. its AC anyhow
Title: All things LA2a related
Post by: Jim50hertz on May 22, 2005, 01:54:28 PM
Thanks Scenaria :grin:

that's what i thought but just wanted to be absolutely sure

cheers

Jim
Title: All things LA2a related
Post by: CJ on May 22, 2005, 02:37:25 PM
Dang, bootleggin the UK also?
That's it. I'm callin UA.  :wink:
Title: All things LA2a related
Post by: Jim50hertz on May 23, 2005, 10:12:49 AM
Och Aye

Even as far north as bonnie Scotland!   :wink:


Steve, is C6 (.01uf) used? Can't find any trace of it in the Bloo build manual.

Jim
Title: All things LA2a related
Post by: Scenaria on May 23, 2005, 10:58:05 AM
its listed on page 5


yes...  you use it
Title: All things LA2a related
Post by: Jim50hertz on May 23, 2005, 11:19:38 AM
Yup, I see it, I've looked through the manual several times to see where it is positioned but can't find it, am i missing something really obvious? I've got cayacosta's graphical representation which suggests it goes somewhere near the variable cap but I can't figure exactly where, can ya help?

Cheers

Jim
Title: All things LA2a related
Post by: Scenaria on May 23, 2005, 11:45:23 AM
that variable..cap your looking at is not the one on the turret... its the one thats used for stereo linking..

same with C6

unless you intend on running two in stereo you can omit C6 and C14

I put it in the list for anyone who wanted to get all of the parts.. but not may seem to be inclined to run la2a's in stereo
Title: All things LA2a related
Post by: Jim50hertz on May 23, 2005, 12:38:06 PM
Aah!  I see....

Thanks again steve

Jim
Title: All things LA2a related
Post by: Jim50hertz on May 23, 2005, 12:50:18 PM
:oops:

Ok I'm gonna ask one more foolish, naive question today and her 'tis...

R23, the 6K8 resistor which goes on the three pole switch is the only 0.5w resistor I'm missing, however, I do have a !/4 watt metal film 6k8 which i could use in this position...

Will i get away with this or should I just be patient and order a 1/2 watt?

(I REALLY need to learn more about circuits and I must thank ya for your patience with all these questions)
Title: All things LA2a related
Post by: CJ on May 23, 2005, 01:24:31 PM
Steve, you got an 1-800 number? I'm having problems with Windows 2000. :grin:
Title: All things LA2a related
Post by: Scenaria on May 23, 2005, 02:29:51 PM
you should wait for the 1/2 watt OR go with a couple 1/4 watt in series.. maybe a 5.7k and a 1k in series or something?

oh and yea call..

1-800-chris-jenrick

 :green:
Title: All things LA2a related
Post by: Jim50hertz on May 23, 2005, 02:57:49 PM
nice one steve :grin:  :grin:  :grin:

gonna order the carbon comp 1/2w 6.8k and use a metal film 5.62k and a 1.24k 1/4 Watt in series for the moment so i can fire up and test tonight.

thanks again

jim
Title: All things LA2a related
Post by: dukasound on May 23, 2005, 03:34:00 PM
Hi
Does anyone measured both voltags on power transfomer?
Please I need off-load voltages.
Thanks in advance
Duka
Title: All things LA2a related
Post by: Jim50hertz on May 23, 2005, 06:39:02 PM
ok, so i finished the wiring tonight and fired her up,

without the tubes in i measured 6.3 AC between A24 and A25 , DC at B21 was 369 and at B20 366 and A1 59V...

I figured they read ok and in the ballpark so i put the tubes in and did some more measuring...  

B21 Read 347V
B20 - 268V
A1 - 59V
A20 - 73V (Should be 131!)
A14 - 221 (Should be 216)
A19 - 93

I reckon most of these are ok apart from A20 which is way off!  
Oh yeah and the neon gave out about half way through takin these readings

i'll double check the component values in that area and look over the whole circiut in the next couple of days.

thanks for the help today steve, should be happily squishing vocals by the weekend if i can work this out.

cheers

Jim
Title: All things LA2a related
Post by: Scenaria on May 23, 2005, 07:30:49 PM
jim, make sure both pots on the back are turned fully clockwise... then re-check that A20 reading
Title: All things LA2a related
Post by: Jim50hertz on May 24, 2005, 09:25:48 AM
will do steve, thanks

unfortunately i can't get to it again till tomorrow evening, will post results and it should hopefully be good news!

jim
Title: Time invested in Bloo LA2A
Post by: QNote on May 24, 2005, 02:42:18 PM
Would anyone care to share the approximate time they have invested in their LA2A kit?  Was curious as to how my timeline is moving along versus the average... :)
Title: All things LA2a related
Post by: Scenaria on May 24, 2005, 02:50:33 PM
depending on skills... 12 to 18 hour build time..
Title: All things LA2a related
Post by: dukasound on May 24, 2005, 03:00:50 PM
Hi
Does anyone have drawing of original front and back of LA-2A.
I made first with comparasion from pics.
Thanks
Duka
Title: All things LA2a related
Post by: on May 24, 2005, 09:15:48 PM
(http://driprecords.com/teletronix.jpg)

 :cool:
Title: All things LA2a related
Post by: AnalogPackrat on May 24, 2005, 10:03:34 PM
QNote,
I'm in for about 8-9 hours so far--taking my time and checking my work as I go.  I'd guess I'm a couple of evenings from first test.  If you go with non-UTC trafos expect to spend a little extra time laying out and drilling your blank plates.  If you use Sowters you might think about mounting a small tag strip on the inside of the plates for easier connections (especially the input one which has some resistors tied directly between the trafo and other points).  This is a fun project--can't wait to use it!

Analog Packrat
Title: Q
Post by: Tom on May 25, 2005, 08:55:56 AM
A couple quick questions for y'all.

I am using Sowter transformers, and found on the Sowter web site a copy of the schematic showing color coded wiring specific to their transformers.  Cool.

In studying this schematic, which appears to be a later revision than the one in the bloo manual, I note an unlabeled variable capacitor that is strapped in parallel across R30 (47k).  This branch of the circuit appears to relate to the stereo parallel option.  What value are you all using here?  Or are you omitting this value.

Also, in an earlier version of the schematic, at pin 2 of the 6AQ5, there is a 1k in parallel with a 50uf cap. (r36||C10).  In the later schematic I downloaded from Sowter, I note the values changed to 470 in parallel with 100uf.  Is there a consensus on a preferred version to use here?

Best regards

Tom
Title: All things LA2a related
Post by: cannikin on May 25, 2005, 09:25:30 AM
(http://homepage.mac.com/cannikin/.Public/sowter_ip_wiring.jpg)

This doesn't answer your question but its good to have, I got this from Cayocosta, :grin:
Title: All things LA2a related
Post by: Jim50hertz on May 25, 2005, 11:23:10 AM
hey steve

tried the rotation of the 1 meg pots and....

same result

A20 - 73V
A19 - 82V

my other readings are as follows...

B21 - 348V
B20 - 270V
A1 - 57
A14 - 220
A17 - 86

are the rest tolerable if I can find the problem r.e. low voltages on A19 and A20?  Don't really want to drop the T4B in and go to audio/meter testing till i solve this one...

Cheers

Jim
Title: All things LA2a related
Post by: Tom on May 25, 2005, 01:15:08 PM
Cannikin:

Thanks for the graphic.  I sort of fudged this connection by using an isolated stud and a ground ring.

I attached the ground ring and isolated stud to the top screw on the input trafo plate.  I connected R7 between the isolated stud and the T4B. Then I soldered R5 and R6 in an "L" shape, soldered the free end of R6 to the stud and the free end of R5 to the ground ring.  I soldered the black and grey wires from the sowter transformer to the ground ring, and connected the ground ring to the chassis ground around the T4B.  Finally, I connected the blue wire to the R5/R6 connection.

I think my solution should work out there, but yours sure does look cleaner.  Thanks for sharing the graphic.

btw, what did you do with the pink wire from the Sowter trafo?  Trim it back?  Cap it off?

Best regards

Tom
Title: All things LA2a related
Post by: cannikin on May 25, 2005, 01:22:59 PM
For the Pink Wire I cut it back to about an Inch then put Heat-shrink on the End so it couldn't make any contact.
Title: All things LA2a related
Post by: Tom on May 31, 2005, 01:13:43 PM
Greetings:

My BLOO kit is coming along, but I ran into a small snag that I hope someone can help out on.  

I have completed the kit assembly instructions.

When I turn the gain up from its lowest setting just a tad, e.g., to a setting of 10, the meter pegs and sort of bounces.  

With tubes but no T4, I get:

B21-331V
B20-244V
A20-130V
A19-114V
A14-197V
A1-60V


These voltages are a tad low, but they appear in the ballpark to me and the neon lights fine.  Any thoughts?

The only thing that I knowingly did that I did not see in the instructions was to provide a jumper between A11 and A14 to link R17 to C3.

I am going to start tracing through the circuit tonight, but hoped someone had seen this before and could provide me a good starting place to begin debugging.

Thanks in advance

Tom
Title: All things LA2a related
Post by: AnalogPackrat on May 31, 2005, 01:54:37 PM
Tom,

Did you put in the 1k grid stoppers on the 12BH7?  Check back through this thread and you'll find a few posts about it.  Apparently the Sowter output trafo wants to cause the white cathode follower to oscillate.

Analog Packrat.
Title: All things LA2a related
Post by: AnalogPackrat on May 31, 2005, 02:01:16 PM
LA2A builders,

I'm on the home stretch on my Bloo kit and have a question.  I'm making all of the shielded connections from the front panel into the chassis and I'm really hating the cable I have :evil:.  I have some belden 2 conductor mike cable which is really thick and the insulation for the inner conductors is really stiff which makes it hard to work with.  I also have some RG179 which is nice and small, but also has the same inner insulation issue.  Does anyone have a source for decent small and flexible insulated cable?

Analog Packrat
Title: All things LA2a related
Post by: sleepingtiger on May 31, 2005, 02:12:34 PM
I used 2-conductor shielded audio cable that's readily available from Radio Shack. Whenever only one conductor was needed I just twisted the two together (the shield is a third conductor). It was easy to use, works well, and with a little heatshrink it looks nice. Luckily, Doug was visiting town and he saw the Belden cable I was about to use and he suggested I try something else. I'd used the RS cable before and since there are no other shops in my area I grabbed a spool and went to work...

You guys are going to love the Bloo... I can't stop compressing things :grin: ...
Title: All things LA2a related
Post by: AnalogPackrat on May 31, 2005, 02:20:19 PM
Thanks, sleepingtiger.  I still buy a few odds and ends from RatShack.  I'll stop by on the way home to see if they've got this stuff.

If they do, I'll be firing this thing up tonight.

 :thumb:

Analog Packrat
Title: All things LA2a related
Post by: Tom on May 31, 2005, 04:29:12 PM
AnalogPackrat:

Thanks for the tip.  Tonight, I will review this entire thread and give the resistors a try.  I gotta believe that I am close.

Btw:  I used mic cable for the shielded wires and hated it too.  The cable is pretty thick and bulky, but it was all I had and stores around me were closed for the holidays.

Thanks and good luck with your build.

Best regards

Tom
Title: All things LA2a related
Post by: AnalogPackrat on May 31, 2005, 04:58:19 PM
Tom,

I see that you never got an answer on the 470R vs 1k question on the 6AQ5.  The choice here depends on what you did with the grid stuff (the later version has the series R and parallel cap on the grid) and what plate resistor you used.  Look back on this thread and you'll find Scenaria's answer to this one, too.  I built the later rev which has the extras on the grid, a smaller plate R (bigger power rating, though) and the 470R on the cathode.

As for the stereo link, I put it in mine.  Scenaria made the sane suggestion that I mount the jack on one of the trafo plates.  With the Sowters I was able to fit the jack on the output trafo plate.  I used a fixed 100pf cap instead of the variable one.  Looks like it's just there to shunt HF grunge on the link to ground.  If I ever build another Bloo I might have to parallel another cap on there if I have trouble with the link.  No biggie.

I hope we'll both have a working unit tonight.

:guinness:
Analog Packrat
Title: All things LA2a related
Post by: Tom on May 31, 2005, 08:40:09 PM
AnalogPackrat:

You were right on.  Those 1k grid stoppers were all that was needed to have what appears to be a fully functioning BLOO.

I plan on doing some recording over the weekend, thus I will know more then, but in my initial tests, the unit works exceptionally well.
Thanks to everyone for their help.

Best regards

Tom
Title: All things LA2a related
Post by: Scenaria on May 31, 2005, 09:31:52 PM
PIX!

I love looking at how each of you wire em up.. they all end up a little different..

congrats

 :guinness:
Title: All things LA2a related
Post by: Butterylicious on May 31, 2005, 10:21:23 PM
What is the going rate for T4B's, and where to get them dang thangs?
Title: All things LA2a related
Post by: CJ on May 31, 2005, 10:55:55 PM
Anywhere from 180 from UA, or down to 99 from a JBL sale they had a while back, also check the black market, or build your own.

I have can/board assemblies so all you need to do is solder in the EL panel and LDR cells. Where do you get those: You can mill around here:

www.vacuumbrain.com

cj
Title: All things LA2a related
Post by: Butterylicious on May 31, 2005, 11:23:52 PM
I love the cardboard box test equip.  I once built a DI box into a cigarette package in one of those sound check crisis situations.  It stayed like that for ages.
Title: All things LA2a related
Post by: AnalogPackrat on June 01, 2005, 12:26:08 AM
Cardboard chassis rule!  My ASM-1 is still in a flip top shipping box.  I really need to rack that thing.  Got my Bloo "done" and started testing (no tubes, no T4B).  B+ looked good, but no neon happiness.  Found a cold solder joint on the turret that linked over to the neon.  Zapped it.  Neon is now on.

B21: 390V  (@ C7A)
B20: 383V (@ C7B)
A1: 60V (@ neon lamp)

I think I'm good to go for step 2 (with tubes).  But first I need a  :sam:.  <kssst>  Ahhh.  Wish me luck...

Oh, and congrats to Tom and thanks to sleepingtiger--the RatShack cable was easier to use on R2.

Back to the bench...

AnalogPackrat
Title: All things LA2a related
Post by: Bluzzi on June 01, 2005, 12:41:39 AM
Quote from: "CJ"
Anywhere from 180 from UA, or down to 99 from a JBL sale they had a while back, also check the black market, or build your own.

I have can/board assemblies so all you need to do is solder in the EL panel and LDR cells. Where do you get those: You can mill around here:

www.vacuumbrain.com

cj


What a beautifully detailed page!

Do you still need a source for the octal housings? I found one and I could dig it up if you need. Let me know.


Jim
Title: All things LA2a related
Post by: CJ on June 01, 2005, 12:53:05 AM
Thanks, Jim!

If you have a link for octal suff, by all means put it up and I can put it on the page.

cj
Title: All things LA2a related
Post by: Bluzzi on June 01, 2005, 12:59:51 AM
OK, I'll look for it and post it here tomorrow.

Jim
Title: All things LA2a related
Post by: AnalogPackrat on June 01, 2005, 01:14:48 AM
OK, tube test went well, inserted T4B, no smoke.  Adjusted meter.  Poked around for some other voltages just for sanity.  Found a couple of odd things.  The output of the 12BH7 White follower is only at 87VDC (I have grid stoppers on it).  Then looking at the feedback line I only get 16V at the top of R22 (supposed to be 27V).  Is this close enough?  I'm going to try an audio test and then pack it for the night.  

Thanks to Scenaria (and anyone who helped--SSLTech?  CJ?) for putting this kit together.  When my wife gets back to CA with the digicam I'll try to get some pics posted somewhere.  CJ's page with Cayocosta's diagrams were also a big help.  Oh, and all the photos (TheDug, SSLTech, Fum) made wiring routing a whole lot easier.  And I'd like to thank the Academy...

:sam:  :guinness:   :grin:
Analog Packrat
Title: All things LA2a related
Post by: Butterylicious on June 01, 2005, 01:48:17 AM
There's a NOS Urei T4B on evilBay.  140 clams buy chit now.
Title: All things LA2a related
Post by: AnalogPackrat on June 01, 2005, 02:02:44 AM
Well, it works...almost.  The pots I used for Gain and Peak Reduction are nice sealed NOS units I got at a local surplus joint.  Either I wired them backwards or they are reverse log taper.  Funky taper.  Like nothing for the 30-40 degrees then BANG it's up to 30k.  I'll be swapping them out for Plan B.  The only other thing that is strange is that my meter shows nothing on either of the output monitoring settings, yet I'm getting output (and plenty of it with my gain pot turned down :shock: ).  Will check tomorrow.  Must be something silly on the connection to the output trafo.

Analog Packrat
Title: All things LA2a related
Post by: CJ on June 01, 2005, 03:06:28 AM
The EL panel in a UREI T4 has to be spent by now, unless it was someones paper weight.
Title: All things LA2a related
Post by: Bluzzi on June 01, 2005, 08:42:58 AM
CJ

I found the URL for the relay cases and sockets!

http://www.lineelectric.com/accessories/index.html

Ask Bill Harris for pricing. I think the socket and housing will come to below $2.00.

There are several sizes. Some are transparent some opaque if I remember correctly.

I found them when I was thnking of building a T4B. I decided to wait until I had the LA2A made with an original one first.

Jim
Title: All things LA2a related
Post by: CJ on June 01, 2005, 02:40:42 PM
Thank's Jim! It's now on the T4 page.

cj
Title: All things LA2a related
Post by: Bluzzi on June 01, 2005, 03:07:41 PM
Glad to make a contribution! I get so much here.

Jim
Title: All things LA2a related
Post by: Tom on June 01, 2005, 08:46:02 PM
Here are some pixs of my new Bloo.

I plan on performing some tests over the weekend, but my first impressions are quite favorable.

(http://circuitmuse.com/Bloofront.jpg)

(http://circuitmuse.com/Blooback.jpg)

(http://circuitmuse.com/Bloodoor.jpg)

(http://circuitmuse.com/Blooinside1.jpg)


The 10uf output cap C5 is a Solen polyprop cap I picked up at parts express.
http://www.partsexpress.com/pe/showdetl.cfm?DID=7&PartNumber=027-568  

I think that the 30uf and 40uf filter caps C7a-d are a little big.  In retrospect, I should have taken the time to look for smaller footprint caps.

Otherwise, everything went together fairly smoothly.  This was my first tube project, and first project of such magnitude.  It took me about 15 hours or so start to finish.   Without this thread, I'd still be scratchin my head as those 1k grid stoppers were just the ticket to get my unit functioning.  Cheers

Any suggestions or comments would be much appreciated.  I look forward to my next project...which will have to wait until I do some recording!


Best regards

Tom
Title: All things LA2a related
Post by: Scenaria on June 01, 2005, 09:14:41 PM
nice work!

and yea.. it seems like everyone keeps trying to squeeze in those atoms  :green:
Title: All things LA2a related
Post by: AnalogPackrat on June 01, 2005, 11:14:56 PM
Woo hoo!  Found my stupid wiring error on the meter switch and replaced the goofy surplus pots with new PECs from DigiKey.  Ran a signal generator into the Bloo while monitoring in/out--everything looks good.  The meter seems to behave and there is a slight difference in the compress & limit settings when sweeping amplitude.  

Time for some real audio tests.  Thanks again for the kit, the photos, and the pointers...I'll try to post some photos next week when my wife gets home with the camera.


AnalogPackrat :green:
Title: All things LA2a related
Post by: cannikin on June 01, 2005, 11:20:36 PM
Thats funny I just swapped out my crappy pots for PEC's, Nice pots makes the unit feel so much better.. :grin:
Title: All things LA2a related
Post by: AnalogPackrat on June 02, 2005, 01:01:36 PM
Yeah, the PEC pots aren't too shabby.  My audio testing last evening was frustrated by DAW interfacing troubles :evil: .  This is the first time I've tried to run outboard gear with my Firepod and Sonar.  It was not fun as neither has particularly great info in their manuals for this.  Anyway, I got some real audio (acoustic guitar and vocals) piped into the Bloo and compared it to the Sonitus compressor plug-in's pseudo LA2A emulation.  No comparison.  The plugin introduces some weird AM artifacts when the signal is near the knee.  The Bloo blew it away--silky smooth.   :thumb:

For fun I inserted Voxengo's SPAN (spectrum analyzer) plugin on the Bloo return channel to have a look at the noise spectrum.  Ignoring the local bumps it has a smile shape with more noise near the bottom and top.  The biggest offender was 60Hz (and harmonics), but even that wasn't bad at reasonable gain settings.  Here's an (incomplete) tabulation of some of the local peaks at various gain settings (average level was maybe 6dB down from the peaks).  Note that is includes any noise from the Firepod's line inputs as well.

Code: [Select]
         Gain 0      Gain 30%         Gain 50%
f (Hz)   noise(dB)    noise(dB)        noise(dB)
60        -91           -83              -75.5
120       -91           -93              -76
540                    -102
1000                   -103
4350                    -94
6240                    -90              -86
15000     -90           -86.5
           

I'm going to take some more numbers with the front panel open to see how much of the AC leakage is coming from the proximity of the GR switch to all the AC stuff on the right side of the chassis.  Probably not too much since it drops 15dB at low gain...must be the heaters.  Not much I can do about that.  Anyway, it's pretty dang quiet.

Analog Packrat
Title: All things LA2a related
Post by: Bluzzi on June 02, 2005, 01:28:20 PM
Congratulation! Everytime another LA2A is born I cannot help being excited with anticipation for mine to be born. I think it will 9 months before I finish.

Jim
Title: All things LA2a related
Post by: AnalogPackrat on June 02, 2005, 01:35:56 PM
Thanks, Jim.  Reading other people's posts as they built and completed their LA2As kept me motivated, too.  Are you still hunting for parts?

Analog Packrat
Title: All things LA2a related
Post by: Bluzzi on June 02, 2005, 01:51:32 PM
No I have the Meters and the transformers (in/out). Left are the T4B (I'll be getting from JBL next). After that all the regular electronic components in one shot. The chassis will be hand made at my friend's machine shop. I'll kget the T4Bs in June, and th erest of the parts in July so I think I will have my LA2As (building 2 for me and at least 1 for my friend) done by September hopefully.

Jim
Title: All things LA2a related
Post by: wave on June 03, 2005, 02:36:56 PM
Hello all!
First let me say I'm a huge newbie. I've been reading these forums for a while and I decided to do a project. Maybe I shouldnt have picked it as my first but I really like LAs so I got a Bloo kit (which is awesome..great job scenaria) I'm starting to source the other parts so my first question is can i get everything (except trafos) at one place (i.e. mouser)? What kind of components should i get ( carbon comp resistors, Xicon brand)? It seems like peeps have trouble with the sowters cause of the grid stoppers should i use other trafos? I think I need some serious help and it would be greatly appreciated.
Thanks,
Wave
Title: All things LA2a related
Post by: AnalogPackrat on June 03, 2005, 04:15:02 PM
Welcome to the forum, wave.  I wouldn't say the Bloo is a really difficult kit, but being point to point (and high voltage) it might not be the best starting point if you've never built anything before.  Have you done much soldering up to now?  Ever modified any of your gear?  Do you have some basic equipment like a decent DMM, scope, and hand tools?  

Many other people have given the following advice to newbies: try a simple project first (passive DI box, distortion pedal, bench power supply, etc.) to get a little experience and confidence before diving into an expensive kit.

The Bloo is a nice kit--looks great and sounds great when its done.  If I were you I would start sourcing parts for it, but also build another starter project while you gather goodies.

As for parts types, you can get most of what you need from DigiKey, Mouser, etc.  I used NOS 12BH7 and 6AQ5 tube that I had around and JJs 12AX7s (google for EuroTubes for the US distributor of JJs--good stuff).  Sowter transformers are fine, but it looks like you need the grid stoppers on the 12BH7--not a big deal.

Read back through this entire thread and pay close attention to the photos people have posted.  You will want to refer to them when you start wiring your front panel.  Check out CJs web page for a nice set of diagrams courtesy of Cayocosta, but note that it is not exactly the same as the Bloo kit.  Also read the meta-meta at the top of the forum page.  Lots of great info there.

You'll soon be an addict like the rest of us. ..

Analog Packrat
Title: All things LA2a related
Post by: wave on June 03, 2005, 05:19:22 PM
Thanks AnalogPackrat!
Ive been soldering for a long time. Mostly making cables, fixing pedals and modifying ps2s and xboxes. I have made a distortion pedal before including etching my own pcb. I have a good DMM and tools but no scope. Speaking of sourcing parts, I just want to make sure I get the right ones.
Thanks for the help!
Wave
Title: All things LA2a related
Post by: AnalogPackrat on June 03, 2005, 06:00:14 PM
Wave,
  Sounds like you've got the basic skills, so go for it.  Just take your time and you shouldn't have any major trouble.  Lots of folks have built these things.  I used a mixture of all kinds of 1/2W resistors--metal film when I could get them at the local surplus shops, but mostly carbon film and some carbon comps, too.  If you're buying all new parts, you might as well go with all metal film.  Caps...I used polystyrene and silver mica for the small stuff (500pf and less), polypro and mylar for the rest (except for the C7s and C10 which are electrolytic).  

Good luck,
Analog Packrat
Title: All things LA2a related
Post by: wave on June 03, 2005, 06:07:27 PM
AP,
Thats the advice I was looking for.... you rock
Wave
Title: All things LA2a related
Post by: AnalogPackrat on June 03, 2005, 06:17:46 PM
Oh, and don't forget the voltage ratings on the caps...
Title: All things LA2a related
Post by: wave on June 03, 2005, 07:43:06 PM
Hi all,
On the Bloo manual page 5 the table lists 1 M preset for R3, R4, and R37 is that a 1m lin pot?
Title: All things LA2a related
Post by: Scenaria on June 03, 2005, 08:17:29 PM
yes

1meg are linear

the 100k's are audio taper
Title: All things LA2a related
Post by: wave on June 03, 2005, 08:38:48 PM
Thanks Scenaria,
Also on page 5 I have no values for C11, 12, or 13.
Can I use the values in Cayocosta's layout rev 5-15-05
C11- .01
C12- .001
C13- 510pf
Title: All things LA2a related
Post by: Scenaria on June 03, 2005, 10:12:42 PM
yes thats fine..

if your ever in doubt always refer to the schematic :)
Title: All things LA2a related
Post by: wave on June 03, 2005, 11:34:26 PM
Scenaria,
On page 2 of the Bloo manual theres a kit checklist it says 1x XLR Cassis mount male and female which I dont have. Can I just use some switchcraft panel mount jacks from mouser (figured I'd could order with my caps etc..)
Wave
Title: All things LA2a related
Post by: wave on June 04, 2005, 12:15:43 AM
If youre ordering from mouser and it says availabilty
12511 ships immediately
estimated factory lead time 13 weeks
does that mean you have to wait 13 weeks if you want 10 of the item?
Title: All things LA2a related
Post by: cannikin on June 04, 2005, 12:20:57 AM
I think it means that if you want to Order 13,000 of them then you have to wait 13 weeks for 489 of them... I think.
Title: All things LA2a related
Post by: Butterylicious on June 04, 2005, 12:26:44 AM
Mouser is damn fast.  As you fill in your order and it says in stock, it will ship immediately.  Ifin your placing your order right now (11:15PM CST, they're in the Dallas Area BTW), your order will be in the shipper's hands tomorrow.  Any order placed B4 noon usually ships that same day.  I love mouser.  I usually use UPS ground and often get next day delivery since I'm just down the street.

Now if the item is out of stock or back ordered, it's out of their control.  I waited about 2 months for a 4PDT PC mount slider switch, but one day it poped up at my door.  Nobody else offered anything even close.
Title: All things LA2a related
Post by: wave on June 04, 2005, 12:37:05 AM
Thanks non macro-noobs
Im gonna paint my face on my avatar green cuz I'm such a noob :wink:
Title: All things LA2a related
Post by: wave on June 04, 2005, 12:38:22 AM
Are Xicon 1/2 watt 1% metal film resistors the right components for me to get?
Wave
Title: All things LA2a related
Post by: wave on June 04, 2005, 01:37:50 AM
I also am not clear on the caps
 some are specified in the Bloo manual to need a power rating of 450V
what should the others power rating be?
I'm looking at mouser and I see orange drop caps rated at .022
can I use those for .02 (ie C1 -.02)
Thanks
Wave
Title: All things LA2a related
Post by: wave on June 04, 2005, 01:45:41 AM
Nevermind I found an LA2A BOM I had downloaded before
It answered my last questions  :grin:
Title: All things LA2a related
Post by: wave on June 06, 2005, 01:35:54 AM
Can you use a .1 600v cap in place of a .1 400v cap?
Title: All things LA2a related
Post by: WJS on June 06, 2005, 02:00:36 AM
should hope so, as long as the quality is the same. if you are going for matching, its better to have identical parts. increasing the voltage rating for a cap means that it wont go up in smoke untill that higher voltage.a cap that will only see a few hundred volts should be rated well over that value (hence 400v) but upping to 600 shouldn't hurt.

I just finished a project which had lots of last minute buying of parts and the local radioshacks ran out of .01 uf caps rated for 500v but  I found ones rated for 2kv which worked just fine.

good luck!!
 and remember
  going up in voltage rating is ok
   be VERY carful going down
Title: All things LA2a related
Post by: wave on June 06, 2005, 02:06:56 AM
Thanks WJS!!
 :guinness:  :guinness:
Title: All things LA2a related
Post by: WJS on June 06, 2005, 02:16:04 AM
:guinness:
cant, i'm 17
but thanks, and best of luck!
(and i guess i can drink digitaly...)
Title: 12BH7A Output Tube replacements
Post by: QNote on June 07, 2005, 01:07:31 PM
Anyone used, or tried, an ECC99 tube for the output driver 12BH7A?  I see Tesla has a new one out and it is supposed to replace the 12BH7A in most applications.  If not, what brand 12BH7A's are ya'll using.  Just curious...
Title: All things LA2a related
Post by: wave on June 07, 2005, 02:32:56 PM
Talked with the guy at triode electronics. He said he was no expert but it shouldnt matter for the LA2. I'm gonna get one of his electro harmonix 12BH7 as well as the jj/tesla ECC99 just in case. Since I live in Chicago I'm gonna take the LA over to his shop so we can check it throughly.
Title: All things LA2a related
Post by: QNote on June 07, 2005, 02:36:00 PM
I just ordered both from Triode, myself.  I'll post any substantial findings.
Title: Re: 12BH7A Output Tube replacements
Post by: AnalogPackrat on June 07, 2005, 03:14:36 PM
Quote from: "QNote"
Anyone used, or tried, an ECC99 tube for the output driver 12BH7A?  I see Tesla has a new one out and it is supposed to replace the 12BH7A in most applications.  If not, what brand 12BH7A's are ya'll using.  Just curious...


I used a NOS GE 12BH7 in mine.  I grabbed a box full of tubes when the lab techs were clearing out some old "junk" from the equipment repair room in the EE department.  I think the 6AQ5 I used came from the same source.  I love it when something I scavenged 15+ years ago is suddenly just what I need  :green:

Analog Packrat
Title: All things LA2a related
Post by: AnalogPackrat on June 07, 2005, 03:58:52 PM
I finally got my sh*t together and have some photos of my Bloo LA2A build.  I used Sowters and added the stereo link option just in case.  I'm still thinking about adding meter illumination, but haven't made up my mind yet.

Rather than use up my free BW limit everytime someone views this thread, lets try this instead (there are about 2M of images):

http://photobucket.com/albums/a173/AnalogPackrat/Bloo%20LA2A/

Password is "Bloo"

Let me know if this doesn't work for some reason.

Analog Packrat
Title: All things LA2a related
Post by: AnalogPackrat on June 07, 2005, 04:01:50 PM
(&^*!!!  Screwed up the link format...

Analog Packrat's Bloo LA2A Build (http://photobucket.com/albums/a173/AnalogPackrat/Bloo%20LA2A/)
Title: All things LA2a related
Post by: wave on June 07, 2005, 06:15:09 PM
Looks Good AP
Title: newbie question on compatable transformers for LA2A
Post by: zhoel on June 09, 2005, 02:23:52 PM
I have all the parts for my first project (LA2A which may take me years to finish) except for the output trans. I found a Triad-Utrad A-65J and was wondering if someone could tell me if it would work and how to wire it in relation to the A-24. I am a very newbie so be patient as I am trying to learn as fast as I can. Here is a link to a pic of one that was sold on Ebay.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=4665&item=7520692299&rd=1&ssPageName=WDVW

It seems there still is not enough info on the sheild to show if it will work. I am looking for more info.
Thanks
Zhoel
Title: Triad transformer
Post by: QNote on June 13, 2005, 09:12:03 AM
Zhoel,

Try this diagram for assistance (note the pri/sec reversal in this case)...

http://www.w5kp.com/n9foy-techinfo/n9foy-2.jpg
Title: HELP
Post by: wave on June 14, 2005, 10:59:43 PM
Gurus,
I just finished wiring up and went for the first test. I got no neon and about .3VAC on turrets a24-25 any thoughts? Also My mains fuse blew.
Thanks
Title: All things LA2a related
Post by: AnalogPackrat on June 14, 2005, 11:11:15 PM
Wave,

Does your power transformer hum (it shouldn't)?  Are you getting 120VAC at the IEC connector?  Be careful in there.  Remember--one hand in your pocket at all times with the HV stuff.  Best to clip onto ground somewhere before powering up and then probe with one hand.  With no tubes in it takes a while to drain those big caps, too, so watch out near them after the power has been on.

Analog Packrat
Title: All things LA2a related
Post by: CJ on June 14, 2005, 11:19:05 PM
grab the hummer with one hand, and put the other in your pocket.
Title: All things LA2a related
Post by: wave on June 14, 2005, 11:27:35 PM
If I probe the hot on the IEC (bottom left from solder side) shouldn't I get my wall AC reading?
Title: All things LA2a related
Post by: wave on June 14, 2005, 11:29:40 PM
Transformer not humming
Title: All things LA2a related
Post by: AnalogPackrat on June 14, 2005, 11:30:25 PM
Yep.  Did you mount your power switch upside down?  If not, check your fuse--probably blown.
Title: All things LA2a related
Post by: wave on June 14, 2005, 11:34:16 PM
I blew one fuse. replaced it. I mounted the IEC ground up and I'm getting 110 VAC at the neutral (top left wired to transformer primary) and I grt no reading on the bottom left of the IEC (wired to B24) Also, even if the fuse is blown shouldn't I be able to probe the top right of the IEC and get 110?
Title: All things LA2a related
Post by: AnalogPackrat on June 14, 2005, 11:42:48 PM
Yes, top right is always hot (if plugged in).  Where is your ground clip?  Do you have a ground on your outlet?  Is your house/apt old?  Seems like you have hot and neutral reversed.  Check the outlet.

It's a safety issue (hot stops at the IEC if fuse blown), but this won't stop your unit from working.  Have you checked the 250VAC secondary of the transformer?
Title: All things LA2a related
Post by: AnalogPackrat on June 14, 2005, 11:44:20 PM
Also, you should be using slo blo fuses.  500mA in the US.
Title: All things LA2a related
Post by: wave on June 14, 2005, 11:53:55 PM
I'm using 500mA slo-blo fuses If I probe the top right I get nothing (when plugged in) if I probe the top left I get 110 VAC is my apt wired backwards?
Why does all my other gear work. This sucks!!!
Title: All things LA2a related
Post by: AnalogPackrat on June 15, 2005, 12:02:33 AM
Dood.  Take your time and be methodical.  Hot and neutral reversed isn't a good thing, but it won't stop your unit from working.  It's AC.  The transformer can't tell.  Check your HV (250VAC) secondary on the transformer.  Probe at B20 and B21.
Title: All things LA2a related
Post by: wave on June 15, 2005, 12:06:55 AM
OK I got out the fluke meter and I'm getting the same reading 128 VAC on both the top left and top right of the IEC...So why do you think I'm blowing my fuses?
Title: All things LA2a related
Post by: wave on June 15, 2005, 12:10:18 AM
I'm getting .005 vac on the red secondaries but i dont have my fuse in because it blows soon after throwing the power switch ( I'll throw the fuse in and test but it's my second to last one) What do you think
Title: All things LA2a related
Post by: bcarso on June 15, 2005, 12:10:39 AM
I once salvaged a batch of power cords from the trash somewhere with the US convention on color code: black hot, white neutral, green safety ground.  Three-wire plug on one end, breakout to a molex 0.156" pitch plug on the other.

I figured out finally why they were being discarded:  black was actually connected to the neutral blade of the plug, white to the hot!!!

YOWWW
Title: All things LA2a related
Post by: AnalogPackrat on June 15, 2005, 12:13:24 AM
So you blew another fuse?  I'm having a hard time following what's going on.    Unplug the thing and check your outlet.  Put one probe in the ground hole and make sure its touching the contact, and then probe each of the other (hot and neutral) referenced to ground.  Then probe between hot and neutral.  What did you see?
Title: All things LA2a related
Post by: wave on June 15, 2005, 12:17:01 AM
122 on what should be neutral. 112 on what should be hot and 122 between hot and neutral
Title: All things LA2a related
Post by: AnalogPackrat on June 15, 2005, 12:23:42 AM
Smells like a 240V outlet.  Check a different one in a different part of the house.  I wish our electrician, Butterylicious, were around.
Title: All things LA2a related
Post by: wave on June 15, 2005, 12:27:49 AM
OK fuse is in I'm getting 122VAC at these points...
IEC neutral
IEC top right
IEC bottom right
IEC hot
Turret B24 connected to
Power switch
Should I throw the power switch again (I'll probably blow the fuse again but I might be able to get a quick reading from A22,A23,A24, A25)
Title: All things LA2a related
Post by: AnalogPackrat on June 15, 2005, 12:31:07 AM
I'm pretty sure that fuse will blow before anything interesting happens.  You haven't changed anything.  Did you measure another outlet or two?  What has been plugged into this outlet before?  

One more post and I'm calling it a night.  Gotta work tomorrow.
Title: All things LA2a related
Post by: wave on June 15, 2005, 12:33:44 AM
Same duplex powers this macintosh that im online with
Thanks alot for the help AP
Title: All things LA2a related
Post by: AnalogPackrat on June 15, 2005, 12:41:21 AM
No problem.  I'm not an electrician, but neutral is tied to ground back at the panel or the service entrance.  You should get near 0VAC between ground neutral on a 120V outlet.  I seem to recall that Macs have auto sensing power supplies so can take anything from 90-240V and figure out what to do with it.

Hopefully someone else will have a better idea tomorrow.

Later,
Analog Packrat
Title: All things LA2a related
Post by: wave on June 15, 2005, 01:12:41 AM
If I probe continuity on the heater lines with the power tranny in place should there be continuity between every point? (i.e. before the power tranny was in place there was only continuity between all of the pin 9 wires but now there is continuity from pin 9 to pins 4-5 on each socket)
Title: All things LA2a related
Post by: wave on June 15, 2005, 02:22:16 AM
I also found that my 6.3 V secondaries off the power trafo, even when lifted off of turrets B24&B25 are making continuity with ground. Is this right?
Title: All things LA2a related
Post by: SSLtech on June 15, 2005, 05:09:32 AM
...It is if you have the center tap connected to ground...

Keith
Title: All things LA2a related
Post by: wave on June 15, 2005, 08:59:38 AM
Yes I do
This is my primary wiring
1 black to B24
1 black to IEC neutral
red/yellow and green/yellow to ground bus (but i used tag 2)
1 red to A22 1 red to a23
1 green to A24 1 green to A25
Gray to IEC ground
I get continuity to ground through the green secondaries but not the red. Could this be the problem?
Title: All things LA2a related
Post by: Scenaria on June 15, 2005, 01:48:41 PM
what do you mean by having used "Tag 2" for the center taps?

A22, A23 and A24, A25 look ok


be sure both of the center taps on the secondaries are tied to ground...

oh and if you really do have 120v sitting on your neutral.. I would suggest having an electrician come out to take a look at your service panel.. even if other stuff is working.. thats a big saftey issue...
Title: All things LA2a related
Post by: wave on June 15, 2005, 02:16:56 PM
OK if I lift the green/yellow&red/yellow from the ground point on the tag strip the mains fuse doesnt blow and i get about 360VDC at A22/A23
and 85VAC at A24/A25 as soon as I reconnect the center taps to ground the fuse goes when the power switch is thrown
Title: All things LA2a related
Post by: wave on June 15, 2005, 02:20:39 PM
By tag 2 I meant where the big 'lytics are the middle 3 tags are wired together and into the ground bus so instead of wiring center taps to the middle tag I wired it to the second from the bottom tag
Title: All things LA2a related
Post by: wave on June 15, 2005, 02:27:29 PM
I've lifted the green wires and the center taps. I get continuity when probing a green and the green/yellow but not when probing a red and the red/yellow is that right?
I'm also testing on a correctly wired outlet now
Title: All things LA2a related
Post by: wave on June 15, 2005, 10:14:19 PM
Alright I'm at a loss...
I rechecked all my wiring at the power supply and I cant' figure out why the fuse blows only when the secondary center taps are connected to ground.
I need some serious help...starting to get depressed :cry:
Title: All things LA2a related
Post by: Scenaria on June 15, 2005, 10:26:42 PM
when you say continuity are you using the one that beeps on your meter?

instead measure resistance...

measure the red to red

and then from one red to the stripped red

and the other red to the stripped red

you should get approx 500ohms between the two red

and 250-275 ohms between the stripped red and each of the solid reds...


Patience! dont get depressed... if this stuff was easy then everyone would be doing it. These things take time...
Title: All things LA2a related
Post by: wave on June 15, 2005, 10:32:51 PM
550 ohms red to red
267 ohms red1 to red/yellow
280 ohms red2 to red /yellow
.4 ohms green to green
.2 ohms green1 to green/yellow
.2 ohms green2 to green/yellow
thanks
Title: All things LA2a related
Post by: CJ on June 15, 2005, 10:33:19 PM
Listen carefully:

First, read my signature at the bottom of this post.

Second:

Unplug the unit.

Short out your filter caps with your least favorite screwdriver to discharge them.

Disconnect all secondary high voltage ac wires coming out of the power transformer.
Including the center tap.

Clip the black lead of your ohm meter to the chassis at a good, non powder coated ground point.

Measure the resistance with the red ohm meter lead of the points where the secondary wires, which have now been dis connected, are supposed to be soldered to and report back immediately because I am getting old.
Title: All things LA2a related
Post by: Scenaria on June 15, 2005, 10:46:26 PM
getting old? or are old?

 :green:

 :guinness:  :guinness:  :guinness:  :guinness:
Title: All things LA2a related
Post by: wave on June 15, 2005, 10:48:12 PM
I'm getting nothing (OL) on the meter
Title: All things LA2a related
Post by: CJ on June 15, 2005, 10:49:08 PM
Keef told me you were on the blue pill.
Title: All things LA2a related
Post by: Scenaria on June 15, 2005, 10:52:40 PM
what the hell is the blue pill? a matrix thing?

scrape some powder coat off the points where you are grounding to chassis... also be sure to dig your probe into the metal a little.. when checking..
Title: All things LA2a related
Post by: CJ on June 15, 2005, 10:54:39 PM
I told him that already, punk!

OK. Nothing is good.

Now measure the resistance on the other side of the diodes.
Your meter should gradually rise in resistance as the filters charge.
Title: All things LA2a related
Post by: Scenaria on June 15, 2005, 10:56:53 PM
I bet he didnt dig the probe into the metal deep enough :)

cj... you need to come down to FL and hit the hookah pipe one of these days...  :green:

where the two tag boards are screwed onto the chassis... be sure that the screws are making good contact with the chassis metal.. not the powder coat. thats why theres star washers... to help dig through the paint :)
Title: All things LA2a related
Post by: CJ on June 15, 2005, 11:02:19 PM
Blue pill=Viagra.

Steve, is he using this layout:

http://www.vacuumbrain.com/The_Lab/TA/LA2/la2a_layout2.jpg
Title: All things LA2a related
Post by: wave on June 15, 2005, 11:05:07 PM
OK
I really scraped off some powdercoat, put on the clip, measured to the IEC ground got .3 ohms so the clip is on good
Measured the turrets where the secondaries were soldered.. I get no reading (OL) on the meter. That means no shorts to chassis right?
Title: All things LA2a related
Post by: wave on June 15, 2005, 11:07:33 PM
I've been religiously checking against that layout (rev 5-15-05 ) the whole build (printed a tabloid size) as it's much easier for me to understand than a schematic
Title: All things LA2a related
Post by: Scenaria on June 15, 2005, 11:09:34 PM
Quote from: "CJ"
Blue pill=Viagra.

Steve, is he using this layout:

http://www.vacuumbrain.com/The_Lab/TA/LA2/la2a_layout2.jpg


more than likely... guess we should ask wave :)

wave: are you using the above layout?

I dont send that layout with the kits as its rons and not mine to use.. rather the manual steps them one connection at a time..

the manual does follow that layout though.. so its good reference
Title: All things LA2a related
Post by: CJ on June 15, 2005, 11:12:21 PM
Yes, no shorts to chassis.
Now get on the junction of those 1N4... "whatever they are" diodes and measure ohms.
Title: All things LA2a related
Post by: Scenaria on June 15, 2005, 11:13:48 PM
4007's

 :grin:
Title: All things LA2a related
Post by: CJ on June 15, 2005, 11:17:12 PM
Dammit Steve! We only have 40 pages left!
Title: All things LA2a related
Post by: wave on June 15, 2005, 11:17:31 PM
If I'm supposed to leave the clip where it is and ohm out the diodes on the b turret side I'm getting 2Mohms if I'm not supposed to measure like that then I guess I dont get what you're saying.
Also thanks for helping me I know you must get tired of troubleshooting this stuff
 :guinness:  :guinness:  :guinness:
Title: All things LA2a related
Post by: CJ on June 15, 2005, 11:21:10 PM
No, I like trouble shooting on the net. It's a nice challenge.

OK, clip your black lead to the minus terminal of one of those filter caps.
I don't think we had a good ground, like Mr. Potatoe head was saying on page 40. Dang, I can't even spell Pototo!

Then re measure the diodes on both sides.

Make sure your red lead is not plugged into the ma socket by mistake. (do it every day at work)
Title: All things LA2a related
Post by: wave on June 15, 2005, 11:29:29 PM
Clip to filter minus
Diodes on turret A side : no reading (OL)
Diodes on turret B side : I got fluctuating readings ranging from 3Mohms to 500Kohms
Title: All things LA2a related
Post by: Scenaria on June 15, 2005, 11:30:10 PM
"The first toy product ever advertised on television was Mr. Potato Head. Introduced in 1952, Mr. Potato Head took advantage of TV's explosive growth to gain access to tens of millions of newly "plugged-in" households."

(http://www.kimberlyyau.com/misc/sins0002.jpg)
Title: All things LA2a related
Post by: CJ on June 15, 2005, 11:33:38 PM
OK, you might have done this already, but do it one more time for a reality check.

Tape up the secondary wires from the transformer so they don't  short out to the chassis. Don't tape them together!

Then re fuse and plug in the unit. Pull all the tubes first.

We just want to get a win by having nothing blow up, and having the pilot light go on.
Title: All things LA2a related
Post by: wave on June 15, 2005, 11:35:00 PM
Including the center taps?
Title: All things LA2a related
Post by: CJ on June 15, 2005, 11:36:59 PM
Yes. We just want to get the primary circuit working so there is something to build on.
Title: All things LA2a related
Post by: wave on June 16, 2005, 12:02:25 AM
OK sorry it took so long
Tubes in, fuse in, switch on, fuse doesnt blow no pops or smoke
Title: All things LA2a related
Post by: CJ on June 16, 2005, 12:05:40 AM
Are the tubes lit?
How about a pilot lamp on the front panel, if using one?
Power switch working?
Title: All things LA2a related
Post by: wave on June 16, 2005, 12:08:56 AM
Nothing is lit (no tubes lit, I'm not using a pilot) but if all the secondaries are taped up how would any voltage get anywhere?
Power switch is working
Title: All things LA2a related
Post by: CJ on June 16, 2005, 12:10:20 AM
OK, cool.  
Lets hook up the heaters. No center tap.
Just two green wires.
Plug just one tube in for starters.

Make it the 6AQ5.
Title: All things LA2a related
Post by: wave on June 16, 2005, 12:23:50 AM
OK
Heaters hooked up all tubes light up  :grin:
Title: All things LA2a related
Post by: CJ on June 16, 2005, 12:25:15 AM
That's one Heineken.

OK, next, hookup the secondary centertap, but not the red wires.
Title: All things LA2a related
Post by: wave on June 16, 2005, 12:27:45 AM
Both centertaps?
Title: All things LA2a related
Post by: CJ on June 16, 2005, 12:30:49 AM
One, then power on and if no smoke, number two.
I know it's a hassle, but .....
Title: All things LA2a related
Post by: wave on June 16, 2005, 12:32:47 AM
Green one first?
(this is definitely not a hassle I'm just glad youre helping or I'd be screwed) :sam:  :sam:  :sam:  :sam:  :thumb:
Title: All things LA2a related
Post by: CJ on June 16, 2005, 12:37:23 AM
OK, lets go with the green one.

This is brute force troubleshooting here, but were close to finding the smoke bomb.
Title: All things LA2a related
Post by: wave on June 16, 2005, 01:01:47 AM
OK
Did green first.... no problems
Then red.. NO PROBLEMS!!!!
 :sam:  :sam:  :sam:  :sam:  :thumb:  :thumb:
Title: All things LA2a related
Post by: CJ on June 16, 2005, 01:03:27 AM
Oh boy. Silence means you found it!
Or your internet provider took a hit.

In case everything is cool so far, ground your center taps and hook a 1 meg resistor (if you have one, 1/4 watt is ok) from one of the red wires to one of the diodes and carefully measure the ac voltage across it.
Title: All things LA2a related
Post by: wave on June 16, 2005, 01:06:38 AM
I dont have a 1 Meg.
Also I thought the red secondary was 250 VDC how do I get AC across the diode?

...I have a 1 Meg pot though
Title: All things LA2a related
Post by: CJ on June 16, 2005, 01:09:01 AM
OK, Plan B.

Disconnect the wire that is on the side of the 4.7k power resistor, where it meets the diodes.
Title: All things LA2a related
Post by: wave on June 16, 2005, 01:13:10 AM
The wire that goes to C7a +
Done
Title: All things LA2a related
Post by: CJ on June 16, 2005, 01:14:51 AM
OK, pull all the tubes and the T4 module if installed.
Title: All things LA2a related
Post by: wave on June 16, 2005, 01:15:38 AM
Done
Title: All things LA2a related
Post by: CJ on June 16, 2005, 01:23:02 AM
Connect one of the red secondary wires to one of the diodes. It does not matter which red wire or which diode.

Hook your ohm meter neg lead to one of the C7 neg leads and the red lead to the junction of the 4.7k/diode connection. We are checking for a dead short before we power this bad boy up.

If no dead short, switch your meter to dc volts.

We are going to briefly plug in the unit for 5 seconds and then un plug it.
Use the power chord, not the switch. Turn on the power switch and then plug the chord in, just long enough to check for about 300 volts dc on the resistor.

Use alligator clips on the meter leads.


Tell me what voltage you measure.
Title: All things LA2a related
Post by: wave on June 16, 2005, 01:34:51 AM
Measuring on the B21 turret I got -130 VDC (minus?)
Title: All things LA2a related
Post by: CJ on June 16, 2005, 01:36:35 AM
Backwards diodes!
We found it. I think. Are your voltmeter leads in the right plug holes?
Title: All things LA2a related
Post by: CJ on June 16, 2005, 01:37:58 AM
See if you can see the little arrow symbols on the diodes.
Title: All things LA2a related
Post by: wave on June 16, 2005, 01:38:52 AM
My black lead to the meter is in common and the red lead is on the V,Ohm,diode capacitance input.
My diodes have a white line on one side is that the positive side?
Title: All things LA2a related
Post by: CJ on June 16, 2005, 01:42:25 AM
The side with the lines should be connected together and to the 4.7k resistor.
Title: All things LA2a related
Post by: wave on June 16, 2005, 01:53:26 AM
Diodes wired correctly now (I feel like an idiot)
I'm getting 340 VDC at the 4.7K junction
Title: All things LA2a related
Post by: CJ on June 16, 2005, 01:56:26 AM
Good Good Good!

Be careful, without the tubes, those caps have nothing to discharge them, the will stay hot for days.

Put a 10k resistor on the end of your voltmeter plus lead and switch the meter to milliamps and carefully short them out.

Then hook up the other lead to the front of that 4.7k.
Title: All things LA2a related
Post by: CJ on June 16, 2005, 02:00:54 AM
Don't feel bad. For the longest time I thought the stripe was the "plate" like in a vacuum tube schematic, and therefore should go to the transformer windings, which is , as we just found out, backwards.

The stripe is the cathode. Let the stripe remind you of a minus sign, which means the arrow is plus and should see the transformer secondaries in order to pass positive voltage.

Plug the tubes in one at a time and see if your fuse stays good.
Title: All things LA2a related
Post by: wave on June 16, 2005, 02:05:26 AM
Will i be able to tell when theyre discharged
Title: All things LA2a related
Post by: CJ on June 16, 2005, 02:08:58 AM
The dc current should slowly drop to zero.

You can also just ground one end of the resistor without the meter and carefully hook up the resistor to that 4.7k resistor at the diodes.

Then check the voltage on all the C7 caps with the voltmeter. 20 volts won't kill you.
Sometiomes they stay charged at a few volts  by themselves.

Thats two Heinekens.

Now were getting to the really fun stuf, getting the VU meter to work.

If you want to continue tommorrow, I see your on Chi Town time.

I have a V72 choke shootout to do at home.
Title: All things LA2a related
Post by: wave on June 16, 2005, 02:24:03 AM
First of all let me say
THANKS CJ!!!!
All tubes are in and lit, fuse is holding.
Should I start probing for voltages?
I can go some more if you can.
I have to go out of town to record some tracks tomorrow so I'll probably work on this thing as long as I can
 :green:
By the way my neon is on!
Title: All things LA2a related
Post by: wave on June 16, 2005, 02:37:51 AM
CJ YOU ROCK!!!! :sam:  :sam:  :sam:  :guinness:  :guinness:  :guinness:  :sam:  :green:  :thumb:

Voltages with tubes in
A24-25 = 3.5 VAC
B21=378
B20=316
A1= 53.7
A20=111
A14= 315.5
A17= 96.8
A19=142
Any thoughts?
Title: All things LA2a related
Post by: Scenaria on June 16, 2005, 07:21:17 AM
when you look on the back of your box...

the two pots that are mounted back there should be turned fully counter clockwise...

make sure both of them are.

your numbers look reasonable...

no smoke is a good thing :)
Title: All things LA2a related
Post by: wave on June 16, 2005, 11:06:50 AM
I believe I did have those pots fully counter-clockwise
Unfortunately I have to go out of town right now and can't resume the build until Sunday.
Thanks for all the help CJ,Scenaria, Analog Packrat
Have a great weekend :thumb:
Title: All things LA2a related
Post by: CJ on June 17, 2005, 10:04:50 PM
Glad to help.

Here is a screen capture of the Jensen pdf that shows how to mod the LA2a for presumbly better sound.

The 1:1 transformer solves the many complaints of distortion with only minimal settings on the gain control. And there are some mods on the output stage.
NYD, alias Kojak, alias New York Dave, alias Coney Island, alias Groucho also has some cool mods for the  output stage. It would be interesting to compare his with the Jensen, but I can't find the link.

This is a jpg if you want to save it so you don't have to search Jensen's schematics.
It is also at the LA2 icon at www.vacuumbrain.com with the cool graphics from Ron.

You don't have to use Jensen iron, but I hear it sounds good.
A 1:1 at the front with enough inductance not to load the source, or even a 1:2 or 1:4 would reduce signal below the stock 1:10 UTC A-10 HA-100X setup.and any good output iron will work.



(http://vacuumbrain.com/The_Lab/TA/LA2/la2_Jensen_mod.jpg)
Title: All things LA2a related
Post by: AnalogPackrat on June 19, 2005, 02:05:07 PM
wave,

No problem on the help.  Troubleshooting remotely is an interesting challenge.  Whenever you get back to it I'm sure someone will be online here to keep you on track for a successful build.  Sounds like you're pretty close already.

Analog Packrat

p.s.  Be sure to have to weird outlet checked out.
Title: All things LA2a related
Post by: wave on June 20, 2005, 09:38:15 PM
Quote from: "wave"
CJ YOU ROCK!!!! :sam:  :sam:  :sam:  :guinness:  :guinness:  :guinness:  :sam:  :green:  :thumb:

Voltages with tubes in
A24-25 = 3.5 VAC
B21=378
B20=316
A1= 53.7
A20=111
A14= 315.5
A17= 96.8
A19=142
Any thoughts?


So I'm Back to it and I'm still getting these readings
Some are close but some are not (like the 3.5 VAC on A24-25)
For the record
R29=4.7K 2W
R34=22K2W
R36=1K
R3/R37 full counter-clockwise
Thanks
Title: All things LA2a related
Post by: AnalogPackrat on June 20, 2005, 10:16:20 PM
Welcome back.  Power the thing down if you haven't already.  Make sure your big caps are drained.  Now carefully check your heater wiring.  It looks like you may have one leg shorted to ground somewhere.  

If you can't find anything, unplug the cord from the wall, and make sure those caps are really drained.  Put your meter in resistance reading mode and measure between A24 and ground and then A25 and ground.  They should both be about equal.  If one is significantly lower than the other, trace that side to your short.
Title: All things LA2a related
Post by: wave on June 20, 2005, 11:30:42 PM
Shouldn't I get a reading to ground at A24-25 if the green/yellow center tap is connected to ground?
Title: All things LA2a related
Post by: AnalogPackrat on June 20, 2005, 11:41:59 PM
Yes, you will get a resistance indicated because the center tap is grounded.  What you need to look at is the actual resistance.  If there is a short somewhere on one of the 6.3V secondaries it will show up as a very low resistance compared to the other one.  So this will reduce your searching by half.  Get it?
Title: All things LA2a related
Post by: wave on June 20, 2005, 11:44:19 PM
I get .3 ohms on each turret (A24/25) :?:
Title: All things LA2a related
Post by: AnalogPackrat on June 20, 2005, 11:53:06 PM
I get 0.4, so maybe you're OK.  My meter's resolution is only good down to 0.2 ohms.  What does yours read with its leads shorted?
Title: All things LA2a related
Post by: wave on June 20, 2005, 11:54:36 PM
.2ohms
Title: All things LA2a related
Post by: AnalogPackrat on June 20, 2005, 11:59:12 PM
OK.  I'm still thinking you've got a short on one of your heater connections.   Check each of the 12AX7s and the 12BH7 to make sure you have pins 4 & 5 linked on each of these.  Then check that one side of the heater wire pair connects to 4 (or 5) and the other connects to pin 9.  Finally make sure that the 6AQ5 has one heater wire to pin 3 and the other to pin 4.  No other components or wires should eb connected to any of these pins.
Title: All things LA2a related
Post by: AnalogPackrat on June 21, 2005, 12:05:19 AM
Also check that there isn't a sneaky short right at one of the turrets.  There's a power trafo mounting screw right behind A24/25.
Title: All things LA2a related
Post by: AnalogPackrat on June 21, 2005, 12:16:08 AM
Looking at your B20/21 voltages you are drawing about 13mA of B+ total, which is a bit low but in the ballpark (should be 15-21mA).  You need to get your heaters fixed before we trace anything else because you gotta heat those filaments to boil off some electrons to make the tubes work at all.
Title: All things LA2a related
Post by: wave on June 21, 2005, 12:16:26 AM
I carefully inspected all these connections and all looks good.
Can you measure on V3 (12AX7) between pin 9 and pin 3 and see what your reading is?
Mine is 1 K ohm and I was just wondering if that's what you have.
Thanks
Title: All things LA2a related
Post by: wave on June 21, 2005, 12:18:48 AM
Quote from: "AnalogPackrat"
Also check that there isn't a sneaky short right at one of the turrets.  There's a power trafo mounting screw right behind A24/25.


I omitted that bolt
Title: All things LA2a related
Post by: AnalogPackrat on June 21, 2005, 12:20:19 AM
You are basically measuring the R31 (1k) plus one side of the heater winding (less than 1 ohm), so you should see about 1k like you have.  Let's try something else.  Your T4B is out, right?  Pull all of your tubes and then power up and measure your heater voltage at A24/25.
Title: All things LA2a related
Post by: AnalogPackrat on June 21, 2005, 12:22:08 AM
Quote from: "wave"


I omitted that bolt


I see that it made you nervous, too.  I shimmed mine with a nut on the backside.  Couldn't find a shorter one in my junk bin.
Title: All things LA2a related
Post by: wave on June 21, 2005, 12:25:22 AM
Tubes out.
3.6 Vac on heater lines :sad:
Title: All things LA2a related
Post by: AnalogPackrat on June 21, 2005, 12:31:04 AM
OK, well, let's prove or disprove my theory.  I think you and CJ did this already, but...

First, power down and make sure your big caps are drained.  Did CJ tell you a way to drain them because it will take a long while with no tubes in?  You need to do this first.

Next, pull your heater secondaries (the green wires from the power transformer) off of A24/25.  Make sure they are sticking up and away from everything.  Clip your voltmeter (set to AC V mode, of course) leads to these wires making sure nothing is shorting to the rest of the circuit.  Turn on the power and read the voltage.
Title: All things LA2a related
Post by: AnalogPackrat on June 21, 2005, 12:31:30 AM
Wait, stop!
Title: All things LA2a related
Post by: AnalogPackrat on June 21, 2005, 12:32:36 AM
When you are measuring 3.5V are you measuring with one lead on A25 and the other on A24?  If you are measuring each relative to ground then you are probably OK on the heater wiring.  Please clarify how you are measuring this.
Title: All things LA2a related
Post by: wave on June 21, 2005, 12:44:41 AM
Lifted from A24/25 both green wires read same as before 3.6 VAC
Yes measuring one side ref to ground
I think maybe I'm too stupid to be doing this
Title: All things LA2a related
Post by: AnalogPackrat on June 21, 2005, 12:49:53 AM
Whoops.  That's what I was afraid of.  When you measure the heater voltage you should measure across the leads--one test lead on A24 and one test lead on A25.  That will give you 7.2 volts AC which is what you want.  Your heater wiring is fine.  Hook it back up.  Sorry 'bout that, but it just dawned on me as I finished typing that message.
Title: All things LA2a related
Post by: AnalogPackrat on June 21, 2005, 01:01:47 AM
So on to the other voltages...

B21 is a bit high and B20 is too high.  Look at the circuit.  You've got 378V onone side of R29 and 316V on the other.  That's 62 volts across 4700 ohms which is:

I = V/R = 62/4700 = 13mA

You should be dropping 85-100V across R29 for 16-21mA.

A1 is fine (my neon was 53.4V).  A20 is a bit too low.  A14 is way too high.  A17 is OK.  A19 is too high.

I'm fading here.  Need to get to bed.  If you're still up for it tonight, make a little diagram of just the parts of the circuit that feed from c7a and c7b out to the plates of the tubes.  Work out the current through each resistor in this part of the circuit using your voltage measurements and the resistor value.  Do it again using the voltages on the schematic and compare currents.  Somewhere you'll find a difference and that will lead you to the problem area.

Have fun.

Analog Packrat

p.s.  If you used Sowter transformers, be sure you hooked up the grid stoppers correctly.  You may have some HF oscillations at this point that are causing your weird readings.  Someone else had this problem a few months back and it's on this thread.
Title: All things LA2a related
Post by: wave on June 21, 2005, 01:20:05 AM
6.8 VAC on A25/25 (meter leads attached to both turrets)
Title: All things LA2a related
Post by: wave on June 21, 2005, 02:45:12 AM
OK new voltage readings
B21=363
B20=304
A1=53.7
A20=106
A14=304
A17=94.4
A19=136.8
Readings with all tubes in / no T4B
I am using sowters w/o grid stoppers in place
I have turned R3 & R37 full each way and differences in reading are very minimal
zero adjust is working fine and when meter swich is in GR mode the needle swings up to zero when I power on
I think I could get my voltages a little better though....
will continue tomorrow
AP thanks for the help!!! :sam:
Title: All things LA2a related
Post by: SSLtech on June 21, 2005, 09:28:07 AM
Quote from: "AnalogPackrat"
... with a nut on the backside ...


Wh-oooah, Nellie!

-THAT's an image I'm going to have to work hard to get rid of!

Keith
Title: All things LA2a related
Post by: AnalogPackrat on June 21, 2005, 10:45:41 AM
SSLTech,

Man, you'd fit right in where I work.  Bunch of non-PC guys who are always "reinterpreting" each other's statements.  Or maybe it's just the MJ trial hangover  :shock:

Analog Packrat
Title: All things LA2a related
Post by: AnalogPackrat on June 21, 2005, 11:24:14 AM
wave,

I think you are close to getting this thing working.  Right now it looks like your 12BH7 is not drawing any current (you have the same voltage on both sides of R17).  Either something isn't right with the wiring or it's oscillating.  If it were oscillating you should see some drop over R17, so I'm guessing something's just not wired right.  What DC voltage do you have at A7?  What about pin 3 of the 12BH7?  Make sure R19 and R20 are the correct values and correctly wired.

Analog Packrat
Title: All things LA2a related
Post by: CJ on June 21, 2005, 03:00:08 PM
Do you have a chart with all these B-21 etc points mapped out?
I am too used to

1plate
2grid
3cathode
h
h
6plate
7grid
8cathode
h

to be of any use here.
Title: All things LA2a related
Post by: audioforge on June 21, 2005, 04:46:35 PM
hello.
I am sourcing components for my la2a clone.except 12bh7a t4b and in/out transfos  missing i have nearly all others components coming from old tube milatary radios surplus in a junk yard.
I have some questions about few caps.i will refer to Ron schematic:
first: i miss C13.i ve read its a mica cap.i have some 300 330 470pf mica's
does the 510pf value is strict or a 470pf or 560pf will be ok?
or can i put a 510pf polystyrene cap there?
Second: C2 and C3: i have old 0.1mfd 400v cornell dubilier film caps .is it OK instead polypropylene orange drop?
third: can the 470pf ceramic (parallel to the neon) be 470pf mica? .
THanks a lot for your help and for the great infos found on this forum.
audioforge.
Title: All things LA2a related
Post by: AnalogPackrat on June 21, 2005, 04:46:37 PM
It's the secret Bloo turret board numbering code. :wink:   I'm officially not supposed to reproduce the manual  :roll: ... I hope Scenaria doesn't mind.

Here's the decoder ring.  Look at Cayocosta's diagram on your site (see, you had it all along).  The top row of the turret board is the "A" row and is numbered 1-25 from left to right.  The bottom row is the "B" row and is numbered similarly.  So:

B21 is C7a
B20 is C7b
A1 is the Neon
A14 is 12BH7 plate
etc.

Analog Packrat
Title: All things LA2a related
Post by: AnalogPackrat on June 21, 2005, 04:54:26 PM
Audioforge,

I used fixed value mica caps for all the pf range stuff except for C4 (it's a trim cap).  I think I used 380pf for C13 and the one parallel to the neon.  Polystyrene would be overkill since these are in the sidechain and the meter circuit.  For C2 and C3 I used some surplus 0.12uf 400V film caps--I'm not even sure whether they are polypro or polyester.  They work fine.

I also used Sowter transformers--you can buy them from Prodigy Pro.  Just click on the icon in the upper left of the forum page.  One note about the Sowter output trafo is that it may cause oscillation in the output driver (12BH7).  To kill this, use 1k grid stoppers on the 12BH7 triodes.  You should be able to find some 12BH7s at your local surplus places.  They seem to be fairly common.

Good luck,
Analog Packrat
Title: All things LA2a related
Post by: audioforge on June 21, 2005, 05:40:37 PM
thanks analog for your answer.
for C4 i have some big 20-120pf and an arco464 (45 to280pf).as CJ (i think) says a  fixed 100pf could do the trick i am sure the 464 will be nice enough.
C13 C12 you said:"Polystyrene would be overkill since these are in the sidechain and the meter circuit".
for sure but i have some with thoses values but not in Mica.
to ask in others words :if i change some ceramic to filmcaps or mkt  i have, in the audio pass will i deviate too much to the "original la2a sound" or as some said: all is in the iron?
thanks . audioforge.
Title: All things LA2a related
Post by: AnalogPackrat on June 21, 2005, 06:04:45 PM
I don't have any experience with a real LA2A, so I can't really answer your question about comparing the sound of various types of caps.  Just looking at the circuit, I think you could use just about anything you wanted for C13, C12, and the neon bypass cap.  

I think if you're going to hear anything from caps it will be C1, C2, C3, and C5.  In the original and reissues C5 is an electrolytic.  I used a film cap that I got surplus.  I believe it is metallized polypropylene.  I didn't compare it to an electrolytic.  To my ears, my Bloo kit sounds very nice.

Some of the other guys here have far more experience with LA2As than I do.  Hopefully some of them will chime in.  I'm curious about some of the same questions you are asking.

Analog Packrat
Title: All things LA2a related
Post by: CJ on June 21, 2005, 06:30:13 PM
The side chain caps drive the EL lite, so there is no real benifit in using  designer caps there, in fact, you may do more harm than good. Response time might change. I do not know the various response times of say, ceramic vs poly, but I bet it is different.
Title: All things LA2a related
Post by: wave on June 21, 2005, 09:41:14 PM
Well the battle has been won! :grin:
Thanks to all your help (CJ,Analog Packrat, Scenaria)
I was looking in the 12BH7 area as per AP's comments and I found that I had accidentally wired
V2 pin 8 to turret B8 and
V2 pin 7 to turret B7 (dumb mistakes)
Here's my new voltages
B21=354
B20=283
A1=53.5
A20=129
A14=87
A19=107
 :guinness:  :sam: BEERS ALL AROUND :sam:  :guinness:
Now I think it's time to plug in the old T4 and get some sound in this box!!
Title: All things LA2a related
Post by: AnalogPackrat on June 21, 2005, 09:47:35 PM
Hold on....everything looks good up to A14.  Do you have a typo in the A14 and you menat A17 (which should be about 100V+/-)?
Title: All things LA2a related
Post by: AnalogPackrat on June 21, 2005, 10:04:47 PM
wave,

Gotta head home.  I'll check in when I get there.  I think you're pretty much done.  Listen out for distorted audio or other oddness which may mean oscillation to be killed with grid stoppers.

Later
Title: All things LA2a related
Post by: wave on June 21, 2005, 10:37:37 PM
Ok
Here's something wierd,
I plugged the unit into my patchbay, took a vocal output from my interface into the Bloo, out of the Bloo back to the bay on to channel 10 of my mixer.
When I plug the cable into the Bloo, the output meter (in GR mode ) drops to zero and looks as if its showing the peak reduction in reverse (rising while compressing). When I pull the output (either from the bay or the back of the unit) the meter works correctly in gain reduction mode. The meter does not work at all in +10/+4 operation.
Other than that, the unit sounds AWESOME! No noise, lots of gain and some serious squash.
Another observation, it seems like I get more compression in compress mode as opposed to limit (i.e. when in compress, I can hear it working more when I've got the peak reduction knob up).
Any thoughts?
Title: All things LA2a related
Post by: wave on June 21, 2005, 10:39:42 PM
Quote from: "AnalogPackrat"
Hold on....everything looks good up to A14.  Do you have a typo in the A14 and you menat A17 (which should be about 100V+/-)?

Yes...sorry
A14=231.5
A17=87
A19=107
 :cool:
I'm gonna disconnect it and get back inside this baby
Title: All things LA2a related
Post by: Scenaria on June 21, 2005, 11:00:34 PM
check your limit/compress switch.. is it upside down?

as for the meter.. this is where a lot of builders make mistakes... triple check your wiring on the rotary switch... its not uncommon to have just one lead attached to the wrong pole.

it should work harder as you turn the peak reduction knob clockwise..

when you switch to GR on your metering.. it should sit at zero and deflect to the left... or downward showing more compression... turn your PR knob all the way down the meter should be at zero.. as you turn the knob up it should begin deflecting to the left (or downwards)  are you sure the rising isnt the compressor returning during its release?

I think all of your metering issues are going to revolve around that rotary switch..

you have no oscillations? did you use grid stoppers? im really impressed if you didnt need the grid stoppers and theres no oscillations (very cool!) :)

and yea... the la2a's have TONS of gain!

congrats!

 :guinness:  :guinness:  :guinness:
Title: All things LA2a related
Post by: Scenaria on June 21, 2005, 11:02:14 PM
oh..

DRAIN your caps before you start poking around in there.. always be safe... double check with a multimeter..

 :razz:
Title: All things LA2a related
Post by: AnalogPackrat on June 21, 2005, 11:28:45 PM
Oh yeah...he's almost there.  I made two wiring mistakes on the meter stuff.  Miswired one of the leads from the output XLR at the rotary and reversed the leads on the meter adjust pot (so GR mode reads backwards).  Another Bloo is born...

Analog Packrat
Title: All things LA2a related
Post by: wave on June 21, 2005, 11:36:39 PM
I put the grid stoppers in.
Does the orientation of the rotary switch matter at all?
I cant seem to find anything wwrong with the wiring of it and I cant use caycosta's layout to my benefit as it is different
Title: All things LA2a related
Post by: CJ on June 21, 2005, 11:41:28 PM
What did I say our next problem was going to be?
Post 5 on page 43.

Get a hi liter pen out and check each and every connection, one by one.
Trace it out on a schematic as you verify.
Good practice.
Start with the VU meter.
Title: All things LA2a related
Post by: AnalogPackrat on June 21, 2005, 11:44:07 PM
It's a three pole three position (throw) switch and you only need two of the pole and contact sets.  It doesn't matter which two you use, but you need to pay attention to which connections are associated with each pole.

I'm going to assume you have the same switch that I have since we're both looking at Bloo kits.  Looking at the back of the switch you should see three connections that have little wipers that contact the metal ring toward the center of the switch.  These are the poles.  The three position contacts that go with each pole are the three immediately counterclockwise from the pole.
Title: All things LA2a related
Post by: AnalogPackrat on June 22, 2005, 12:02:53 AM
Quote from: "CJ"
What did I say our next problem was going to be?
Post 5 on page 43.


No doubt.  Rotary switches are easy to F up  :oops:

CJ, I think he's got it working and can't stop using it to give us a victory post.  Whaddaya think?
Title: All things LA2a related
Post by: wave on June 22, 2005, 12:13:35 AM
AP You said it!
My dumb ass had the twin shield wired 1a-3a instead of 3a - 3b :shock:
I'm surprised I actually got it (actually I looked at photos of AP's build and caught my mistake-)
Mass beers for everyone!
 :guinness:  :guinness:  :sam:  :guinness:  :guinness:  :sam:  :sam:
This thing SOUNDS GREAT.
Reminds me of everytime I've used one (or had to rent one) in the studio.
The meter is behaving correctly now. I'm sure some people would probably really calibrate the thing but I think it's good enough for ROCK!
Let me also just say thanks alot for all the help from AP,CJ, and Scenaria.
I'm way green and this was my first DIY project except for an Electra distortion pedal made from Shack parts :wink:
I couldn't have done it without you guys, Cayacosta's great layout and Scenaria's awesome kit!
Cheers :sam:
Wave

p.s. I'll take this bad boy to work tomorrow and have our photographer snap some shots for you.
Title: All things LA2a related
Post by: AnalogPackrat on June 22, 2005, 12:17:05 AM
Cheers back at ya!   :guinness:   And congrats on your first "real build."  You did good. Now make some music with it before you get hooked into another project  :green:

Analog Packrat
Title: All things LA2a related
Post by: wave on June 22, 2005, 12:19:56 AM
Hear ya!
I've already got this baby patched on a vocal makin some sweet sounds!
Gonna have to drop the mix! :grin:
Title: All things LA2a related
Post by: CJ on June 22, 2005, 12:57:08 PM
Coolsville!
Drop a 6072 in for V1 and then tell us how you like it.
 :thumb:
Title: All things LA2a related
Post by: AnalogPackrat on June 22, 2005, 01:05:22 PM
Hmmm...I need to try that, too.  Have a few 12AY7 in the collection.  It's not like the thing is short of gain, eh?
Title: Finally!
Post by: QNote on June 30, 2005, 11:25:18 PM
Ok... I finally finished construction and...it's blowing fuses.  It seems at this point, without me doing a lot of troubleshooting (lack of time today), that the problem may be the two FRED's that I used instead of 1N4007's.  They are in correctly, but they may be underrated--voltage (600V PIV).

Anyone used any FRED's for the two rectifiers?  Which ones did you use?  I'm gonna try a couple of 1N4007's on Saturday, after I take a peak of what the FRED's are doing on an oscilloscope.  I'm thinking they might be breaking down on reverse peak voltage, causing an ac load at the caps.  I ordered these in a hurry and should have selected a much higher PIV rating. ??

Just in case your wondering, I did correct their orientation as the table in Bloo's manual is incorrect.  Also, there are NO shorts anywhere--isolated and checked thoroughly.  It's at the rectifiers and caps.
Title: All things LA2a related
Post by: CJ on June 30, 2005, 11:40:09 PM
Just did a fix on this for wave if you look back a few pages.. Which way is the bar on the diodes facing?
Title: All things LA2a related
Post by: AnalogPackrat on July 01, 2005, 01:12:43 AM
I used UF100x fast recovery diodes in my Bloo kit with no problems.  I can't remember if they're 600 or 1000PIV parts.  Which part # are you using (there are several flavors)?  Seems like 600 PIV would be enough.  Check CJ's debug of wave's fuse blowing episode.

Analog Packrat
Title: All things LA2a related
Post by: SSLtech on July 01, 2005, 07:49:09 AM
The positive side of both diodes should be on the 'B' row (the row nearest the front panel) and should be joined together.

I was not aware of an error in the Bloo instructions, everyone else has built theirs following the orientation as marked, and they've worked?

FWIW, if you're going to see +380Volts or so at the output, then the peak must be at +350V. This means that when the AC is at its most negative part of the cycle, it will be at -380VDC.

+380VDC at one end, with -380VDC (instantaneously) at the other end means instantaneous 760V reverse bias. I'd have gone with 1000V.

My thoughts FWIW: Fast Recovery diodes in tube gear? Sounds a little over the top to me! :wink:

Keith
Title: All things LA2a related
Post by: AnalogPackrat on July 01, 2005, 12:38:25 PM
Keith,

Just checked--I used UF1007 which are 1000 PIV parts.  I vaguely remember doing the voltage calc before ordering  :oops: so I blew it on the 600PIV statement.  These FREDs were $0.38US parts and seemed worth it for the potential noise reduction.  I'm not going to start a FRED war here...  :razz:   They do work fine in the Bloo, though.  Oh, and I also recall no errors in the manual regarding the rectifier orientation.  Can you clarify what you meant, QNote?

Analog Packrat
Title: All things LA2a related
Post by: SSLtech on July 01, 2005, 01:57:05 PM
Cool. there's nothing I can find actually wrong with using FReDs, but I can't measure an improvement, so it's a bit of a non-issue for me. -At under a buck for something that can't make anything worse, there's nothing wrong with using them though.

However, 600V is too low. And if Qnote has "corrected" their orientation, then they're very likely the wrong way round also. Replace them with 1000V diodes and put them the right way round... I reckon you'll stand a better chance of not sending quite so many fuses to meet their maker... :wink:

Keith
Title: All things LA2a related
Post by: QNote on July 02, 2005, 08:32:32 AM
My Bloo manual reads on page 15 (of 27)...

A23, B23

To me, this indicates that the anode should be on B23, but this cannot be the case, since the anodes should connect to the transformer secondary.  In retrospect, I'm assuming the

As for the FRED's, they are Fairchild ISL9R460P2, 4amp 600volt.

Also, from my memory, voltage peak across these diodes would be calculated at...

PIV = 2 x Vpk - Vd,

...which would NOT be based on the +380VDC figures, but the AC peak voltage values from the secondary.  This would indicate higher values than you originally suggested.

After a few more minutes this evening in troubleshooting, I should find the reason and I will post it here.

Thanks guys!  Oh, I did go back and read the "fuse blowing" chat earlier.  :wink:
Title: All things LA2a related
Post by: Scenaria on July 02, 2005, 09:01:24 AM
cathode always needs a higher potential in order to forward bias the diode... think of the arrow pointing in the direction of current flow.. increase the potential at on the side that has the line (cathode) and it allows current to flow... a higher potential :) or the +

in either case I guess I could see how its confusing..


 :guinness:
Title: All things LA2a related
Post by: QNote on July 02, 2005, 10:18:16 AM
Quote from: "Scenaria"
cathode always needs a higher potential in order to forward bias the diode... think of the arrow pointing in the direction of current flow.. increase the potential at on the side that has the line (cathode) and it allows current to flow... a higher potential :) or the +

in either case I guess I could see how its confusing..



You're close, but no cigar!

It's the anode that must have a higher voltage potential than the cathode, by the amount of voltage as it's Vf, or forward voltage rating, which for most silicon diodes is approximately 0.7V.  Diodes can be biased in the reverse direction, but this is usually not a good thing (except for zeners).  This value is the diode's Vrb, reverse breakdown voltage.

If you have a higher voltage potential on the cathode than on the anode, then a normal diode would be biased off, assuming the reverse breakdown voltage hasn't been met or exceeded.  If you have to bias the cathode on a particular diode in order to acheive current flow, then either it is defective, incorrectly marked (with the band), or a zener, but this is unlikely, since it (a zener) behaves like a normal diode in the forward bias direction.

<<school bell rings>>

  :thumb:
Title: All things LA2a related
Post by: QNote on July 02, 2005, 10:33:24 AM
FWIW, I did just notice something on the original LA2A schematic that I feel may be contributing to the confusion...the little + symbols next to the diode cathodes.  What were they thinking?  These ARE NOT the positive sides of the diodes, but it IS where the positive DC voltage resides after rectification.  Scenaria, was this what was throwing you off?

Oh, well.  This is still not the issue in my case.  To be continued...
Title: All things LA2a related
Post by: QNote on July 02, 2005, 12:33:52 PM
UPDATE!...

The FRED's were breaking down from reverse voltage, or PIV.  Since they were 4 amp, there wasn't enough current to evidently destroy them before popping the fuse.  Again, my fault from overlooking this detail when ordering them.  I should have known better!

Replaced them with standard 1N4007's and all is well--at least the basic, no-tube voltages look good!  Now for the final test and alignments...
Title: All things LA2a related
Post by: QNote on July 02, 2005, 03:57:58 PM
Well, the tubes and T4 are in anf the unit passes audio and compresses!  BUT...

The frequency response of the unit sucks!  I have about a 10dB/octave roll-off up at 1000 Hz (high pass filter), so there is a huge reduction in the low-end.  The high-end (above 1000 Hz) is pretty flat.  No problems on linearity above 1KHz.

Adjusting C4 seems to do little--if anything.  All voltages around tubes line up with Scenaria and Jahnsen's book.  I have also gone back through the unit verifying resistor/cap values, placement, etc.  I can zero the meter and it shows gain reduction when compressing.  This problem also exists without the T4 module inserted.  I'm using Sowter transformers, and I have the two grid-stopper resistors on V2 (12BH7A).

Any ideas?  Time for some serious o'scoping, huh?  BTW, I don't have an extra 12BH7A tube--wish I did.  Originally, I found I was missing the link between A11 and A14.  Could this have damaged V2?  I did swap the two 12AX7's with no luck.
Title: All things LA2a related
Post by: Scenaria on July 02, 2005, 04:09:06 PM
check the caps you used for C13, C9, C11, C1

im not talking the value rather the voltage rating.. If I remember correctly keith and I experienced a similar problem with lack of low freq... found that the caps we used for those designations were simply too small..
Title: All things LA2a related
Post by: QNote on July 02, 2005, 04:28:43 PM
C13 - 500V
C9   - 400V
C11 - 600V
C1   - 400V

They are all Orang drops except C13 (standard poly).
Title: All things LA2a related
Post by: Scenaria on July 02, 2005, 04:33:12 PM
I would scope it starting from the input on the wiper of the input level pot... move forward from there to see where its rolling off..
Title: All things LA2a related
Post by: QNote on July 02, 2005, 08:16:58 PM
I think I found it!  :grin:   :cry:

0.00 uF reading on C5.  It's not shorted, but open.  I was loosing the lower freqs between it and the output transformer primary.  Sure wish I had another one handy...

Oh well, maybe Mouser will send me a replacement.
Title: All things LA2a related
Post by: Scenaria on July 02, 2005, 08:26:46 PM
c5 is the coupling cap right?

the 10uf/450v one?

just run down to radio shack and grab something equivelent to see if it works..

you can use a 20 or 30 uf too if you cant find a 10uf..
Title: All things LA2a related
Post by: QNote on July 02, 2005, 11:56:56 PM
Yep!  It is the coupling cap.  I did have a 47uF at 400V, so I put 'er in there and...voila!  Everything is great!

I put her on the analyzer and the specs are quite impressive.  Flat frequency response from under 20 Hz to about 22 KHz.  Noise floor better than printed specs.  THD+N peaks at 0.46%, and that is with 3dB of reduction at 1 KHz.  Interestingly, THD+N peaks at most frequencies when there is about 3dB of compression--lower and higher reduction values have less THD, for some reason.  Nominal THD+N stays around 0.15% on average.  Spectrum activity shows the lovely 2nd order and 3rd order harmonics (vary with peak reduction)!  :thumb:

Thanks to everyone in the forum.  Great info here.  I will post a link of the pictures in the next few days (when I have them uploaded).  Now, off to plan the next DIY project...
Title: All things LA2a related
Post by: Scenaria on July 03, 2005, 12:03:02 AM
well congrats!

I know you have been waiting a long time to finish this one  :green:

cant wait to see some pix :)
Title: All things LA2a related
Post by: SSLtech on July 03, 2005, 12:17:38 AM
Do try to "wash" the T4b. leave it with a signal across it for a few hours and you should find that the THD+N goes down.

The '+' end of the diode is the end with the white band. If the white band is at the 'B' end then everything is hunky-dory. There might be some argument over which way current flows, but basically if the white bands are pointed towards the front panel, everything is hunky-dory.

 :thumb:

Keith
Title: All things LA2a related
Post by: QNote on July 17, 2005, 04:31:00 PM
Well, here are some pictures...

http://www.quarternotestudio.com/LA2A.html
Title: All things LA2a related
Post by: Scenaria on July 17, 2005, 04:54:56 PM
wow!

looks good :)

<-- little biased

but it still looks good... nice and tidy
Title: All things LA2a related
Post by: CJ on July 18, 2005, 03:32:28 PM
This is pretty funny:

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=7337593537&category=23793&ssPageName=WDVW&rd=1
Title: All things LA2a related
Post by: Bluzzi on July 27, 2005, 02:12:39 AM
OK, I got all my audio transformers and meters and I just ordered my passives but I still can't locate a few parts.

One is the 2 trimmer capacitors at 50-580pf. Can I go for less range?
Any sources in Canada or U.S.?

The other is C13 at 510pf. I found it but it comes in Ceramic or Mica. Does it matter?

My pocket is empty again but when I get the passives and turret boards at least I can start to solder! Hopefully my pockets will fill again for the balance of parts by then. I've never been so patient in my life and its killing me!

Thanks

Jim
Title: All things LA2a related
Post by: rove on July 27, 2005, 10:40:06 AM
Quote from: "SSLtech"

The '+' end of the diode is the end with the white band. If the white band is at the 'B' end then everything is hunky-dory. There might be some argument over which way current flows, but basically if the white bands are pointed towards the front panel, everything is hunky-dory.

 

Is this right?  I thought it was the negative end on the diodes that had the white band...
thanks.
Title: All things LA2a related
Post by: SSLtech on July 27, 2005, 11:39:09 AM
It is absolutely dead-serious right.

Keith
Title: All things LA2a related
Post by: AnalogPackrat on July 27, 2005, 11:39:37 AM
Quote from: "Bluzzi"

The other is C13 at 510pf. I found it but it comes in Ceramic or Mica. Does it matter?

Thanks

Jim


I used silver micas for the pf range caps (except for the one trimmer).  No problems...
Title: All things LA2a related
Post by: rove on July 27, 2005, 11:58:08 AM
Quote from: "SSLtech"
It is absolutely dead-serious right.

Keith


Thanks!
Title: All things LA2a related
Post by: dissonantstring on July 27, 2005, 02:15:34 PM
hi,
i read on past post that a 100pf cap works well instead of using the trimmer caps.  i'm wondering because i'm having the same problem as Bluzzi finding a trimmer cap with enough range.  what is the minimum voltage of the trimmer cap if used?
thanks,
grant
Title: All things LA2a related
Post by: Bluzzi on July 27, 2005, 02:51:23 PM
Quote from: "AnalogPackrat"
Quote from: "Bluzzi"

The other is C13 at 510pf. I found it but it comes in Ceramic or Mica. Does it matter?

Thanks

Jim


I used silver micas for the pf range caps (except for the one trimmer).  No problems...


Thanks AnalogPackrat (can I call you Analog?). I can order in peace now.
Now I will look further for the trimmers.

Jim
Title: All things LA2a related
Post by: Bluzzi on July 27, 2005, 03:04:43 PM
Quote from: "dissonantstring"
hi,
i read on past post that a 100pf cap works well instead of using the trimmer caps.  i'm wondering because i'm having the same problem as Bluzzi finding a trimmer cap with enough range.  what is the minimum voltage of the trimmer cap if used?
thanks,
grant


I can probably find them but I am trying to order from as few suppliers as possible so as to avoid minimums and transport costs etc.

Mouser is waiting for me to add a few components and I think what I will do is just order a range of values in Ceramic or Mica (50, 100, 150, 200, 250, 300, 350, 400, 450 and 500 all in pf). It may be overkill but this way I know I will be able to find the correct value.

So unless someone can let us know what the usual setting is maybe I can  purchase a smaller range. So for example if the trimmer is almost always  at maximum pf then probbaly 300 to 500 pf would do.

Jim
Title: All things LA2a related
Post by: rove on July 27, 2005, 05:31:12 PM
Hey maybe I can help, when I thought my trimmer cap was busted, I was told a cap somewhere between 300pf and 510pf would work fine swapping out for a higher value if you experience too much HF rolloff in testing.
Title: All things LA2a related
Post by: dissonantstring on July 27, 2005, 06:06:14 PM
here's what i found on another thread:


Quote
thats an adjustable trim cap. hard to find and harder to install. you can put in a 100 pf and be alright.
cj


this is in reference to C4 and C14.

i'm still building mine, so i'll try this and see what happens.
-grant
Title: All things LA2a related
Post by: Scenaria on July 27, 2005, 07:25:05 PM
hmm

if you are building from the kit the trim cap should be in there.. look for a small ceramic trim type cap with a couple of solder lugs on it.. they arent that small :)

if you are talking about the one for the stereo link then yes.. I dont include that one unless someone orders a pair and specifies that they intend on using them in an actual stereo link.
Title: All things LA2a related
Post by: dissonantstring on July 27, 2005, 07:41:22 PM
hi steve,
you're right! :oops:
i do remember the trimmer cap being in one of the bags.  
thank you for reminding me!
(lack of sleep)
i guess no worries for me since i got your wonderful kit!
 :grin:
thank you steve!
Title: All things LA2a related
Post by: Bluzzi on July 27, 2005, 10:17:47 PM
Does anyone have a source for the trimmer caps in the required value?

Thanks

Jim
Title: All things LA2a related
Post by: Scenaria on July 27, 2005, 10:40:21 PM
seriously...

just drop a 100pf cap in there and be done with it..

if you need to tweak it up then increase the value.

I know theres a few places out there that have the trimmer caps.. (cant remember the URL)

but in all honesty.. it is not needed...

as a matter of fact I was just talking to ssltech about the trimmer cap earlier today with that fact that im thinking in the future of just including a few values to test and try instead.
Title: All things LA2a related
Post by: Bluzzi on July 27, 2005, 11:06:39 PM
Yeah, sounds good. I'll get a few more values in case.

Thanks Scenaria

Jim
Title: All things LA2a related
Post by: rove on July 28, 2005, 01:57:10 AM
Couple of Bloo LA2A transformer questions for anyone with the time:
Re: input transformer (Sowter) I am not sure where to connect R5 and the Grey wire.
Re: output transformer (Sowter) where do I connect the Green wire?
Re: power transformer, where exactly do the 2 black wires and the Grey wire connect (unclear on which is "live" and which is "neutral").
Searched the group for this info and haven't been able to turn it up, so forgive me if this is old hat!  Also I have the Sowter schematic, but it isn't helping at this point.
thanks
Title: All things LA2a related
Post by: SSLtech on July 28, 2005, 08:09:47 AM
On the IEC power socket, there are three pins. the one at the top is ground. The one just below it is Neutral. The one (should be on the right looking inside the box) which doesn't some straight out, but ducks down to the fuseholder part at the bottom, then re-appears on the left side is the live.

So: from top to bottom:

Top = Ground
Middle = Neutral
Bottom = Live

Keith
Title: All things LA2a related
Post by: DrFrankencopter on July 28, 2005, 09:31:24 AM
Regarding the trimmer cap, why not just take a rotary switch and put say 6 different values on it, add the control to the front panel, and label it 'colour', and have it range from dark to bright.

Cheers,

Kris
Title: All things LA2a related
Post by: Bluzzi on July 28, 2005, 10:01:40 AM
Quote from: "DrFrankencopter"
Regarding the trimmer cap, why not just take a rotary switch and put say 6 different values on it, add the control to the front panel, and label it 'colour', and have it range from dark to bright.

Cheers,

Kris


Kris, good idea! I just ordered a few values last night. I still have a ways to go before even starting to solder but I will be trying each value to see its effect. If it is very noticable it may be a good thing, otherwise just having the value that seems to leave the frequency response intact would be enough.

Jim
Title: All things LA2a related
Post by: rove on July 28, 2005, 11:58:44 AM
Quote from: "SSLtech"
On the IEC power socket, there are three pins. the one at the top is ground. The one just below it is Neutral. The one (should be on the right looking inside the box) which doesn't some straight out, but ducks down to the fuseholder part at the bottom, then re-appears on the left side is the live.

So: from top to bottom:

Top = Ground
Middle = Neutral
Bottom = Live

Keith

So maybe what is confusing to me is that the "live" and "neutral" refer to the IEC power socket and not to the black wires coming out of the Power transformer?  Does the grey from the power transformer then attach to ground on the IEC?
Thanks!
Title: All things LA2a related
Post by: CJ on July 28, 2005, 02:19:24 PM
I was just tweaking around with that trimmer and thinging the same thing as Kris.
You can change the freq rep on that box quite a bit with one of that trimmer. Then you might take off the cap on the T4 so you could reach down itno the lows a bit more.

cj
Title: All things LA2a related
Post by: SSLtech on July 28, 2005, 03:15:37 PM
rove,

Which transformer are you using? -The only ones that I've built (and I'm buildinig another at the moment) are all 110/120/220/240V multiple primaries...

Check with the transformer information sheet/wire color info for your paricular transformer, with specific reference to the primary wires.

Keith
Title: All things LA2a related
Post by: Scenaria on July 28, 2005, 03:42:19 PM
Hey Rove,

If you look on the transformer you will see sticker that tells you everything you would ever want to know..

I believe you have a 110/120 tranny right?

if so then you should have this:

the two BLK come from the IEC power socket (this is your primary)
the two reds are your 600vt winding
the two greens is your heater winding
the yellow stripped and grey wire all need to be tied to chassis ground (the same ground that is tied to the IEC socket)

 :guinness:
Title: All things LA2a related
Post by: wave on July 29, 2005, 10:52:19 PM
Quote from: "rove"
Couple of Bloo LA2A transformer questions for anyone with the time:
Re: input transformer (Sowter) I am not sure where to connect R5 and the Grey wire.
Re: output transformer (Sowter) where do I connect the Green wire?
Re: power transformer, where exactly do the 2 black wires and the Grey wire connect (unclear on which is "live" and which is "neutral").
Searched the group for this info and haven't been able to turn it up, so forgive me if this is old hat!  Also I have the Sowter schematic, but it isn't helping at this point.
thanks

Rove,
Sowter input :Gray, black, and one side of R5 go to ground bus
Sowter output : Green and black go to ground bus
If you search back in the thread you can find one of cayacosta's handy diagrams for the sowter input wiring
Title: All things LA2a related
Post by: CJ on July 30, 2005, 12:46:03 AM
I ate the reds. What should I do with the greens and blacks?
Never mind.
Title: All things LA2a related
Post by: rove on July 30, 2005, 06:53:22 PM
Thanks everybody, things are becoming more clear to me now regarding the transformers.  I keep second guessing my ability to read the schematics, but so far it seems I shouldn't have.
I dug up that Sowter input diagram by cayacosta and it was also helpful.
Title: All things LA2a related
Post by: Paul B on August 15, 2005, 08:10:54 PM
Hi everyone,

I just finished a rebuild of my Bloo and arrived at the exact same situation that I had before I ripped it apart and re-did it.

Passes audio, has plenty of gain.  Meter zeroes fine in gain reduction mode.  Absolutely no compression or needle deflection no matter how high I turn up the Peak Reduction.

In +4 and +10 modes, I can barely get the VU to register and that?s only if I turn the gain to 10.  

Using Sowters, grid stoppers in.  Using the 1k, 22k  4.7k config.

Voltages were a little hot, nothing too far out:

A24-25   6.5VAC
B21      367VDC
B20      290
A20      105  (a little weak)
A14      238
A17      94      
A19      139

V1:  pin 1   121
        Pin 3   1.1
        Pin 6   130   
        Pin 8        1.16
V2:  pin 1   109
        Pin 3   4.2
        Pin 6   236
        Pin 7   82
        Pin 8   114
V3   Pin 1   93
        Pin 3   .88
        Pin 6   92
        Pin 8    .88
V4    pin 2     92
        Pin 5   105
        Pin 6   139

I have tried taking the cover off the T4B and shining a light on the panel.  There was no delfection on the meter (in gain reduction mode) when I tried this.

I have checked and re-checked everything 50,000 times, checked for grounding, continuity, blah blah blah.  Any help in where to look for the problem is greatly appreciated,

Thanks,

Paul
Title: All things LA2a related
Post by: CJ on August 15, 2005, 09:34:11 PM
Does the EL panel flicker with the music?
Title: All things LA2a related
Post by: Paul B on August 16, 2005, 12:50:06 AM
Hi CJ,

The flickering barely becomes visible with peak reduction at 10 o'clock.  when turned up full the flickering is as about as bright as a strong glow in the dark toy.  Still no needle deflection on the VU.

At +4 I have to turn the gain all the way up and then the VU only jumps to a max of about -1db.  at +10, I can only get it up to about -7db.

Any tips on how to, er, get it up?

thanks for your help

Paul
Title: All things LA2a related
Post by: CJ on August 16, 2005, 02:11:55 AM
Sounds like the EL panel is doing it's thing. Is there a volume reduction when you crank the GR up to 11?

And what does the meter do when it's reading level, not GR?
Title: All things LA2a related
Post by: Paul B on August 16, 2005, 08:00:53 AM
Even with GR cranked to 11, ZERO volume reduction, no VU deflection.  When measuring level, have to turn gain ALL the way up to get any register in the meter, only goes up to peak level of -1db.  With the T4B taken out, I get a stronger signal thru the meter
Title: All things LA2a related
Post by: Scenaria on August 16, 2005, 08:35:21 AM
four things to tripple check

octal socket wiring

T4b assembly

the rotary switch wiring

and meter turret assembly

when looking at these things its easy to *assume* something its wired correct without tripple checking it.

just look those things over carefully. if your meter isnt really showing any deflection at all then I suspect its in the rotary switch area


can you hear the audio getting louder (when turning up the gain) and compressing when turning up the input?

in other words do you think the compressor is working and the meter isnt showing it?
Title: All things LA2a related
Post by: SSLtech on August 16, 2005, 09:14:55 AM
Using a resistance meter, with the LA-2 powered off, the T4B plugged in with the screws removed to allow you to slip the cover on & off the T4, and the front panel open:

Sounds like the unit might actually be compressing... that might explain the low signal level... We'll get back to it.

In a not-too brightly-lit room, do a resistance measurement between R27 (the end that connects off to the octal socket... the other end should connect to meter trim pot on the front panel) and ground. Once you have a steady reading, take the cover off the T4 and shine a flashlight on the  optocells.) you have the glowing side of the El panel facing the optos, -right?) and see if the reading changes significantly.
Title: All things LA2a related
Post by: Paul B on August 16, 2005, 09:53:04 AM
Scenaria,

I will re-check those areas. I will also try hard-wiring aroung the rotary switch.  To answer the question, I dont think it is compressing OR registering on the meter.

SSLtech,
I will try that later this afternoon when I get home.

thanks guys for your help

Paul
Title: All things LA2a related
Post by: Paul B on August 16, 2005, 11:34:31 PM
OK I did the test that SSLtech recommended.  I got a change from 48k to 2.1k when I shined a bright light on the opto resistors.  The EL panel is facing towards the optos.
Title: All things LA2a related
Post by: Paul B on August 16, 2005, 11:46:22 PM
I forgot to mention that I have the sowter input tranny mounted on the inside of the box - let  me know if that is a bad idea
Title: All things LA2a related
Post by: AnalogPackrat on August 17, 2005, 12:05:15 AM
Paul,

Sowter input tranny on the inside is fine.  That's how I built mine--no problems.  Keep checking the other places Scenaria and Keith mentioned.  I had one incorrect wire on the rotary switch when I first built mine.  It's easy to screw up there.  I'd especially re-check R24 (3.9k).  That resistor affects all meter settings.  Are you sure the T4B is wired correctly?  I would take a fresh look at the internals and make sure nothing is shorting to the can when you close it up.

Good luck,
Analog Packrat
Title: All things LA2a related
Post by: AnalogPackrat on August 17, 2005, 12:15:03 AM
Paul,

One other thing...have you tried running it with the T4B out?  What do the +4 and +10 meter readings look like now?  Is the output level higher?

Analog Packrat
Title: All things LA2a related
Post by: Paul B on August 17, 2005, 01:06:38 AM
Hi Analog,

Thanks for responding.  I did try it earlier with the T4B out and yes the levels are quite a bit higher, still seems on the weak side.  Still need to crank the gain all the way up to get a response.

Let me ask you this; I put my DMM to the end of the pink and the gray wires of the output tranny, and  the resistance was almost zero - does this sound right?

paul
Title: All things LA2a related
Post by: AnalogPackrat on August 17, 2005, 03:11:44 PM
Paul,

When you say that with the T4B out the levels were higher, do you mean the meter readings or actual measurements at the output jack?  Do you have an signal generator that can output a sine wave?  If so, set it up to output about a 1Vp-p sine and inject that into the input of your Bloo with the T4B removed.  Turn peak reduction to 0 and measure the output using the AC V setting on your meter (or look at it on an o-scope if you have one).  Vary the gain and write down the range of outputs you see.  Power off and put the T4B back in.  Repeat the above (leaving peak reduction at 0).  Try again with peak reduction set at 50 and again at 100.  Write down you settings and the output voltages.

What I'm trying to do is determine whether your signal path is behaving and if it is affected by the T4B as it should be.  If that all looks OK, then we'll look at the meter stuff.  Divide and conquer.

BTW, when you zeroed your meter in GR mode, you set it at "0" on the meter scale, right?  Because the meter moves to the left to indicate gain reduction.  Just making sure...

Analog Packrat

p.s.  About the output tranny DCR reading...I'm no expert on tranny stuff, but looking at the Sowter specs it should be about 15 ohms.  If you get less than that, you may have a wiring error on the meter switch that's shorting the output leads.
Title: All things LA2a related
Post by: Paul B on August 17, 2005, 09:27:55 PM
Analog and others,

I meant the meter readings.

I have something that will generate a sine wave, but Im limited to either 440     A or 1k - let me know which one would be better.  Im sure by now that no one would mistake me for an electrical engineer, and I have no idea what a 1Vp-p sine is.  I'll just try using what I have.  Off to the rehearsal studio to go grab it.  Oh, and I did set the meter at zero in GR mode.

I appreciate the help,

Paul
Title: All things LA2a related
Post by: Scenaria on August 17, 2005, 09:58:21 PM
have you actually listened to the output?

the thing is...

everything might be working as expected but your meter might be wired in wrong via the rotary switch

I wouldnt depend on the meter to tell you if its compressing or if theres gain..

toss signal into it (even from a cd player is fine)

and listen to the output... is it distorted? is there alot of gain? can you hear the music being squished?

if yes. then we can narrow things down.. if no then we need to look elsewhere

1khz would work fine
Title: All things LA2a related
Post by: SSLtech on August 17, 2005, 10:09:09 PM
...so would 440.

What's your signal aource anyhow? Let me see if an SM57 is enough...

Keith
Title: All things LA2a related
Post by: SSLtech on August 17, 2005, 10:20:19 PM
I'm back...

Yes an SM57 plugged in direct through no mic preamp is all it takes to drive the VU meter to the endstop with only moderate effort close to the capsule.

Remember that you need to connect a line signal to pin 2 (hi) and pin 3 (low) and not pins 1 and 2 or 1 and 3... likewise for the output.

Then it's what Scenaria said... what's it sound like?

Keith
Title: All things LA2a related
Post by: Paul B on August 17, 2005, 10:31:36 PM
I've listened to it many times.  Ive used an SM58 and a tele straight in and with a FMR preamp.  The output sounds strong and starts to distort at higher levels.  There is absolutely no squash soundwise and no needle deflection in GR no matter what the peak reduction setting.

I dont know what high or low is, regarding the pins on the in and outputs.  I have it wired per the schematic on the Sowter site:  input, green to 2 and brown to 3.  output pint to 2 and grey to 3.

I will run the tests shortly

Paul
Title: All things LA2a related
Post by: Paul B on August 17, 2005, 11:31:47 PM
OK, here's the results I got.  They were virtually identical at all settings:

t4b out and t4b in with Peak reduction set at 0, 50 and 100

no register until 40
GAIN
40:    .1VAC
50     .2
60     .2
70     .6
80     1.5 -1.3  as the PR is turned up
90     2.4 - 2.2
100    2.7-2.5

the input signal was strong and the VU meter did peg from about 75 and up.
Title: All things LA2a related
Post by: AnalogPackrat on August 17, 2005, 11:41:12 PM
Paul,

These were with your signal generator (tuner tone)?  Can you directly measure its output level with your meter so we have an idea what was going into the unit?

Analog Packrat
Title: All things LA2a related
Post by: Paul B on August 17, 2005, 11:57:27 PM
I dont have a way of doing that.  Or I just do not know how to do that.  My lack of knowledge is not making this very productive.  :?
Title: All things LA2a related
Post by: Paul B on August 18, 2005, 12:06:48 AM
I have some other things to try.  I will reposition C5 because it might be slightly interfering with the mini turret board when the box is closed.  Also need to hard-wire around the rotary switch.
Title: All things LA2a related
Post by: AnalogPackrat on August 18, 2005, 12:22:01 AM
Paul,

Hang in there.  You'll get this thing working.  I assumed that you used your signal generator to make those measurements, right?  If so, just connect your meter directly to the signal generator output and measure VAC so we can see what's going into the LA-2A.

I wish I could measure my Bloo and compare to yours, but my stuff is all packed up while I finish up some work on my house.  More DIY    :?
Title: All things LA2a related
Post by: Paul B on August 18, 2005, 11:42:13 PM
Ok, I have some results.  The input signal on the above test was measured at .2VAC.

I cranked the signal up to .7v and got the following results.  I?d also mention that the above measurement was done before the signal went into a passive direct box.  I got no measurable voltage after the signal passed thru the direct box. The signal into the box was strong enough to get the following results at the output

.7 v input signal, With +4 setting for the VU:
T4B out            T4B  In
      
             PR at 0:          PR at 0:            PR at 50            PRat100
Gain   VAC   VU   VAC   VU   VAC   VU     VAC    VU
10   0   -20   0   -20   0   -20   0   -20
20   .2   -9   .2   -9   .2   -9   .1   -15
30   .4   -5   .4   -5   .4   -5   .2   -9
40   .6   -2   .6   -2   .6   -2.5   .2   -7
50   .8   -.5   .8   -.5   .8   +.5   .4   -5
60   1.1   +1.2   1.0   +1   1.0   +1   .5   -3.3
70   2.3   peg     2.2   peg   2.4   peg   1.1   +1.5
80    4.8   peg   5.0   peg   4.8   peg   2.5   peg
90   8.0   peg   7.2   peg   7.2   peg   4.2   peg
100   8.8   peg   8.4   peg   8.4   peg   4.6   peg

I know the chart didnt space correctly, I hope you can sort thru it.
it appears that it wants to squash but somewhere along the line I?m losing signal.

The Gain coming out of this thing, soundwise, is off the charts.  I cant imagine having to turn the thing past 30-40 when the thing works!

Thanks for hanging in there with me

Paul[/url]
Title: All things LA2a related
Post by: AnalogPackrat on August 19, 2005, 12:26:53 AM
Paul,

Yeah, the LA-2A is no slacker when it comes to gain.  It sounds to me like at least most of your main audio path is fine since you are getting reasonable output (yeah, you probably won't be going past 40 on the gain knob very often).  Your meter also appears to work when monitoring the output, but may be a bit off.  It's hard to say unless we know for sure that your signal generator is outputting a sine.

That leaves the part of the meter switch that relates to gain reduction, the T4B, and the sidechain amp.  Lets check the T4B first.  Pull your T4B and measure resistance between pins 7 and 8 with the cover still on.  Then measure resistance between pins 4 and 5.  Now take the cover off and make these measurements again holding the T4B under a bright light.  What happens to the resistance?

Analog Packrat
Title: All things LA2a related
Post by: Paul B on August 19, 2005, 12:58:16 AM
OK, I know what to do now  :grin:  :mad:  :oops:  :evil:

I have some weak connections on the circuit board and the pins.  All the pushing in and pulling out of the T4B has weakened some points that measured just fine last time I checked them.

Ill report back tomorrow
thanks again

Paul
Title: All things LA2a related
Post by: AnalogPackrat on August 19, 2005, 01:03:28 AM
I just thought of something.  Gain is applied after compression, so its possible that you aren't hitting the input with enough level to really get far up the compression knee.  Can you increase the ouptut of your generator any further?  Set the PR pretty high (50 or more) and the gain to something that doesn't peg the meter at +4 (say 30 or so).  Now meter the GR and vary the output of your generator.  Does the meter register?  Try switching to limit mode and check again.

I've got to turn in for tonight.  Sorry this is taking so long, but it ain't easy debugging a circuit by email...

Analog Packrat
Title: All things LA2a related
Post by: Paul B on August 19, 2005, 01:08:47 AM
Thanks Analog,

Ive done that already with varying results.  the shorts in the t4b would help explain that, I would think.
Title: All things LA2a related
Post by: Paul B on August 20, 2005, 11:40:35 AM
I fixed the connections in the t4b.  Analog, per the test you suggested I got the following:

xxxxxxxxxcover onxxxxxxxcover off w/ bright light
resistance
pin 7&8xxx468kohmsxxxxxxxxxx286 ohms

pin 5&6xxxxinfinityxxxxxxxxxxxx262 ohms

you did mean pins 5&6 and not 4&5, right? I got infinity when I measured 4&5 cover on and off.  5&6 and 7&8 are the ones that go to either side of the opto resistors.

the results have not improved with the fixing of the t4b.  so its obviously a component failure and not me.     :wink:

seriously tho, I did also hard-wire around the rotary switch, and that didnt change anything. let me know what you think.

Paul
Title: All things LA2a related
Post by: AnalogPackrat on August 20, 2005, 12:08:21 PM
Paul,

Yeah, I meant for you to measure across each opto. They both seem to work, though the infinity looks suspicious.  The thing is, the "light on" condition is when compression should be happening.  Try looking over the V3 & V4 wiring as that is the sidechain amp for the EL panel.  Maybe it's not getting enough drive.  Also, on the 8-pin T4B socket, the crossed wires are soldered to each other, right?  I'm getting ready to leave the house for the day.  I hope Scenaria or Keith can help you out some.

Cheers,
Analog Packrat
Title: All things LA2a related
Post by: Paul B on August 20, 2005, 12:58:53 PM
Just me being stupid again.  Its not infinity, its a resistance beyond 2000k, which is the capacity of my cheap DMM

so cover off they measure about the same about 280 ohms.

cover on, pin 7-8 resistance slowly climbs to about 468k ohms
resistance 5-6 goes beyond 2000k ohms.

I obviously have a huge difference in the resistance.
Title: All things LA2a related
Post by: SSLtech on August 20, 2005, 04:55:53 PM
Don't worry, that all looks good. The circuit has tracking adjustment built in to compensate for differences in opto resistance. (R25)

Keith
Title: All things LA2a related
Post by: Scenaria on August 20, 2005, 07:53:32 PM
ok so your LDR's are showing that they work when light is applied...

but if you take the cover off the T4b when in the octal socket.. and shine a light on the LDR's the VU meter does not show deflection in GR?

This still suggests to me that something is wrong between the Octal socket, rotary switch or the meter turret board..
Title: All things LA2a related
Post by: Paul B on August 20, 2005, 08:37:58 PM
Ok good,

the needle deflects back and with a bright light I can get it to peg to the left.

Audiowise, I put the cap back on the t4b and  it takes a ridiculously strong input signal to make the meter deflect in GR mode and to get a meter reading on +4

I rechecked V3 and V4 per Analog's suggestion and it looks good.  Yes, the T4 cross is soldered at the 'X'.

Hope this gives a better clue where to look next.  I appreciate all the help.

Paul
Title: All things LA2a related
Post by: Scenaria on August 20, 2005, 09:07:55 PM
did you solder the cross on the octal socket to ground?

I thought you mentioned the other day that the EL panel flickered?

im a little confuse as if the EL is lighting and the LDR's are working there should be some decent compression...
Title: All things LA2a related
Post by: Paul B on August 20, 2005, 10:16:08 PM
Not as confused as I am  :?

The output has gain for days, but to get the VU meters to register anything (GR and +4) it needs an input signal so strong it is unusable, and turn both the Gain and PR to at least 60-100.  The cross on the octal socket is grounded.
Title: All things LA2a related
Post by: AnalogPackrat on August 21, 2005, 12:07:49 AM
OK, I'm back home.  This is very confusing.  Maybe there are two problems--on affecting the metering and something else messing up the compression.  Are you sure that R24 is correct?  Can you tell us how the meter reacts in the +10 setting?  Maybe you got the +4 and +10 wired backwards.  The only thing I can think of with the lack of compression is that your EL drive amp stuff isn't driving the EL panel hard enough.  Does the EL panel still flicker with the input level?  I've never looked at mine to see how bright it is.  Scenaria or Keith may have a hint for you there...

:?:
Analog Packrat
Title: All things LA2a related
Post by: Paul B on August 21, 2005, 12:49:33 AM
It doesnt get particularly bright - about as much as a glow in the dark toy at its brightest.

R24 is correct.  +10 meters about 6db behind +4.  The +4 setting bypasses R23 and +10 sends the signal thru R23.
any advice on troubleshooting the EL drive amp stuff?
Title: All things LA2a related
Post by: AnalogPackrat on August 21, 2005, 01:09:08 AM
Quote from: "Paul B"

R24 is correct.  +10 meters about 6db behind +4.  The +4 setting bypasses R23 and +10 sends the signal thru R23.
any advice on troubleshooting the EL drive amp stuff?


Right, just making sure we didn't miss something obvious.  Did you set the back panel trim pots per the instructions (I can't rememebr which way they're supposed to go)?  Make sure they aren't wired backwards--swapping the outer two connections (everyone wires pots backwards occasionally).  Other than that, recheck all of your voltages on V3 and V4 and compare to the schemo at the back of the Bloo manual.

<checking your old post with voltages>

Looks like V4 may be the problem.  What are your DC voltages there?

Analog Packrat
Title: All things LA2a related
Post by: Paul B on August 21, 2005, 01:32:34 AM
Back panel trim pots are full clockwise.  Theyre wired correctly. the voltages at V4 are pin 2, 9V, pin 5- 105V, pin 6- 140V
Title: All things LA2a related
Post by: Paul B on August 21, 2005, 01:53:20 AM
Posted: Wed Jan 19, 2005 1:44 am    Post subject:
 

 
JU, the EL should flicker with the music during GR. Follow the ac volts from the 6AQ5 coupling cap to the T4 socket. You need about 100 vac to get that thing lit.

Here's an old post from CJ.  Can you tell me exactly where to measure this?

Paul
Title: All things LA2a related
Post by: AnalogPackrat on August 21, 2005, 12:47:57 PM
Houston, we have a problem.  V4 pin 5 is the plate and this should be around 135V while the screen grid (pin 6) should be lower at around 100V.  V4 won't work properly with the voltages you have.  Earlier you said:

"Using Sowters, grid stoppers in. Using the 1k, 22k 4.7k config."

This is the older config with R34 = 22k and R36 = 1k, right?  You're not using C13 and R38 which were added in a later rev, right?  Recheck R34 and R36 as well as R35.  Something isn't right with the wiring around V4.

BTW, I built the newer config with the extra stuff on the screen grid of V4 and 10k R34, 470R R36.

As for CJ's comment about tracing the EL panel drive...  The AC signal from V4 is on the side of C11 that is away from the tube.  Put your meter in ACV, clip the back to ground and probe that end of C11 which should be soldered to pin 3 of the T4B socket.  There should be a wire from pin 5 of V4 over to the insulated lug where the other end of C11 is soldered.  The thing is, I don't think V4 is wired correctly, so there's no point in tracing the output voltage yet.

Analog Packrat
Title: All things LA2a related
Post by: Paul B on August 21, 2005, 01:13:44 PM
Analog takes hammer, hits nail on head.

I have been using the 1k 22k 4.7 config WITH C13 and R38 in place!

Are you then saying I either need to remove R38 and C13 or get the 470 and  10k config?  If that is the case which would you recommend, or does it matter.  I hope this is it...    :razz:
Title: All things LA2a related
Post by: AnalogPackrat on August 21, 2005, 01:23:08 PM
Aha!  Yes, you either use the R34 and R36 you chose and leave out C13 and R38, or you swap our R34 and R36 for 10k and 470R and leave in C13 and R38.  I built mine with C13 and R38 figuring there was a reason they did the rev.  Other folks ahve built them the other way.  I'd say whichever is easiest to do at this point...  You're almost there!
Title: All things LA2a related
Post by: Paul B on August 21, 2005, 01:27:04 PM
maybe this is insanity but I have to ask it:

my voltages for pin 5 and 6 on V4 are exactly reversed of what they should be - what if I just conected pin 5 to A19 and pin 6 to A20 - or is that idea completely stupid??
Title: All things LA2a related
Post by: AnalogPackrat on August 21, 2005, 01:56:17 PM
Nope.  Can't do that.  V=IR.  Do you have parts for the R34 & R36 change?  If not, I believe you can just bypass R38 and leave C13 in place to get the thing working.  Theres a wire that runs from C7D (lower left one) to R38.  Remove the end at R38 and solder it to pin 6 of V4.  You can leave R38 in there, but it will be shorted and won't affect anything.  Now you've essentially got the "old rev" wiring.
Title: All things LA2a related
Post by: AnalogPackrat on August 21, 2005, 02:03:09 PM
To clarify my V=IR comment...  The DC voltages at those pins are defined by the DC currents flowing through the resistors at each pin (plus a bunch of fun thermionic stuff int he tube itself).  Swapping the leads around will result in even wackier voltages and may even damage the tube.
Title: All things LA2a related
Post by: Paul B on August 21, 2005, 02:22:53 PM
I removed R38 and C13 and I still have the same voltages - 105 at pin 5 and 139-40 at pin 6.  :?
Title: All things LA2a related
Post by: AnalogPackrat on August 21, 2005, 02:28:45 PM
Recheck R35.  Make sure you don't have a 22k in there instead of a 220k.  For fun, measure R38 which you just removed.
Title: All things LA2a related
Post by: Paul B on August 21, 2005, 02:35:24 PM
R35 is 220k.  R38 measured at 22K
Title: All things LA2a related
Post by: AnalogPackrat on August 21, 2005, 02:43:07 PM
Strange.  With the cover open and the power on, carefully probe DCV at the other ends of R34 and R35.  You should get the same voltage (roughly 275V).  Have you listened to it since you removed R38 and C13?
Title: All things LA2a related
Post by: Paul B on August 21, 2005, 02:57:59 PM
they measure exactly the same, 288V.  It sounds the same as before - need a horribly strong and distorted input signal to get any reaction out of the VU meter in either mode - with  PR at 100 can get the meter to dip 7db, thats all.
Title: All things LA2a related
Post by: AnalogPackrat on August 21, 2005, 03:03:03 PM
Well, I'm stumped.  If I think of anything else, I'll post again.   :?
Title: All things LA2a related
Post by: Paul B on August 21, 2005, 03:08:42 PM
thanks for all your help Analog.  :thumb:

anyone else?   :sad:
Title: All things LA2a related
Post by: CJ on August 22, 2005, 12:51:34 PM
too lazy to backpaddle, where are you guys with this?

your signal input level should be like a cd player plugged in. a mic will not give you much gr.
Title: All things LA2a related
Post by: AnalogPackrat on August 22, 2005, 01:11:41 PM
Paul,

Well, I had a couple of potential ideas.  Looking at the currents in your 6AQ5 and comparing to those on the schematic, it seems like R34 may be a problem.  If you have a spare 22k 2W resistor (verify its value with your DMM before using it), remove your existing R34 and put in the new one.  The other possibility is a bad tube.  Do you have another 6AQ5?  Another possibility is a weak EL panel, but I know nothing about these things.

Analog Packrat
Title: All things LA2a related
Post by: AnalogPackrat on August 22, 2005, 01:17:20 PM
Welcome back, CJ.  Maybe you can bail us out.  Paul has resoldered his T4B pins to eliminate cold joints and intermittents.  He also accidentally had a combo of old and new circtuitry around the 6AQ5.  Now he's back to the old version with no R38 or C13 (his choice).  What we see for DC voltages on the 6AQ5 look wrong to me.  The screen grid is at 140V and the plate is at 105V.

Paul has also taken the cover off of his T4B with the unit in place and the power on.  Shining a light into the thing made his meter (in GR mode) move much more than his input signal would even with max peak reduction setting.  Maybe you're right and he's just got too weak of a signal.   :oops:

Analog Packrat
Title: All things LA2a related
Post by: AnalogPackrat on August 22, 2005, 04:53:16 PM
I was just looking back through this thread for clues and it looks like Paul has the same problem that wilebee had back in Feb.  Anyone remember what the solution was?
Title: All things LA2a related
Post by: Paul B on August 22, 2005, 06:45:19 PM
Thanks guys,

I have another 22k2w and will try that.  I will also get my hands on another 6AQ5.

As far as input levels go I have used a mic, guitar, (preamped and not, )and CD player.  They have all needed a horribly strong and unusable input signal to register anything on the VU.
Title: All things LA2a related
Post by: wilebee on August 22, 2005, 09:15:37 PM
the GR switch was miswired ! Wil

Wilebee
Title: All things LA2a related
Post by: Paul B on August 22, 2005, 09:18:08 PM
I changed the 22k2w and put in a 6AQ5W.  I now have 101V at pin 5 and 132  on pin 6.

Is there a consensus that these voltages are a likely source of my problem?  The voltages should be about exactly opposite.  It makes sense to my ignorant brain that if pin 3 on the t4b is not getting enough voltage, the EL panel wont get enough juice.  or something like that.   :?
Title: All things LA2a related
Post by: CJ on August 22, 2005, 09:36:23 PM
Don't worry about the 6aq5 voltages. They are not that critical.

Sorry if you have done these already, but:

Forget what the meter says. Are you "hearing" any compression? Does the volume go way down when you turn up the GR?

Mess with R3, the useless stereo pot that might be turned the wrong way.
It could be shunting your gr signal to ground.

What is the part number on the LDR's in the T4? You might need to pull them out a bit to find out. They will bend back in so don't sweat it.

You will get far less GR while operating the T4 with the lid off in a bright room. So put the lid on when testing for GR.

Fire up the compressor and turn the GR up all the way. Measure the ac volts on the 0.1 uF plate cap which is hooked to the 6AQ5. You can measure it on either side. The side opposite the tube plate is better, as you will not have the high voltage dc on there, but it dosen't really matter.
Measure it at Pin 3 of the T4 socket also. You need about 100 vac to lite up your life.


If low ac volts on the AQ5 plate output cap, hook ac meter to the cap comming off V3, which is 0.02 on the schematic. This is the 12AX7 that has it's sections running in parallel. Measure the ac volts while the GR knob is cranked up.

Disconnect one side of your VU meter and listen for gain reduction. You can hear it without the meter. We want to eliminate the meter as a trouble spot.
Title: All things LA2a related
Post by: Paul B on August 23, 2005, 12:25:35 AM
OK first the good news...   IT SQUISHES!!  :grin:   the music is definitely compressing and it sounds nice, but...

The bad news is that it takes an overbearing input signal to get the thing going, and with PR turned all the way up, it still compresses a max of 7-10db.  
I also have a lo level grounding hum...

when you say to measure the cap of the plate of V4, you mean C11 right? the one between V4pin 5 and pin 3 of the t4b)   isnt that a .01 uf and not a .1uf??
with PR all way up I am measuring 220VAC before the capacitor (up to 260 with input) and negligible on the pin 3 side.  with music on, pin 3 is getting about .3VAC max.  hope that tells you something good....

 VAC coming off the .02 cap on V3 (C9) I am measuring 215 VAC on the tube side and 0 on the R37 side.
Title: All things LA2a related
Post by: Paul B on August 23, 2005, 12:27:02 AM
Another question, what would you consider a good input signal VAC-wise?? both peak and average.

thanks
Paul
Title: All things LA2a related
Post by: Paul B on August 23, 2005, 12:40:55 AM
OK now Im REALLY confused and maybe this is it.....

Schematic in bloo manual shows C11 as .1uf.  page 5 of bloo manual does not state value for C11.  the instruction for installing C11 on p.5 does not state the value.  

Schematic at Sowter, cayacosta, jahnsen and 1968 schematics show C11 as .01 uf

which is it?????   :?
Title: All things LA2a related
Post by: Scenaria on August 23, 2005, 08:02:42 AM
the key is to stay with the revsion of schematic that you prefer to use.

the bloo manual is not based on the same revision as what caycosta used (which is why I dont put his layout in the manual)

I think where people make mistakes is trying to use the bloo manual in conjuction with that layout. This is ok if you identify all the differences but where you end up in trouble is if you dont pay attention to the differences. then you end up with a mix-n-match between the two.

you could put a .01 in there if you want... I dont think this is going to fix your problem..

I really believe the first thing to figure out is what your box is actually doing verses what the meter is showing.

try not to even use the meter for any reference at this point.

are you certain that you have wired your input transformer in correctly? we need to know that you have plenty of signal getting in there first/

-steve
Title: All things LA2a related
Post by: CJ on August 23, 2005, 12:46:51 PM
Paul, I think you are measuring DC on those caps. 0.1 or 0.01, don't make no diff.
try it again only with your meter set on ac volts.
Also, compare say, a cd into the LA2 and without the LA2, into a stereo or some other playback amp. Make sure you can get unity gain out of the LA2 without cranking up the Gain.

Go.
Title: All things LA2a related
Post by: Paul B on August 23, 2005, 04:19:18 PM
Hi Steve / CJ,

Those were actually AC measurements at C11:  220VAC and 100VDC.  and 215 VAC at C9 (on the tube side).  I will check on the unity gain when I get home.

Steve, the sowter input tranny is wired per sowter schematic and that small cayacosta diagram.  I know that they are wired correctly but have no idea if Im getting plenty of signal. But I will check it again.  Do you have any reference voltages for the input and output trannies?

I really appreciate you guys hanging in there with me.

Paul
Title: All things LA2a related
Post by: CJ on August 23, 2005, 06:18:54 PM
Unplug the unit and blead the filters.
Then put an ohm meter on the low side of those plate caps on V3 and V4 where your ac voltage is disappearing.
I will make some resistance and AC voltage measuements  on my LA2 tonight and get back to ya. am.
cj
Title: All things LA2a related
Post by: Paul B on August 23, 2005, 09:18:58 PM
Filters bled.  

checked resistance at pin3 t4b (that is the low side, right?) and got something over 2000k (beyond the capacity of my DMM)

checked resistance on the other side of C9. (V4) With R37 turned all the way up, resistance = 1086k and as I turn it up the resistance climbs until it goes beyond the capacity of my DMM at 2000k at about a 9 oclock setting.
Title: All things LA2a related
Post by: CJ on August 23, 2005, 09:30:05 PM
Hmmm, I wonder why you only get a good AC reading on one side of those caps.

You are measuring from the chassis ground to the cap, right?

Measure that AC volts coming off the plate of the 6AQ5 with the T4 thing removed. Measure on both sides of the coupling cap, C11.
Title: All things LA2a related
Post by: Paul B on August 23, 2005, 11:39:15 PM
$#[email protected]%^&*! sorry I keep getting interrupted,

I have 222VAC on the tube side of C11. I have zero voltage on the t4b pin3 side of the cap...
Title: All things LA2a related
Post by: AnalogPackrat on August 23, 2005, 11:44:55 PM
Paul,

Not to confuse your session with CJ, but what are your test conditions?  Input source, PR setting, meter sw setting, etc?  Have you tried CJ's idea of disconnecting one end of the meter (tape up the lead so it doesn't short out o n anything)?

Analog Packrat
Title: All things LA2a related
Post by: Paul B on August 24, 2005, 12:42:42 AM
I am using a digital recorder as the source and am outputing a signal peaking in the 300mV area into a passive direct box and then into the bloo..  

The PR has no effect, either meter or soundwise until about 70, and at 100 it compresses to a max of about 7-10db.

Gainwise, I have parity at a gain setting of 10  :shock:  :shock:
Shows ya how much this sucker is putting out!

However, on +4, parity gain barely makes the needle jump - less than a 16th of an inch.  To get the meter to read near 0db, I have to turn the gain to 70 and it is a horribly distorted sound.

Like I said tho with the gain at 15 and the PR set between 70-100 I hear a  warm compression that tells me when I get this box right its going to be a beautiful thing!

by the way yes I did try it with the VU disconnected and it had no effect on the sound.
Title: All things LA2a related
Post by: Scenaria on August 24, 2005, 07:54:44 AM
300mv is pretty low

you need to get around 3.4v p-p or 1.2 volts RMS

it doesnt matter if your getting alot of gain.. if the box isnt seeing much signal its just not going to compress..

why the DI box?

get something that has a +4 level output and send it into the compressor...

the box has plenty of gain... (it COULD be used as a preamp) BUT the threshold of the compressor is based on the amount of signal being sent into it... think of the Gain knob as "make-up" gain... it has no affect on the amount of compression.. its after the compressor.

therefore if you have a weak signal going into the box it might not ever even cross the threshold.. or you might have to turn the PR all the way up to 90 or 100 to even get a little bit of compression..

my feeling is that your compressor is probably working fine.. once you get a nice strong signal going into it.

I just tried 300mv on the AP..

even with 300mv input and your meter switched to +4 you  should see it hit 0db around the "40 position on the gain"

if you have to crank it all the way up to 70+ then we need to figure out if its the meter or the gain of the box..

my guess is if the meter is registering rather low (which seems to be the case wether its looking at GR or gain) then its something to do with the metering circuit.

try this.

send a 3.4v p-p signe wave into the box (measure the output of the generator) then switch the compressor meter to +4

put a meter on the output of the compressor and turn up the gain knob until your meter reads 3.4vp-p (this will be unity gain) read the meter on the compressor and tell us what it is showing.

while your doing this make sure the PR knob is turned fully counter clockwise.

if your gain knob is somewhere around 20-35 then your gain structure is looking good.

now put a scope on the output of the compressor and bring up the Gain knob.. as you do this you should see the sine wave getting bigger... there will be a point where you will begin to see some distortion.. (the peaks will begin to square off) take note of where your gain knob is when this happens.. my guess is its going to give you a LOT of gain before this occurs.. now look at the VU meter of the compressor (again while its switched to +4) what does the meter say?if its barely registering then I'd say we have a wiring problem in the meter turret or the rotary switch

you actually might be getting alot of compression but just not hearing it... while the meter isnt showing it (our minds can play tricks on us sometimes)

im typiing this early in the morning so my idiot disclaimer is turnes on (im not liable for any mis-information at this time of day) haha

let us know what you find


-s
Title: All things LA2a related
Post by: AnalogPackrat on August 24, 2005, 12:29:45 PM
Paul doesn't have a sig gen or a scope, so we have to get creative.  Hey, Paul, what digital recorder do you have?  Do you have a soundcard for your computer?  The reason I ask is that there are some nice test signal wavs at:

http://www.binkster.net/extras.shtml

If you could grab a couple of the sine wave files and play them into your LA2A we'd have a more controlled source.  Just a thought.

Analog Packrat
Title: All things LA2a related
Post by: CJ on August 24, 2005, 12:56:58 PM
I measured the ac volts on C11 this morning. As you turn up the GR, the ac volts should gradually increase. At max GR, I had about 60 volts ac on both sides of C11. This makes sense as 60 volts is about 160 volts peak to peak, which is within the limits of the B+ voltage.

220 VAC is Way too high, means someting is goofy with your 6AQ5 circuit. Maybe oscillation is causing that  big voltage. I doubt you got your power supply transformer ac on the plate, but check and see just in case.

You might have a bad filter cap, which would explain the hum. Check the ripple voltage on your supply filters, C7a and C7b. Set the meter to ac volts, ground your black lead, put one hand in your pocket for saftey, and get on those caps.

Medium GR only reqiures about 30 vac, I guess audio lites up the EL panel easier than 60 cycles, as you have all those other freqs in there.


In desperation, I usually grab a cup of coffee and sit down with a hi-liter pen and trace out every darn wire and component on a schematic, regardless of how many times I have checked things over. If there is a wiring wrror, this method will find it.

Kind of like this:

(pic on the way)


cj(http://vacuumbrain.com/The_Lab/TA/LA2/la_trouble.jpg)
Title: All things LA2a related
Post by: Scenaria on August 24, 2005, 01:23:13 PM
if you are using sowters and have not put the grid stoppers in or in correctly you will get a lot of oscillations on the output.. .which would explain the hi AC reading..

hmm is there some scope software out there thats free that we can have him put in his pc? so he could atleast see if theres oscillations on the output...
Title: All things LA2a related
Post by: AnalogPackrat on August 24, 2005, 01:54:25 PM
Good idea.  If he's got a soundcard he can try some of the stuff in the meta:

http://www.groupdiy.com/index.php?topic=8387

Maybe we should have a meta on circuit tracing and debugging with minimal tools.  Seems to be a fairly common problem.

Analog Packrat
Title: All things LA2a related
Post by: CJ on August 24, 2005, 03:19:22 PM
The grid of V3 is fed off the input iron so grid stoppers might not be it, unless the osc. is flying around the circuit, which can happen sometimes.

It might be the AQ5 doiny it by itself.

Increase R35 to 1 meg to drop screen volts, that might help if it is V4 in osc.
Title: All things LA2a related
Post by: Paul B on August 25, 2005, 12:19:02 AM
thanks guys,

CJ, I am measuring 222VAC on the tube side of C11. With no audio it stays at 222 as I turn the PR and with audio it peaks at about 255 by the time PR is all the way up.  On the pin 3 t4b side I get 0V without audio and a max of about 1V with audio and PR maxed.  I definitely have a problem in this area so I will trace out with highlighter per your suggestion.

The grid stoppers are in on pin 2 and 7 of V2.  I will try a 1meg as R35 tomorrow.

I am measuring 804VAC at C7a and 631V at C7b.  just for grins, C7c is 492VAC and C7d is 285.

Steve - I have listened to the signal very, very carefully to see if in fact maybe I was tricking myself with my ears.  Im not an electrical engineer (oh,... really???) but I do have a very good ear and I literally am not hearing any compression or volume reduction at all until a setting of about 65-70 on PR.  I have tried different inputs, preamped or DI'd or not, with signal levels (audiowise) that would blow the doors off anything else.  I am getting the same results - strong input signal, tiny compression. overly strong signal, more compression as the input signal gets stronger/worse(the higher/worse the input level, the earlier the meter shows reduction as I turn PR up)

Steve and Analog, I will have time to mess with the software this weekend. It looks interesting but Im not sure how effective in the hands of someone like myself.  The learning curve could be quite flat.  I do want  to see if I can measure for the oscillations that you are talking about.

Let me know what you think of the C7 voltages and if you have any more ideas about, well, anything.

Paul
Title: All things LA2a related
Post by: Scenaria on August 25, 2005, 12:23:12 AM
HOLY CRAP

did your really write 804V AC? and 631

dont turn this thing on anymore..

if you are really seeing those numbers... theres some things we need to address.. for the life of me I cant even think of how your seeing those values..

let me sleep on it
Title: All things LA2a related
Post by: CJ on August 25, 2005, 12:55:13 AM
Paul, be sure you are not reading millivolts ac.

If you are using an auto ranging meter, and measuring low voltage, it will go to mv but you might not see the mv in tiny letters.

I pretty sure you are, otherwise your house would have burned down by now.

If it is AC Volts, check the pwr supply first.
Title: All things LA2a related
Post by: AnalogPackrat on August 25, 2005, 01:06:10 AM
:shock:  :shock:  :shock:

Two things.  First, what brand/type of meter are you using?  Can you please measure ACV at the outlet you are plugging into (put the black probe into one slot and the red probe into the other)?  

Wave had a wacky outlet in his apartment and maybe you do too.  I once lived in a duplex that had a 220V box for a window mounted air conditioner, but someone had put a standard outlet in it!  Luckily the only thing we plugged into it were a string of christmas lights.  They seemed awfully bright, so I checked the voltage.  220!

Analog Packrat
Title: All things LA2a related
Post by: Paul B on August 25, 2005, 08:13:08 AM
The DMM is a cheapo GB and it didnt even occur to me that its capacity is 500VAC so I dont know how I am getting these readings. It is not showing me mv - there is a small 'HV' in the left corner.  I am measuring 121-125VAC at the power outlet.  Maybe I need to get a better DMM :?:

ugh   :mad: I am sorry if I am wasting your time.  Despite the tribulations and aggravation, I've had a great time building the Bloo and Ive learned a helluva lot, but I need to get back to playing.  I am considering sending the unit to Steve to have a look at it.  Let me kmow if you have any more ideas.

Thanx for your patience,

Paul B
Title: All things LA2a related
Post by: AnalogPackrat on August 25, 2005, 11:41:37 AM
Paul,

Hey, we're all volunteering our time.  Don't feel guilty.  Everyone makes mistakes and I, for one, learn more from mistakes than anything else.  CJ has said before that he enjoys the challenge of remote debugging.  I do too.

It seems like your DMM works since you got a reasonable reading on your outlet.  If you have a manual you might look up what the "HV" indicates besides the obvious "high voltage."

The only other idea I have is to disconnect the power tranny HV secondary and check the voltages there (be very careful not to let the leads touch anything else and keep one hand in your pocket).  Something is really wierd.  Have you looked at photos of anyone else's build and compared them to yours?  I have some at:

http://photobucket.com/albums/a173/AnalogPackrat/Bloo LA2A/

Use "Bloo" as the password to view them.  Check the right end of the turret board where the PS stuff is...

Analog Packrat
Title: All things LA2a related
Post by: CJ on August 25, 2005, 01:21:27 PM
Measure from the center tap to each leg of the secondary of the power transformer. If you measure across the full secondary, you might zap the meter.

Caps disconnected. We can fix it here, but if you want to send it off, that's your choice. I think if we get you a good power supply working, the thing might work right. Don't get mad, get even!
Title: All things LA2a related
Post by: Paul B on August 25, 2005, 09:07:50 PM
Well since you guys enjoy the challenge of remote troubleshooting, then I wouldnt possibly think of denying you guys such a GREAT time   :grin:

Now bear with me, I'll remind you all that you are helping an electrical ignoramus, so write to me like Im in first grade.  i dont know terminology like center tap or secondary leg, you will need to say something like "the grey wire from the power transformer"

CJ, are you saying to take out the caps before I make these measurements? (see what I mean?!)

A case of your favorite beer to everyone involved when this thing is fixed.  And I mean that!

Paul
Title: All things LA2a related
Post by: CJ on August 25, 2005, 09:12:28 PM
OK, lets do the pwr supply first. Leave the primary side connected to the power plug/fuse.

Leave the heater wires connected. These are green wires, right?

How many wires are left coming out of the pwr transformer and what color are they?

I suspect two reds and one red/"striped with something" wire. The striped would be grounded as your center tap. No need to unsolder it. Just unsolder the two red wires that go to the diodes, those two small black thingy's with a stripe or arrow on them, that then lead to the big capacitors.

Do you have a digi camera?

What make/model pwr tranny?

We have yet to be skunked on troubleshooting here at the forum.

The harder the problem, the more fun it is, because then we get more of a cyber self esteem boost and get to act like were cool beens for five minutes.

You did read my signature at the bottom, right?  :razz:
Title: All things LA2a related
Post by: Scenaria on August 25, 2005, 09:31:55 PM
his power tranny should be from allied

6K88VG

two blk's are primary

two red's are 250v

red/yellow is 0v (center tap)

two Grn's are heaters

Grn/Yel is the center tap

gray is the shield
Title: All things LA2a related
Post by: Paul B on August 25, 2005, 09:41:03 PM
The power tranny is the Allied 6K88VG.  The other wires out of it are the yellow striped red, yellow striped green and grey.  those 3 are to ground.

no digi camera but I can get one.  will disconnect reds now.
Title: All things LA2a related
Post by: CJ on August 25, 2005, 09:53:15 PM
Cool.
While your under the hood, check the polarity of the diodes. What side are the stripes on? The cap side or the transformer side?

Also, what kind of filter caps, they should say 20 uf at 450 volts or siimilar.

How is the minus lead labeled? Black stripe? Where is it soldered?

Hook up one lead of your meter, which I know is set to AC volts with the red jack not in the ma socket, to chassis ground, via an alligator clip lead or alligator on the end of the probe.

other lead gets another alligator flown in fresh from a florida hurricane attached to one of the red wires. black meter lead is fine on the red, since ac is non polarized. (trying to teach ele 101 at the same time)
stand back and lite that bi-atch up!
Title: All things LA2a related
Post by: Scenaria on August 25, 2005, 10:10:18 PM
light up?

oh you mean the compressor

 :green:
Title: All things LA2a related
Post by: Paul B on August 25, 2005, 10:12:30 PM
Diode stripes on the cap side.  C7a & b are 47uf 450V, c& d are 33uf 450V.  
The caps have an indentation on one side.  that IS the + side isnt it?
 :sad:  the minus sides of the caps are to ground.  measuring each disconnected red wire to the ground where the center taps are, I measure 289 VAC on each.  
I apologize, Im not understanding your instruction to attach the black lead to ground, take the red lead out of the ma socket, and do what now??
Title: All things LA2a related
Post by: CJ on August 25, 2005, 10:13:02 PM
How's the weather, punk?
Yeah, you will have to lite one after I get done cleaning up your customer's mess.
You get the coin, and I get the customer support.
I guess you didn't fall off the hay wagon yesterday, now did ya? :grin:

Don't worry Paul, I was talkin to Austin.
Title: All things LA2a related
Post by: Paul B on August 25, 2005, 10:21:49 PM
Yeah I'll be lightin one of those when this sucker works.  sorry to be such a mess.   :evil:
Title: All things LA2a related
Post by: CJ on August 25, 2005, 10:22:46 PM
OK, that ac is a lil high but no big.
We can mod your supply to get it down a bit.

Those caps should charge to peak voltage of 289 times 1.4 equals 400 volts. Dangerous, but not lethal. We don't wear panties with lace on them like some of the paranoid saftey fanatics around here, so not a problem. Right?

Paul? Come in, Paul!   :grin:

The circuit load will draw down the voltage to about 1.3 times 289 which is 375. Plenty of headroom on the filter cap voltage.

Hook up the red wires and measure dc volts on all four caps.

Oh yeah, un plug it first. Don't just turn off the pwr sw.
Title: All things LA2a related
Post by: Paul B on August 25, 2005, 10:52:00 PM
No sissies here.  I picked myself right up after that 289 V went across my chest....

Im getting C7a- 366VDC, b- 288V, c- 227V, d- 131V.  Still getting the bizarro AC readings of 809, 636, 498 & 287 respectively; maybe best to throw this info to the side for now (?)
Title: All things LA2a related
Post by: CJ on August 25, 2005, 10:58:13 PM
Yeah, forget about it. It's rf messing with your meter.

Your probably driving the next door neighbor's dog nuts, but thats tough on them.

OK, how are you sittin as far as having a few resitors laying around the house?
Title: All things LA2a related
Post by: Paul B on August 25, 2005, 11:04:03 PM
I gots a bunch, all 1/2 watt tho.  Which do you have in mind?
Title: All things LA2a related
Post by: Scenaria on August 25, 2005, 11:06:01 PM
those DC voltages look good :)

they are almost the number exactly what I just measured on one of mine
Title: All things LA2a related
Post by: CJ on August 25, 2005, 11:07:31 PM
What are you using for R16? On the schematic, it shows 68k, color code wise, thats blu-gry-org if Im not color blind. It connects to your voltage line after the R29 47 k 2 watter.
Title: All things LA2a related
Post by: CJ on August 25, 2005, 11:09:35 PM
OK, we are going to disable the side chain temporarily to see if that wacky ac stuff goes away.

there shpuld be a wire going to ther R34, R35 junction that feeds the AQ5, and this same wire will go also to R33, the 220k plate resistor on V3.

Dis connect the wire.
Title: All things LA2a related
Post by: CJ on August 25, 2005, 11:14:30 PM
Crap. Am I toolate!

Just pull the tubes, V3 and V4 out.

Doh on me.
Title: All things LA2a related
Post by: Paul B on August 25, 2005, 11:27:28 PM
R16 is 68k.  Pulled V3 and 4. even more whacko - a-837V. b- 754, c- 590 d- 718.
Title: All things LA2a related
Post by: CJ on August 25, 2005, 11:31:57 PM
OK, pull V1.
Title: All things LA2a related
Post by: Scenaria on August 25, 2005, 11:39:14 PM
Paul... how tidy would you say your wiring is? im just curious... is it nice and short wires with good routs and bends? or is it just kinda all over the place? im not questioning your workmanship but am wondering as I have had a few on the bench with oscillations due to some messy point to point...
Title: All things LA2a related
Post by: Paul B on August 25, 2005, 11:41:28 PM
pulled V1. even worse:  848, 775, 764, 741
Title: All things LA2a related
Post by: CJ on August 25, 2005, 11:43:58 PM
OK, pull the last tube and tell me what the neon bulb is doing.
Title: All things LA2a related
Post by: Paul B on August 25, 2005, 11:54:34 PM
pulled last tube.  neon is glowing.  "volts" now 880, 870, 856, 834
Title: All things LA2a related
Post by: CJ on August 26, 2005, 12:02:49 AM
OK, read dc on caps c7 a b c. d.
Title: All things LA2a related
Post by: Paul B on August 26, 2005, 12:11:26 AM
I got 401, 397, 395, 387 VDC
Title: All things LA2a related
Post by: CJ on August 26, 2005, 12:16:40 AM
OK, thats good.

Do you have any caps around like a  0.1 at 400dc?

If not, what is the voltage rating on C11, the cap on the AQ5 plate?

We want to shunt the "rf" to ground via the cap to see if you have some stuff on your dc voltage line or if it is just your meter acting funny.

If C11 is 300 or more, pullthe T4 module, and connect pin 3 of the T4 socket to ground via a jumper wire or whatever you have. Then re check for the weird ac readings.

Then measure your heater voltage to see if your meter is working on 6.3/12.6 ac.
Title: All things LA2a related
Post by: CJ on August 26, 2005, 12:44:26 AM
Paul, do you have a 1 k and 100 k or so resistor? 1/4 ok.

If so, solder them together to form a voltage divider, clip each end across your heater wires (grn-grn) and measure the ac on the 1 k. This will tell you if your meter is funny, if it does not display millivolts, then we have solved the phantom ac mystery.
Trouble shooting test equipment is all part of the game.
Title: All things LA2a related
Post by: Paul B on August 26, 2005, 12:49:01 AM
the  voltage at c11 was "884AC" and 397 DC.  pulled t4b and shorted pin 3.  still getting the wacky readings at the c7 caps and was "876AC" and 401 DC at c11.

I am still getting no voltage ac (or dc) on the pin 3 side of c11....  I know you said you got 60 VAC on both sides with PR cranked. so this is way off.
Title: All things LA2a related
Post by: Paul B on August 26, 2005, 12:53:10 AM
oh, and on the heater wires 3.4VAC individually to ground and 7.1 on both greens together.
Title: All things LA2a related
Post by: CJ on August 26, 2005, 12:53:42 AM
no, thats ok, don't worry about c11.

lets clear the meter thing up

can you find a pair of resistors that are 1/50th or 1/100th the value of each other? does not have to be exact.

see if you have a 1 k  or 10 k and we can go from there.

that high ac volts would fry those caps in a second, so i know it's not real. I just want to show you that the meter is not really reading ac, but millivolts ac, but not telling you.
Title: All things LA2a related
Post by: Paul B on August 26, 2005, 12:56:16 AM
I have a 1k and 100k, do I solder them in series or parallel? _ think you  mean series, and then just measure on both sides of the 1K?
Title: All things LA2a related
Post by: CJ on August 26, 2005, 01:01:55 AM
exactly, series.

then unplug unit, tack them across heater greens with lite solder, hook up across 1 k resistor...and should read 63 without a millivolts reading.

then from here on in, its a piece of cake, clear sailing, duck soup, cream of wheat, made in the shade with cool-ade.

can you dig where i'm coming from?

can you relate to my trip, man?

how many freaks are there in the world?
Title: All things LA2a related
Post by: CJ on August 26, 2005, 01:17:26 AM
BTW, don't hook it up to the high volts DC!

Or you will have tweakers incense permeating your domicile.
Title: All things LA2a related
Post by: Paul B on August 26, 2005, 01:18:02 AM
oop.  no 100K.  used a 3.9k and a 330 k .  measured zero VAC around the 3.9.  too high of values?
Title: All things LA2a related
Post by: Paul B on August 26, 2005, 01:29:50 AM
well Im fading away here. gotta be up at 4 again tomorrow.

I appreciate you also having no life and being around to help.  Or maybe its what ever is turning you into one of the furry freak brothers. hey, got any more?  type at ya later.
Title: All things LA2a related
Post by: CJ on August 26, 2005, 01:30:10 AM
lets bump that 3.9 k up to 10k and try that.

or even 20k or 30k if you have that.

make sure meter is back on ac volts.

when you see this hi volts ac, is it stable or is the display bouncing around?
Title: All things LA2a related
Post by: CJ on August 26, 2005, 01:09:39 PM
OK Paul, a couple of thoughts. Do you have another piece of tube gear laying around? You could measure the high volts dc and see if your meter does the same thing. It might be the dc playing havoc with the a/d converter when on ac volts. I have seen this before.

Also  check your grounds. Hook your meter ground right to that res/yel centertap. There were some paint-in -the -way of- ground issues a while back.
Title: All things LA2a related
Post by: Paul B on August 27, 2005, 02:01:04 PM
I found the 100k's strapped the 1k-100k around A24 and A25 and I still got a zero voltage reading. :?   The wierd readings I got were steady save for C7d which would slowly drop about 20V and then stay steady.

other tube gear is an old marshall head at the rehearsal studio, I can get it but I wouldnt know where to measure the high volts dc.  

Grounds should be ok, paint has been scraped at all star washer points and pots as well and have been checked for continuity.
Title: All things LA2a related
Post by: Paul B on August 28, 2005, 12:43:45 AM
Ok I replaced the cheap DMM and the bad readings went away.  Here's what I am getting now, all VAC:

C7a- 10.12 VAC 7b- 9.88 7c- 2.45  7d- 980mVAC

C11 tube side:  963mVAC pin3 t4b side: 18-24mVAC

C9 tube side:  960mVAC  R37 side:  1.2mVAC

How about those low AC readings on C11 and C9?  CJ said he got 60VAC on both sides of C11.
Title: All things LA2a related
Post by: SSLtech on August 28, 2005, 09:38:06 AM
The C7 a-d readings look reasonable.

The C9 and C11 look as though the caps might possibly be be too low value... what are they?

If C11 is too low value, you'd probably find that it compresses better at HF than LF (Kinda like a de-esser) Likewise C9. Try with R37 wound one way then the other and see if the difference between thr two ends of C9 is any different.

Keith
Title: All things LA2a related
Post by: Paul B on August 28, 2005, 11:17:45 AM
Hi Keith,

C11 is .01uf and c9 is .02uf.  On the tube side of C11 I got 1060mVAC, slowly dropping a few millivolts every few seconds- I got this no matter where I turned R37.  On the R37 side of C11 I got .9 mV when turned full up and 1.6mV R37 all the way down.
Title: All things LA2a related
Post by: Paul B on August 28, 2005, 03:14:06 PM
A little good news - I replaced C12 and my ground noise went away.  The voltages at C11 and C9 barely changed, a little higher.
Title: All things LA2a related
Post by: AnalogPackrat on August 28, 2005, 10:14:11 PM
Paul,

The tag-team debug continues...  Glad you got a better meter.  There's nothing worse than not being able to trust your tools when you're debugging.

What's this about "ground noise?"  Do you mean that you had hum/buzz on the output before you replaced C12?  If so, that's strange since C12 is in the sidechain.  Have you taken CJ's advice and printed a copy of the schematic so you can trace it against your build using a hilighter pen?  I'm wondering if maybe you got a grounding problem somewhere.  Have you checked your ground tree?

Analog Packrat
Title: All things LA2a related
Post by: Paul B on August 28, 2005, 10:30:45 PM
Well the noise is "back", or maybe the disappearance was just a figment of my hallucination.  :? I mentioned the low level hum in one of my earlier posts.  Might be a power conditioning situation, going to check it at another power source -

In fact I did print out and trace the schematic - it was a good lesson.  Fortunately (or not) I didnt find any problems with the wiring.
Title: All things LA2a related
Post by: AnalogPackrat on August 28, 2005, 10:41:58 PM
Well, the hum may be a clue.  Mine's pretty quiet.  Have you messed with C4 (the big trimmer cap that soldered onto the terminal strip)?  I set mine to ~300pF before I installed it.  Try cranking it CW (clockwise) if it's not all the way CW already.

So your situation is still that the meter has weak response and you don't get any noticable compression, right?  You can get a strong and clean output without compression (PR at 0), right?

Did you try CJ's suggestion to disconnect one of the meter leads?  <grasps handfull of straws>

Analog Packrat
Title: All things LA2a related
Post by: Paul B on August 28, 2005, 10:49:47 PM
Thanks for reminding me!!  Before plugging it in,  i did in fact screw C4 all most all the way counter clockwise - I will put it back where it was.

Situation is strong output, weak compression (7-10db max w/ PR at 100)  and weak needle response (no PR until 70, on +4 must turn Gain to 70 (horribly distorted) to get meter to 0db)

It seems there is definitely a situation around C9 and 11.  Not getting anywhere near the 60 VAC that CJ got at C11 on both sides.

No change in the noise as I adjust C4  :?
Title: All things LA2a related
Post by: AnalogPackrat on August 28, 2005, 11:36:45 PM
Have you taken a look at CJ's T4 page?

http://www.vacuumbrain.com/docs/t5.html

He has some nice photos of the module.  Does yours look like these?  I'm still wondering if maybe you've got a minor error inside the T4 itself.  You can just remove the cover without yanking the whole unit out since that has caused trouble before.  Check the way the cap and EL panel are wired.

Do you have C6 and R30 in your build?  They're related to the stereo link, but they are in the sidechain circuit.
Title: All things LA2a related
Post by: Paul B on August 29, 2005, 12:18:29 AM
I actually used those pictures to help me with my t4b.  I checked that area again earlier today.
I do have C6 and R30.  for C14 I am using a 100pf.
Title: All things LA2a related
Post by: CJ on August 29, 2005, 02:53:30 PM
Cool on the new meter!

When I measured 60 vac on C11 both sides, that was with a music input and the GR turmed up all the way. Just to verify...

Let's get a good sounding line amp first. Do you have some 100K 1/2 watts laying around?  Change the two 220K plates resistors on V1 with 100K 1/2 W.

Do you have a 12AY7 laying around? This tube makes a much better input stage than the 12AX7 for two reasons:

1) On the average it will sound nicer than the harsh 12AX7
2) It will reduce your gain so you can turn up the Gain pot above 1 with a CD type level input.

It has the same pinout as the 12AX7, so all you do is pop it in.


There is a mil version of the 12AY7 which is very cool.
Fogot the number. 6001 or something.
Anybody?
Title: All things LA2a related
Post by: wilebee on August 29, 2005, 03:05:22 PM
6072a
Title: All things LA2a related
Post by: CJ on August 29, 2005, 03:15:17 PM
Thanks Wilebee!

6072A 19.95 at www.tubesandmore

I am sure you can find cheaper. And you can change the tube later, so just do the resistor mod for now and we can hook up the VU meter on audio outout and that will leave us the side chain to trouble shoot.
Title: All things LA2a related
Post by: Paul B on August 29, 2005, 11:09:15 PM
OK, the 100k's are in.

Strangely, I am finding  myself more and more concerned with your tag line...
 :wink:
Title: All things LA2a related
Post by: CJ on August 29, 2005, 11:49:34 PM
OK, put in just the line amp tubes in (V1 and the 12BH7) and try some audio thru there. Crank it up and see if you like the distortion a little better.

Measure V1 pins 1 and 3, 6 and 8.
Title: All things LA2a related
Post by: AnalogPackrat on August 29, 2005, 11:53:09 PM
I wouldn't worry too much about CJ's advice until he starts telling you to peel the covers off of your transformers  :shock:

I have a hunch what he's up to with the 100k's but I'm not going to risk guessing.  You'll have to wait for him to get back here...
Title: All things LA2a related
Post by: Paul B on August 30, 2005, 12:41:30 AM
With V1 and 2 only, no t4b, the hum is there.  sounds about a 120hz.  The voltages at V1 are pin1 180 VDC, pin3- 1.57VDC, pin6- 188 VDC and pin 8 - 1.74VDC.  The distortion seems a little less harsh maybe.
Title: All things LA2a related
Post by: CJ on August 30, 2005, 12:55:22 AM
We will kill the hum later.

 The 100 K's are just to lower the gain a bit and make the clipping more symetrical. The 220K's clip one half way before the other.

Voltages look cool on V1.

Measure the voltage on R16, both sides.

Thanks.
Title: All things LA2a related
Post by: Paul B on August 30, 2005, 07:50:21 AM
Youre thanking me!?! Well, you're seriously welcome.

Measuring R16 at B2 on the turret board (the R13,R9,C7c side) I got 255VDC, other side 347 VDC
Title: All things LA2a related
Post by: CJ on August 30, 2005, 12:49:30 PM
OK, good.

B+ is high because side chain tubes not in yet.

I think were done with the line amp.
Check Pin 3, 1 and 8 on the 12BH7 just to be sure.

Was your VU meter working fine on the +4 setting?
We will get that dialed in next for level monitoring, then thae last thing left is the pesky side chain.  

Just curious, where did you get the filter caps C7 * * * *?

What kind of transformers are you using?
(I/O)

Thanks!
Title: All things LA2a related
Post by: Scenaria on August 30, 2005, 12:52:52 PM
he's using sowters for the audio and an allied for the power

sowters are the reason for the grid stoppers on the 12BH7a.... otherwise theres a lot of unstable oscillation on the output
Title: All things LA2a related
Post by: CJ on August 30, 2005, 12:54:24 PM
Yeah, I know that, hairbrain!
Tell me somthing I Don't know, alright?
And pass the Hookah.  :grin:
Title: All things LA2a related
Post by: Scenaria on August 30, 2005, 01:02:35 PM
here ya go

 :green:

(http://img.photobucket.com/albums/v293/scenaria/DSC003391.jpg)
Title: All things LA2a related
Post by: CJ on August 30, 2005, 01:13:56 PM
That looks like Chappelle's crackpipe!
Title: All things LA2a related
Post by: SSLtech on August 30, 2005, 01:25:15 PM
That's me in the mirror in the picture, to the left of the camera flash, red sweat-shirt.

I'd just had a hit myself, and had to stand up and walk it off... :wink:

Keef
Title: All things LA2a related
Post by: Paul B on August 30, 2005, 08:52:17 PM
C7a&b are Xicons, c& d are KyOEI or something like that.

V2 is measuring 129VAC at pin1, 5.26 pin3, 136 pin8

the meter on +4 was the same: barely register  from 0-40 or so, need to turn to 70 and above to get to 0db.
Title: All things LA2a related
Post by: CJ on August 30, 2005, 10:10:20 PM
OK, line amp looks good.

Lets see if we can get that meter to read your output level on the plus 4 db setting.

You have a 8940 with grey hot , pink cold on the 600 ohm out.

Straight wire the meter plus to the grey transformer lead connection.

Minus meter goes to a 3.9k and straight into the pink lead/connection.

Try that with some music running and see how the meter works. We don't care about gain reduction. If your switch already works good in the +4 setting, forgot the above.

We can move to fixing the sidechain.
Title: All things LA2a related
Post by: Paul B on August 31, 2005, 12:21:03 AM
Something occured to me while I was starting the above instruction.  I have one hole and grommet for the output tranny wires.  The hole is on the opposite side of the grey, pink and black wires.  These wires are wrapped pretty closely around half the tranny before they get inside the box.  I imagine its not ideal to have the ouput wires wrapped around the tranny  :?

I'm thinking about yanking the tranny, drilling another hole for the output wires and insulating them.  Let me know what you think.
Title: All things LA2a related
Post by: CJ on August 31, 2005, 01:22:50 AM
That would be a good idea. There is not a problem electrically, but if something were to catch those leads, you don't want them ripped out of the transformer.

Will make hookup a bit easier I imagine.
Title: All things LA2a related
Post by: Paul B on August 31, 2005, 09:48:53 PM
Ill do that later, then.  I hooked the output tranny straight into the VU as you said and am getting the same results.  Have to turn gain near 80 to get the VU up to 0db on the peaks.
Title: All things LA2a related
Post by: CJ on August 31, 2005, 10:59:21 PM
What kind of meter?
Title: All things LA2a related
Post by: Paul B on August 31, 2005, 11:10:43 PM
Its the Beede meter that Steve ships standard (I think) with the kit.
Title: All things LA2a related
Post by: AnalogPackrat on August 31, 2005, 11:13:36 PM
Scenaria provides a nice Beede VU meter with the Bloo kits...standard A scale.

<edit>
Title: All things LA2a related
Post by: CJ on August 31, 2005, 11:17:33 PM
Paul, check the ohms on the resistor that is in series with the vu meter.
Disconnect one end of the meter so you don't slam the needle with the ohm meter. (it uses a battery inside for ohms)

Do you have a CD player handy?
Wire one output into the LA2. Might have to hack up an RCA cable.
Title: All things LA2a related
Post by: Paul B on August 31, 2005, 11:22:09 PM
I got 3.88K  Gonna go hack that RCA cable now.
Title: All things LA2a related
Post by: CJ on August 31, 2005, 11:27:00 PM
That's good. Are you going thru the VU switch?
If so, try disconnecting it and just use two wires from the meter to the resistor and transformer. This elimnates anything connected to the switch from loading the meter down.
Title: All things LA2a related
Post by: Paul B on August 31, 2005, 11:32:16 PM
Yep Ive got the VU wired straight into the output.  Gotta rehook the output jack.  gonna run out of the cd into an adapter jack into a DI box and then into the Bloo.
Title: All things LA2a related
Post by: CJ on August 31, 2005, 11:36:02 PM
No DI box  necessary, but if you want...
Title: All things LA2a related
Post by: Paul B on September 01, 2005, 12:07:51 AM
With the CD player I still need to blast it to distortionland to get it to register at 0db...
Title: All things LA2a related
Post by: CJ on September 01, 2005, 12:12:49 AM
What are you feeding the output of the LA2 into?
Just for the heck of it, disconnect it and just drive the meter.
Title: All things LA2a related
Post by: Paul B on September 01, 2005, 12:16:04 AM
There's no change when I try that..
Title: All things LA2a related
Post by: Paul B on September 01, 2005, 12:19:23 AM
Does this tell you anything at all:  V2 and V4 burn hotter than a pistol, but the AX7's are a faint glow and do not burn very hot at all.  The voltage is running 6.5V across all the heaters...
Title: All things LA2a related
Post by: SSLtech on September 01, 2005, 12:24:01 AM
Quote from: "Paul B"
Does this tell you anything at all:  V2 and V4 burn hotter than a pistol, but the AX7's are a faint glow and do not burn very hot at all.  The voltage is running 6.5V across all the heaters...

Both of mine are like that and they're both normal.

Keith
Title: All things LA2a related
Post by: CJ on September 01, 2005, 12:26:11 AM
That's normal. They use a lot more juice than the AX's.

Measure the output of the transformer with your voltmeter on ac volts when you are getting 0 db on the meter.
Title: All things LA2a related
Post by: Paul B on September 01, 2005, 12:46:49 AM
With the gain set on 70, I'm getting peaks that occasionally peg the meter.  The voltage at the output was topping out at 1082 mV
Title: All things LA2a related
Post by: CJ on September 01, 2005, 12:54:29 AM
That sounds about right.

OK, we have a healthy pwr supply, a good line amp, a meter that could be trimmed up a bit and a unit that is passing audio, albeit a bit distorted when driven hard. Which is normal for this circuit.  12AY7will help a bit.

Side chain time.

The side chain signal is taken off the input transformer via some resistors.

Check the two 68 k's and the 2.7k resistors against the schematic to make sure they are ok.
Title: All things LA2a related
Post by: Paul B on September 01, 2005, 01:21:42 AM
the 68k's & 2.7 look good against the schematic..
Title: All things LA2a related
Post by: CJ on September 01, 2005, 01:38:22 AM
OK, disconnect the output amp you are listening thru, put on a cd, crank up the gain till you get 0 db average on the meter, take out the T4 module, turn up the GR pot all the way,and see if you can measure any millivolts of cd signal pumping thru the input tranny. You can take the measurement  off of the junction of the 68k and 2.7 k resistor, or off the top of the 100k GR pot.
The bottome of the pot will be grounded.
Title: All things LA2a related
Post by: Paul B on September 01, 2005, 08:03:36 AM
do I put in V3 and V4 yet or am I still leaving those out?
Title: All things LA2a related
Post by: Paul B on September 01, 2005, 08:41:53 PM
I did the above with V3 and 4 in.  With the gain turned to 70, I got an average of 0db on the meter.  Whether the PR was up or down the measurement at the 68k-2.7k juncture averaged about 85-95 mV and peaked (with the VU pegging) at 152mV.

I know nothing,  but that seems awfully low....
Title: All things LA2a related
Post by: CJ on September 02, 2005, 01:40:12 PM
That's a good enough signal to work with. Make sure the millivolts you have at the resistor junction makes it to the top of the GR pot.
Title: All things LA2a related
Post by: Paul B on September 02, 2005, 03:13:05 PM
Yep its getting there.  Measured about the same with a 155mV peak.
Title: All things LA2a related
Post by: CJ on September 02, 2005, 03:28:00 PM
OK, lets fix this dang thing.

All we have left to do is follow the siganl thru two amplifier stages, into the EL panel, and then verify EL>LDR circuit in the T4 module. Once we are to that point, we will verify that the T4 is controlling the input signal and registering this reduction on the VU meter.

So plug in all the tubes and get on that first stage:

1) V3- 12AX7a - voltage amp.

Volt meter on DC Volts > Measure pins 3 and 8 to determine the health (bias) of this voltage amp.

Anywhere from 0.5 to 1 volt is OK.
Title: All things LA2a related
Post by: Paul B on September 02, 2005, 03:36:14 PM
V3 pin 3 & 8 measure .84 V
Title: All things LA2a related
Post by: Paul B on September 02, 2005, 03:53:54 PM
By the way, (with the same gain setting as above) on either side of C11 I am getting 29 VAC with PR set at 50, 102VAC peak with PR at 100
Title: All things LA2a related
Post by: CJ on September 02, 2005, 04:53:41 PM
OK, thats promising.

Lets take a few measurements along the way just so we have a referance for future DIY'ers with trouble.

Measure DC volts on the plate of V3, pins 1 and/or 6 (they are strapped) with no GR.

Without any GR, inject a signal and turn up the gain to give a 0 db output on the VU meter set at +4 db.

Then turn up the GR full blast and measure the AC volts on both sides of the V3 plate cap, C9 on my schematic, 0.02 connected to pins 1 and 6 on the 12AX7a.
The 12AX has a gain of about 100, so I expect to see about 10 volts ac on that cap.


BTW, I find it helpful to pencil in these voltage measurements on a copy of the schematic for future use. I circle AC volts and underline DC volts. You got to keep em seperated, like the song says.
Title: All things LA2a related
Post by: Paul B on September 02, 2005, 05:42:25 PM
V3 pin 1 getting 99VDC.  with PR at 100, am getting an average of approx. 2.5VAC on tube side of C11, 4.1V as a peak.  on the R37 side of C11, am getting what looks like an average of about 1.5 VAC, with peaks at about 3V.
Title: All things LA2a related
Post by: CJ on September 02, 2005, 05:52:39 PM
Plate volts look good, 0.88 bias divided by 1000 ohm cathode resistor means 0.88 milliamps flowing thru V3.

0.88 ma times 220k plate resistor is  0.00088 amps times 220,000 ohms equals 193 volts across R33. So 193 plus 99 measured across the tube equals  292, which is I believe close to your suply volts to V3.

Just for the heck of it, swap V1 with V3 and see if you get the same numbers on the plate and cathode (dc) and on C9 (ac).

.
Title: All things LA2a related
Post by: Paul B on September 02, 2005, 07:27:08 PM
I got 95 VDC at pin 1, .87 at pin 3 and nothing changed at C9.
Title: All things LA2a related
Post by: CJ on September 02, 2005, 07:34:41 PM
OK, unhook one end of C9, the end that does not go to the plates of V3, and measure the ac volts again.

You might want to hook a 1 meg resistor from the cap to ground to give V3 an ac load during measurements.
Title: All things LA2a related
Post by: CJ on September 03, 2005, 02:32:43 AM
Paul, I am going to be either on a river somewhere, or sleeping in a tent in the woods for the next three days. So try by passing the Lim Response circuit by connecting C9 cap directl;y to the top of the R3 stereo adj pot. Disconnect c^ also. I think you are losing GR signal inbetween C9 and the output of C11 somewhere, so trace it out with the hi liter.

cj
Title: All things LA2a related
Post by: Jim50hertz on September 05, 2005, 04:24:36 PM
Hi Guys

A while back i got my Bloo2 up and running but discovered some voltages were off, particularly at A20 and A19  :cry:

I've had a good look at the circuit, checked all my component values and swapped out the R3 and R37, the 1 Meg pots in case that was the problem...

With the tubes in but no T4B l've still got wrong voltages at A20 (76V - nearly 60V out) and A19 (84  - 20V out)...

B20 reads 275V and I'm using 22k at R34..

Any ideas what may be wrong?

My other voltages are as follows, still out but not as far

B21 - 353V
A1  - 56V
A14 -225V
A17 - 86V

Thanks in advance!

Jim
Title: All things LA2a related
Post by: Scenaria on September 05, 2005, 05:33:43 PM
Hi Jim,

look this scan over

and tell us which one you wired yours up like...

a lot of guys get different revisions mixed together which sometimes causes odd voltages.

(http://img.photobucket.com/albums/v713/jdr555/la2arevisions.jpg)
Title: All things LA2a related
Post by: Jim50hertz on September 05, 2005, 06:09:39 PM
Hi Steve

Ok, this is making a bit more sense now...

I'm using the A10 so i guess i'm looking at the 1979 LA2A but i've got

R7 = 2.7 K
R34 = 22K
R38 = 22K

I'll try swapping these values out tomorrow and see what difference that makes to the voltages, thanks for posting that steve!  

Cheers

Jim
Title: All things LA2a related
Post by: Jim50hertz on September 05, 2005, 07:27:31 PM
Hi Steve

I decided to go for it tonight and swapped out the values, chaining resistors in series to get the right values....

Hallelujah i think we're nearly there!

Got

B21 - 360V
B20 - 267V
A1 - 56V
A20 - 140V
A14 - 219V
A17 - 119V (the only one significantly out)
A19 - 109V

I'm thinking of chucking in the T4B and seeing if she'll pass audio...

Here goes
Title: All things LA2a related
Post by: Scenaria on September 05, 2005, 07:32:00 PM
dont put the T4b in yet...


just check to see if you have clean gain first


(you dont need the T4b to pass audio) that way if anything is wacked you wont destroy the t4b :)

by the way your voltages are looking right on target... what were the resistor values you changed?
Title: All things LA2a related
Post by: Jim50hertz on September 05, 2005, 07:54:49 PM
Ok, it passes audio but i have a weak, buzzy signal...

Also, when i put the meter position to gain reduction it whacks right up to the red... Hasn't been adjusted yet tho

The values i changed were

R34 from 22k to 10k
R38 from 22k to 220k
R7 from 2.7k to 1.2K

Reckon there might be grounding issues, closer listening sounds almost like mains hum, gonna swap out this gain pot cause it sounds noisy too
Title: All things LA2a related
Post by: Scenaria on September 05, 2005, 07:59:33 PM
triple check your rotary switch wiring..

a lot of people mess up in this area..

becareful with that meter.. they are very expensive to replace
Title: All things LA2a related
Post by: Jim50hertz on September 05, 2005, 08:18:58 PM
That may be it, i appear to have wired it correctly but positioned upside down

How do i know which pole is Pa and which is PB as i may also have referenced them incorrectly?

Cheers

Jim
Title: All things LA2a related
Post by: Paul B on September 05, 2005, 10:34:55 PM
I know everyone is on the edge of their seat regarding my situation, so here's what's new:
I havent disconnected C9 yet, but I took new readings with a more controlled input source.  With a 1k sine wave, I turned the Gain until reading 0db at +4 setting.
Source at input jack:  7.72 VAC
Output signal:  grey (pin 3 of output jack)  105mVAC, pink (pin 2) 1344 mVAC  :?
C11, tube side:  11.1 to 75.1 VAC as PR is swept from 0 to 100
t4b side:  11.0 - 74.8 as PR is swept from 0 to 100

C9:  tube side 0-2.21 VAC as PR is swept 0 to 100
R37 side:  0-2.08 VAC as PR is swept.

The voltage at the 68k-2.7k juncture at the input tranny (insulated turret) AND at the top of R2,  is 223mVAC
Title: All things LA2a related
Post by: AnalogPackrat on September 05, 2005, 11:41:38 PM
Paul,

What's your new voltage source?  Try reducing the input to 1-2VAC.  What AC voltage reading do you get at R5-R6 node?

A P
Title: All things LA2a related
Post by: Paul B on September 06, 2005, 08:13:37 AM
Hi AP,

The signal source is the same 1k tone I was using before. Most of the past readings were with a CD input source.

When I measure straight off the input source I am getting 1.42VAC, which is strong enough to give me a 0db reading with the gain at 44.  I get the 7.72 when I measure off the pins of the input jack to ground.  That is after going thru a DI box.  I'm using the DI box for the XLR out to get into the Bloo, by the way.  In any case, I am getting 446mV at R5-6 junction and 213 mV at R6-7

thanks again,
Paul
Title: All things LA2a related
Post by: CJ on September 06, 2005, 03:58:33 PM
Voltages look good on your 6AQ5 output!

Something is fishy on you input circuit. The Sowter has a 1:4 ratio, so you should be seeing an increase in ac volts as your source goes thru the input transformer. Also, if using an XLR jack, you measure your signal between pins 2 and 3. Pin one is usually the shield.

Try ditching the DI box and just hook up a cd signal straight into the LA2. Measure input vs output on bpth sides of the input transformer. Also make sure your lim/comp sw is wired right.

cj
Title: All things LA2a related
Post by: rove on September 07, 2005, 06:33:47 PM
Hey I just got my bloo to the testing stage.  Plugged it in the other night and powered up.  nothing caught fire.  Measured voltages (w/o tubes) and they were closer to 400 for both b21 and b20, but I figure that is ballpark.  A1 is 59V (IIRC) and I had 7+ V btwn A25 and a24.  so I power down throw in the tubes, and power back up, tubes heat up, but then the fuse goes and that is it.
Today I got some new fuses and they blew up ifrst try with out the tubes in.  any ideas where to start?
thanks.
Title: All things LA2a related
Post by: Scenaria on September 07, 2005, 06:39:45 PM
well..

I would put in one tube at a time

see if it blows with one tube.. if not then try another.. see if you can narrow it down to one of the tubes.. then I would look around to see if you have short somewhere.

be very careful... as it sounds like you might have a shorted lead...

always always allow time for the caps to discharge.

dont put the T4b in yet...
Title: All things LA2a related
Post by: rove on September 07, 2005, 07:04:14 PM
well if I turn the unit on now without any tubes it just blows a fuse.  I looked around for any shorts, I'll look closer.
any help appreciated.
thanks
Title: All things LA2a related
Post by: CJ on September 07, 2005, 07:27:42 PM
Are you using slo-blo fuses?
What amps rating?
Check the two diode polarities.
Ohm out filter caps and see what they say.
Title: All things LA2a related
Post by: rove on September 07, 2005, 07:40:04 PM
I am using 250 volt 500mA slo-blo fuses.
the diode white bands are nearest the front of the chassis.
c7a=7.6k
c7b=12+k
c7c=80k
c7d=250k

thanks!
Title: All things LA2a related
Post by: CJ on September 07, 2005, 07:51:13 PM
disconnect the red wire going to the front of the 4.7k pwr resistor and plug it in.
check polarity on c7 a-d
Title: All things LA2a related
Post by: rove on September 07, 2005, 08:31:48 PM
Quote from: "CJ"
disconnect the red wire going to the front of the 4.7k pwr resistor and plug it in.
check polarity on c7 a-d

there are the two red wires coming from the Power transformer through the diodes, should I disconnect both of them?
thanks.
Title: All things LA2a related
Post by: Scenaria on September 07, 2005, 08:52:28 PM
nono

what cj is talking about is the lead that goes from the capacitor down to the front of the 4.7k 2watt power resistor

leave the two red leads from the power tranny hooked up

 :green:
Title: All things LA2a related
Post by: CJ on September 07, 2005, 09:01:22 PM
Yeah, what Baby Face Nelson said.
http://www.fbi.gov/libref/historic/famcases/babyface/babyface.htm
 :green:
Title: All things LA2a related
Post by: rove on September 07, 2005, 09:13:30 PM
I just disconnected the + of c7b and blew another fuse, then disconnected the + of c7a (reconnecting the first) blew another fuse.
thanks again for the help.
I get 3 more tries tonight, then back to the fuse shop tomorrow.
Title: All things LA2a related
Post by: CJ on September 07, 2005, 09:51:35 PM
Please disconnect the red wire.
Thanks.
Title: All things LA2a related
Post by: rove on September 07, 2005, 09:58:00 PM
Quote from: "CJ"
Please disconnect the red wire.
Thanks.

I don't know what wire you mean as the "red wire"  sorry.
Title: All things LA2a related
Post by: CJ on September 07, 2005, 09:59:04 PM
OK, never mind.

And disconnect one of the green heater wires on the pwr trans.
Not the one that might be grounded.
Title: All things LA2a related
Post by: rove on September 07, 2005, 10:04:52 PM
ok, disconnect a heater wire, reconnecting the above link I unsoldered or leaving it disconnected?
then try to power up with the heater wire disconnected?
sorry for any communication failures...
Title: All things LA2a related
Post by: CJ on September 07, 2005, 10:07:37 PM
Yes, we are seeing if your hetaer circuit has a short.
Actually, before you blow another fuse, you could ohm out the place where the green wires get hooked to, looking for a dead short. Pull the tubes for this test, as they will seem like a dead short to the meter.
Title: All things LA2a related
Post by: Scenaria on September 07, 2005, 10:40:49 PM
umm

dont power up when you ohm it out

 :razz:
Title: All things LA2a related
Post by: rove on September 07, 2005, 10:57:34 PM
meter shows .4Ω resistance when I measure between the heater wires, power off.
Title: All things LA2a related
Post by: CJ on September 07, 2005, 10:59:50 PM
Disconnect both green heater wires and fire it.
Title: All things LA2a related
Post by: rove on September 07, 2005, 11:11:11 PM
Quote from: "CJ"
Disconnect both green heater wires and fire it.

tried it.  blew fuse.
Title: All things LA2a related
Post by: Scenaria on September 07, 2005, 11:18:13 PM
ok dont take this the wrong way...

but did you remember to put the insulator under the main turret board?

also check to make sure none of the turret mounting screws are touching any of the tube sockets..
Title: All things LA2a related
Post by: rove on September 07, 2005, 11:46:51 PM
insulator is under the turret board.  None of the mounting screws appear to be interfering with anything.  there did appear to be a bad solder joint on C7c (neg), I fixed that, reconnected the heater wires, turned it on, blew another fuse. uggh.  Got one more fuse tonight...
Title: All things LA2a related
Post by: CJ on September 08, 2005, 12:15:56 AM
Ohm out your power chord prongs with the pwr switch on.

Not plugged in, of course.
Title: All things LA2a related
Post by: rove on September 08, 2005, 12:23:55 AM
shows about 12Ω when power switch is on.
Title: All things LA2a related
Post by: CJ on September 08, 2005, 01:53:48 AM
That's good.

I know you don't want to do this, but can you disconnect all the wires coming out of the transformer except the two black pri wires and plug it in?
Tape upe the loose leads with a turn of elect. tape

 :mad:
Title: All things LA2a related
Post by: rove on September 08, 2005, 02:25:00 AM
done, fuse did not blow.
now what?
thanks.
Title: All things LA2a related
Post by: CJ on September 08, 2005, 02:27:22 AM
first, put back the heater wires and the light bulb.

then smoke test.
Title: All things LA2a related
Post by: rove on September 08, 2005, 02:34:28 AM
Quote from: "CJ"
first, put back the heater wires and the light bulb.

then smoke test.

I'm sorry, the lightbulb? you lost me.
I'll be getting those heater wires hooked back up.
Title: All things LA2a related
Post by: CJ on September 08, 2005, 02:37:35 AM
Pilot light, pwr light, whatever you want to call it.
Check the socket for a short before you try your final fuse.
Title: All things LA2a related
Post by: rove on September 08, 2005, 02:42:20 AM
Quote from: "CJ"
Pilot light, pwr light, whatever you want to call it.
Check the socket for a short before you try your final fuse.

perhaps that has something to do with the meter (which I am testing without as I do not have it yet)? or do you mean the neon?
the socket still shows about 12Ω with the switch on, no power connected.
Title: All things LA2a related
Post by: CJ on September 08, 2005, 02:49:02 AM
OK, no light on the front panel. No worry then.
Plug it in. If no probs, plug in tubes, see if they glow, but still leave other wires taped.

i suspect backwards diodes at this point, but we won't hedge our bets.
go for the heater circuit check.

i have to buzz home soon, but will be back AM

cj
Title: All things LA2a related
Post by: rove on September 08, 2005, 03:06:09 AM
I should also call it a night.  I put the tubes in and powered on and got a weird fizzy sound (tubes frying?) though no smell or heat and only the 12bh7 showed any appreciable glow.  The fuse, once again, did not blow.

it would be "funny" if it were backwards diodes as I posted earlier on this very thread regarding just which direction the diodes should be.

thanks, looking forward to getting to the bottom of this.
Title: All things LA2a related
Post by: Scenaria on September 08, 2005, 08:28:18 AM
as long as the white lines on the diodes are forward on the turret board.. as in closest to the front (the B row of turrets) they are not backwards.. but if the white lines are facing the rear of the chassis (on the A row) they are backwards.

ive built over 15 of em now.. and would have some serious problems if they were all in backwards :)
Title: All things LA2a related
Post by: rove on September 08, 2005, 10:08:40 AM
Quote from: "Scenaria"
as long as the white lines on the diodes are forward on the turret board.. as in closest to the front (the B row of turrets) they are not backwards.. but if the white lines are facing the rear of the chassis (on the A row) they are backwards.

Diodes appear to be physically facing the right way.  I actually did have them backwards long ago, but figured out to turn them around (long before testing).
upon closer inspection the 6aq5 is lighting up a bit as well as the bh7, but the other 2 do not appear to be glowing.
And hey, all the help is very much appreciated.  I am sitting here with my soldering iron whenever anyone wants to chime in with some advice.
thanks again.
Title: LA2a DIY Simple help
Post by: yodermr on September 08, 2005, 01:21:10 PM
Hi all. I'm new here and also to DIY projects of this magnitude.

Anyway I am in the process of building a LA2A off of Jahnsen's book. I have read and read and read anything and everything. Nice thread BTW. I catch on quick but there are some things easier asked.

Everything seems to be going well, I'm up to putting in the caps. Realized I was missing a few and then also realized that I don't know enough to get what I need. So, I thought it was time to ask some basic questions...

I read somewhere that C12 and C13 are color dot mica caps. I can't seem to be able to find these @ mouser so I thought I might not have the name right. Can anyone clue me into what these are otherwise called or where can be found?

Another basic question. I purchased jensen tranformers for this project (JT-11P-1 and JT-10K61-1M)The Jensen schmatic has some nomenclature that I am not familiar with. It lists C2 as 100nf. Is this nanopharid? What's the conversion to, hell something I could find - micro, pico, etc...

On the same schematic, C3 is listed as a 220n. Is that something different than C2's nf?

C8 on the LA2A schematic shows two caps in paralell .01 & .02uf. I read somewhere that these should be ceramic. Not sure what wattage I need.

I hoped to not bother you guys with this simple stuff. But its better to be sure than try to find the problem later.
Title: All things LA2a related
Post by: rove on September 09, 2005, 03:51:23 PM
Well, just to keep y'all updated.
things seem fine with just the heater wires hooked up ie the AC voltage measures out ok.  I went back and cleaned up any spots that looked questionable in my soldering (well many of them anyways...) and also discovered some grounds weren't properly connected so I did that too.
Checked the heater wires voltage again, still ok.  Decided to reconnect the remaining wires from the PT and power up again.  Blew a fuse again.  So should I try it without, say the ground wires or the red PT wires and add each in pairs to determine where upstream my short might be?
going to get more fuses now.
thanks.
Title: All things LA2a related
Post by: CJ on September 09, 2005, 04:18:59 PM
Try hooking up the two red wires to the two diodes, but leave the red striped wire centertap ungrounded and see what you get. There will not be any dc voltages since the return path is not connected, but this might tell us what is going on.
Title: All things LA2a related
Post by: rove on September 09, 2005, 05:04:03 PM
With all the wires attached but the red striped centertap from the power transformer, I once again blew a fuse.
however, when I take the red wires off as well as the red center tap, I can power on with out blowing a fuse, but the ac at a24-a25 is incorrect or non-existant.  The only difference from earlier when I was running just the heater wires is the Green centerr tap is hooked up. oh and all this is without tubes at this point.
Title: All things LA2a related
Post by: CJ on September 09, 2005, 05:29:42 PM
Bingo. Your diodes are blown.

Ohm them out from front to back and back to front.
If not blown, they should read high ohms one direction, and low ohms the other way.
Check them without any wires connected.
Title: All things LA2a related
Post by: rove on September 09, 2005, 06:25:58 PM
That was it.  One (it appears) of the diodes was died.
I put in replacements and all is pretty good.
Voltages as follows: A20 is supposed to be 131V (avg) and I get 100V.  A19 is supposed to be 105V, I get 78V.
the others (b21, b20, a1) are good with a couple a bit off: A14 off by +15V and A17 off by +8.  I realize the published numbers are averages, just not sure what the tolerances are.  Oh and this is with the tubes IN.  Should I put in the T4B?
thanks again for the online troubleshooting.
Title: All things LA2a related
Post by: CJ on September 09, 2005, 06:52:25 PM
Try just the line amp first (no T4).
Pump in one channel of your cd player.
I like subbing in 100K plates resistors instead of 220K on V1.

I will try and stock up on 12AY7's so I can send out some samples for you to try for V1.

Now where's that Paul character?
Title: All things LA2a related
Post by: rove on September 10, 2005, 01:33:58 PM
funny, my name is paul, too.  different guy, though.
Plugged in with no t4b, seemed ok.  Plugged in with t4b, also seemed ok.  Very transparent gain reduction, can't wait to get the meter in so I can watch it work.
there is still a tiny bit of hum that I would like to try to get rid of, if anyone has any suggestions there.
thanks again.
Title: All things LA2a related
Post by: Paul B on September 10, 2005, 04:12:41 PM
OK, other Paul character is back.

Quote from: "CJ"
Voltages look good on your 6AQ5 output!

Something is fishy on you input circuit. The Sowter has a 1:4 ratio, so you should be seeing an increase in ac volts as your source goes thru the input transformer. Also, if using an XLR jack, you measure your signal between pins 2 and 3. Pin one is usually the shield.

Try ditching the DI box and just hook up a cd signal straight into the LA2. Measure input vs output on bpth sides of the input transformer. Also make sure your lim/comp sw is wired right.

cj


I wired the input straight into the bloo.  The input signal measred an avg. of about 300-320 mV with peaks at about 450.  This is about the strongest signal I can put into it without it distorting at the output, even at the lowest levels.  By the time  I turn the gain to 10 it is already about 3 times as loud as not running throught the bloo, and starts distorting at about 20.

With this 300-320 mv avg, I was getting a reading of about 1100mV average at the R5-R6 junction (w/ the blue sowter input tranny wire), with peaks at about 1640mv.

Using a 313mV strong 1k sine wave straight into the bloo, I got a reading of 1153mV at R5-R6.  So it seems Im getting more like 3.7:1 ratio rather than 4:1.

with that 313mv sine wave I got these readings also:

495 mV at R6-R7 (insulated turret) and at top of R2
114VAC on both sides of C9 with PR at 100, 58.4V w/ PR at 50

as far as output, with these gain readings I got the following output readings at the output jack:


gain at 10:  215 mV
15:  712mv
20:  1268mV  (reading 0db on the VU meter)
25:  1672mV
30:  2.6V
The VU meter was pegging by the time I got to 30 with the sine wave so I didnt test it any higher than that.  

With the signal hooked straight into the bloo, it seems to only lower the distortion threshold on the gain knob.

thanks,

Paul
Title: All things LA2a related
Post by: wilebee on September 11, 2005, 12:05:11 PM
hey there everyone la2a #2 is up and running passing signal and has a few quarks I doesnt seem to be getting a signal at the floresent panel optos ae working because when I take the housing off the t4b the gain goes down but nothing when its on at hitting it with +4 @1k. also It doesn't seem to be regestering at +4 or +10 setting.

1st what should I be seeing at the t4b feeding the floresent panel ?

any thoughts on the +4/+10 meter setting ?

Wil

Wilebee
Title: All things LA2a related
Post by: Scenaria on September 11, 2005, 12:34:52 PM
hey wil,

im off to watch some football BUT

you have a good situation there.. in that you have one that works that you can take some measurments off of...

thats where I would start
Title: All things LA2a related
Post by: wilebee on September 11, 2005, 12:59:16 PM
We'll that figures  football
Title: All things LA2a related
Post by: wilebee on September 11, 2005, 04:47:09 PM
ok Wilebees advice for the day  dont trust that someone elses transformer jumpers are good ! that being said I've got my levels to +4 and +10 now that I'm using my entire Secondary. But I still don't have gain reduction meter deflection  any thoughts anyone ? Wil
Title: All things LA2a related
Post by: AnalogPackrat on September 11, 2005, 06:39:36 PM
Wil,

If you have two and one works, follow Scenaria's advice.  You say you've got the output metering working, but are you getting any gain reduction on the output signal?  Does your EL panel blink with the music?  If it still doesn't then your meter isn't going to read anything GR since there is no GR happening if the EL panel doesn't light up.

If the problem is the EL panel, then you're going to have to start comparing voltages around V3 and V4.  What are your DC voltages (pg 25 of the Bloo manual)?

A P
Title: All things LA2a related
Post by: AnalogPackrat on September 11, 2005, 06:51:06 PM
Quote from: "Paul B"
OK, other Paul character is back.

I wired the input straight into the bloo.  The input signal measred an avg. of about 300-320 mV with peaks at about 450.  This is about the strongest signal I can put into it without it distorting at the output, even at the lowest levels.  By the time  I turn the gain to 10 it is already about 3 times as loud as not running throught the bloo, and starts distorting at about 20.


That's strange and seems to contradict your measurements with the sine wave input where you put in 300mV and got out 215mV with gain at 10.  Out of curiousity, what pot (brand, type) did you use for gain?  Is it linear or log (audio) taper?

Quote

Using a 313mV strong 1k sine wave straight into the bloo, I got a reading of 1153mV at R5-R6.  So it seems Im getting more like 3.7:1 ratio rather than 4:1.

with that 313mv sine wave I got these readings also:

495 mV at R6-R7 (insulated turret) and at top of R2
114VAC on both sides of C9 with PR at 100, 58.4V w/ PR at 50

as far as output, with these gain readings I got the following output readings at the output jack:


gain at 10:  215 mV
15:  712mv
20:  1268mV  (reading 0db on the VU meter)
25:  1672mV
30:  2.6V
The VU meter was pegging by the time I got to 30 with the sine wave so I didnt test it any higher than that.  

With the signal hooked straight into the bloo, it seems to only lower the distortion threshold on the gain knob.


You mean that changing PR made the distortion worse?  What happens to your output levels at PR 50 and PR 100 for 300mV sine in and gain set at 10?

A P

p.s.  What pot did you use for PR?
Title: All things LA2a related
Post by: wilebee on September 11, 2005, 07:32:29 PM
nope voltages are al right on and the t4b works in the other la2a I'm quite stumped !
Title: All things LA2a related
Post by: AnalogPackrat on September 11, 2005, 07:40:49 PM
Wil,
You compared DC voltages at all nodes around V3 and V4 and at the T4B socket?  They ALL match between the two units?  Carefully compare all of the wiring in those areas.  Try swapping T4Bs between them.  Something is different or they would both behave.

A P
Title: All things LA2a related
Post by: wilebee on September 11, 2005, 08:13:08 PM
I've done that I think ill give it the night off re attack it tommorrow
Title: All things LA2a related
Post by: Paul B on September 11, 2005, 09:05:21 PM
Hi AP,

the pots for R1 and R2 are the alpha 100k log. they "ohmed out" just fine before I put them in - about 102k for R1&2 and 998 for R3, 4 and 37.  they are all grounded fine.  let me know if you think I should change them.

with the 313mV sine wave in and the gain set at 10 and getting the 215 mV at the output, the VU meter is barely registering.  And it is already about 2 times as loud as compared to bypassing the Bloo.
Title: All things LA2a related
Post by: AnalogPackrat on September 11, 2005, 09:30:46 PM
Strange...what does the output measure with your DMM on ACV?  Does the output change at all when you turn up the PR?
Title: All things LA2a related
Post by: Paul B on September 11, 2005, 09:58:49 PM
gain at 10:  215 mV
15:  712mv
20:  1268mV  (reading 0db on the VU meter)
25:  1672mV
30:  2.6V
The VU meter was pegging by the time I got to 30 with the sine wave so I didnt test it any higher than that.  


thanks,

Paul[/quote]

these were all AC readings at the output jack.
Title: All things LA2a related
Post by: AnalogPackrat on September 11, 2005, 10:10:28 PM
Right.  So hook up your meter at the output and set the gain at 10 or 15 and fiddle with the PR knob.  Does anything change when you do that?  It seems like things are working pretty well for the most part.
Title: All things LA2a related
Post by: Paul B on September 12, 2005, 10:35:58 AM
Ok now I understand - Ive been in "no t4b mode" for a while.  With my 313 sine wave in, and gain at 15, I get 795mV at the output and -4db on the VU. With t4b in, when I raise PR to 61 I get half voltage, about 655mV.  Sure enough, VU was down 6db at -10.

Raising the gain to 20 where I get 0db on the VU, I get 1295mV at the output.  Turning PR to 61, I get half voltage (655) and -6 on the VU.  At PR = 71 I get another half in voltage (325) and about -12 on the VU.  With PR at 100 I get 172mV at the output, so the total gain reduction is less than 18db.  

Problem is, that 313 mv input gives me a very distorted signal on the output.  Everything seems to be fine except that I can' t put in a strong enough input signal to both drive the t4b and not get a distorted sound at the output.

I find myself suspecting C5.  I will run out later this morning and replace that sucker.

thanks again for your help.
Title: All things LA2a related
Post by: AnalogPackrat on September 12, 2005, 11:53:38 AM
I'll try to dig some of my test equipment out this week and reproduce your set up for comparison.  Can you describe the distortion you're hearing?  Does it sound like an overdriven guitar amp or is more like a ring modulator?  Is it buzzy?  Just trying to figure out if it's really the audio path being overdriven or something else.

Have you ever used an oscilloscope?  Do you think you could find a way to borrow one (and maybe some of the owner's help, too)?  I think it would really help to have a look at the signal to see where things are going wrong...

A P
Title: All things LA2a related
Post by: CJ on September 12, 2005, 02:47:50 PM
I will do a complet ac and dc voltage chart on an LA2 schematic with a given input and GR on 5.  This would be a good referance to have here and might be all that you need to troubleshoot.
Paul, time to get out the highliter pen as a last resort. It's not that tough, give it a shot.
cj
Title: All things LA2a related
Post by: Paul B on September 12, 2005, 04:13:32 PM
AP,
I would describe the distortion as "puffy" and "splatty".
I will see what I can do about the O-scope.

CJ,
I've already traced out the schematic with a highlighter. I didnt find anything wrong.

I replaced C5 with a 22uf 450V.  The output is cleaner with less distortion at higher levels.  My other problems remain, tho. I will wait to see what readings you and AP come up with.  If that doesnt narrow it down, then I'm looking at a complete re-work on the thing.
Title: All things LA2a related
Post by: CJ on September 12, 2005, 04:46:27 PM
You can also ohm out the circuit to make sure there isn't a bad joint somewhere. (Remember when a joint was a bad place to hang out?)
You are esentially tracing the schematic with the ohmeter instead of the highliter. Sometimes I have turned up wires that I forgot to solder on tube sockets and things.
At least you will know every nook and cranny on this thing. So the next one will take ten  minutes to fix.
Title: All things LA2a related
Post by: AnalogPackrat on September 12, 2005, 04:59:31 PM
Paul,

So the distortion sounds similar both with the T4b in and with it out?  That is, the T4b doesn't change the character of the distortion, right?  If so, it seems like something is awry in your audio path (maybe a bad joint [seeds?] as CJ pointed out).  What kind of caps did you use for C1, C2, and C3?    Can you make the distortion go away with really low gain settings or is it always there?

A P
Title: All things LA2a related
Post by: CJ on September 14, 2005, 12:56:30 AM
OK, here is a voltage chart for the LA2. I used my own machine, which utilizes a 12AY7 for V1, but that should not make that much difference for troubleshooting purposes. Just expect more gain from V1 from the 12AX7a.
Also, my 6AQ5 is really a 6V6GT, the exact same tube, different bottle, so that should be ballpark also.

I am using an external pwr supply, so the B+ comes straight into the box, thus, I did not include pwr supply ac volts.

File is a bit large thus I am linking it. Filp it left or right to get it to print on one page. Colors will be lost on B/W printout.

I injected real music from a CD player, set the GR on 5, set the gain pot so I was reading 0 db on the meter output. GR registered about minus 20 on the meter. My VU meter is not perfectly calibrated, so allow 20 percent difference in all of these readings, which is normal for a tube circuit anyway.

Now go get that problem, Paul! Rework be damned!
 :thumb:

http://vacuumbrain.com/The_Lab/TA/LA2/la2_voltages.jpg
Title: All things LA2a related
Post by: rove on September 14, 2005, 01:47:41 PM
Quote from: "CJ"
I used my own machine, which utilizes a 12AY7 for V1, but that should not make that much difference for troubleshooting purposes. Just expect more gain from V1 from the 12AX7a.
Also, my 6AQ5 is really a 6V6GT, the exact same tube, different bottle, so that should be ballpark also.

Can I just put in a 12ay7 or 6072 in place of v1 without any other alterations?
thanks.
Title: All things LA2a related
Post by: CJ on September 14, 2005, 02:35:25 PM
Yes. Drops right in.
A lot of those min dual tri's have a mantra that you should recite:

plate- grid-cathode----plate- grid- cathode---
if you memorize that, it will save you a lot of time fetching the tube manual.

the heater wires divide the two sections, so an id of elements is quick and easy.


12AT7, AY7, AX7, all the same.
Title: All things LA2a related
Post by: Paul B on September 14, 2005, 10:57:25 PM
Quote from: "AnalogPackrat"
Paul,

So the distortion sounds similar both with the T4b in and with it out?  That is, the T4b doesn't change the character of the distortion, right?  If so, it seems like something is awry in your audio path (maybe a bad joint [seeds?] as CJ pointed out).  What kind of caps did you use for C1, C2, and C3?    Can you make the distortion go away with really low gain settings or is it always there?

A P


The distortion is the same with t4b in or out.  C1-3 as well as both C8's and C9 are orange drops.  The distortion goes away at low gain levels.  The basic problem remains:  Little/no compression because input signal is weak (more than 320mV or so at the input and it starts distorting even at low gain levels).

Now on CJ's chart (thanks for the chart, CJ) he's got 4.0V at R5-R6.  Does that mean the input level pre the A-10/HA100X is about 1.0V??  Is that 4.0 a peak or an average (or something else?)

Everything seems whacked at the input tranny.  The signal is weak when I come in thru the input jack and when I wire straight in the signal is stronger but must be kept weak to avoid distortion.

CJ, how far did you turn the Gain up to reach 0db?
Title: All things LA2a related
Post by: AnalogPackrat on September 15, 2005, 12:05:05 AM
Paul,

Well, maybe there is something amiss with your input tranny.  Those orange drops should be fine, though (assuming they're the 400 or 630V ones).  The HA100x is wired as a 1:10 on the original, I believe.  I'm guessing that's how CJ has his set up.  So he's actually got more like 0.4V on the primary.  He said he used music for a signal so you'll have to ask him whether it was Black Sabbath or classical harp music  :grin:

I still haven't managed to get my test equipment set up this week like I promised :oops: .  Today is my wife's birthday.  Maybe tomorrow evening...

A P
Title: All things LA2a related
Post by: CJ on September 15, 2005, 01:07:06 PM
Yes, HA 100X wired 1:10, so about 0.4 on the input jack. I was using music so the signal moed around a bit, but was stable enough to get an average reading on a digital voltmeter. I also double checked with the scope at the same time.

Everybody says their LA2's distort at about anything past 5 on the gain dial with a hot (modern cd signal input, so that part of your project might not be a concearn. The 12AY7 will help that out a lot.

It sounds like you have a problem with the input trans and/or input jack.
Make sure you both are wired correctly.

I think I remember the gain knob being set at about 3 or 4 to get o db out.

How is your compression when you straightwire the thing?

Fill out that voltage chart along side my numbers so we can have a gander. I can post a pic if you can get someone to take a digi pic.

Just for the heck of it, you might want to make sure that your nrgative feedback is reducing the gain of your amplifier. This can be done by disconnecting the wire that leads from R11 (68 K) to the cathode of V1a.

Do an A/B test with the amp running. No high voltage at this part of the circuit.



cj
Title: All things LA2a related
Post by: Paul B on September 17, 2005, 08:50:57 PM
I filled in my readings next to CJ's.  How do I post the chart?

I did the readings with a 750mV input signal.  I could only turn the Gain to about 15 before it got to 0dB and really started to distort.  BUT it is a strong enough input to get about a 20 dB reduction when I turn the PR to 50.  When I crank PR it REALLY gets quiet!!

Some of the voltage readings are completely whacked.  I can understand why some of the AC readings are low because my gain was only at 15, but the higher AC voltages I dont understand.

It appears the unit will function just fine at these higher input levels, once I can tame the output.  By the way, the distortion is more of a rolling warm tubey distortion with this higher input level.

Pls tell me how I can get the chart up. It's a .gif format.  thanks.
Title: All things LA2a related
Post by: AnalogPackrat on September 17, 2005, 09:51:24 PM
Hi, Paul.  Just open a free account at photobucket (http://photobucket.com/), upload your picture and then post the link here.  I think CJ is out kayaking this weekend, but I have a couple of hours this evening to help you out.  As a bonus, I dug out some of my test equipment so we can try to trace a nice simple sine wave input for comparison.  My Bloo also uses Sowters, so we should be good.  You changed your 220k plate resistors on V1 to 100k, right?  We'll have to keep that difference in mind in our comparisons.  I have to eat dinner at some point, but we should be able to make some progress on this thing.

A P
Title: All things LA2a related
Post by: Paul B on September 17, 2005, 10:11:57 PM
OK let's see if this works....

http://i8.photobucket.com/albums/a5/PaulB_/PaulsVoltages.gif
Title: All things LA2a related
Post by: AnalogPackrat on September 17, 2005, 10:29:05 PM
OK, I'm checking vs. your gif.  I have 750mV in, gain at 15 and PR at zero.  Set to compress.  At the input transformer secondary I get 2.89VAC and between R6 and R7 I'm getting 1.25VAC.  At Output +4 meter setting I get -4.3dB.  Right off, it looks like you have a problem somewhere around R5, R6, or R7.  

A P
Title: All things LA2a related
Post by: AnalogPackrat on September 17, 2005, 10:31:23 PM
Or an alternate theory is that your EL panel is lit even with PR at 0 and that is killing your input.  Can you pull the cover off, dim the lights and see if the EL panle is on at all?
Title: All things LA2a related
Post by: AnalogPackrat on September 17, 2005, 10:35:27 PM
Interestingly, I have to turn my PR up to 80 to match your 0.268VAC after R7.  Are you sure you don't have a linear pot for PR?
Title: All things LA2a related
Post by: Paul B on September 17, 2005, 11:28:00 PM
Quote from: "AnalogPackrat"
OK, I'm checking vs. your gif.  I have 750mV in, gain at 15 and PR at zero.  Set to compress.  At the input transformer secondary I get 2.89VAC and between R6 and R7 I'm getting 1.25VAC.  At Output +4 meter setting I get -4.3dB.  Right off, it looks like you have a problem somewhere around R5, R6, or R7.  

A P


are we all comparing the same thing, tho?  My reduction from 2.86 to .345 across R6  is about 8.3 times difference, where CJ's is 6.7 times difference, yours is 2.3 times difference....

yep, I still have the 100k's in for R9 and R13...

also, The EL panel is dark when PR is turned down.
Title: All things LA2a related
Post by: AnalogPackrat on September 17, 2005, 11:32:03 PM
Sorry for the confusion.  My measurements were with PR = 0.  To match your number at R6-R7 I had to turn my PR up to 80.  I think we should try to debug this thing with the simplest setup.  PR = 0.  Let's make sure your amp (v1 & v2) works.  I'm not sure it does looking at your voltage readings near the back end.  Can you set you PR to 0 and measure at R6-R7?  Then turn your gain up until you get CJ's voltage at V1 pin 2.  I need to eat dinner.  Be back in about 30 minutes.

A P
Title: All things LA2a related
Post by: Paul B on September 17, 2005, 11:32:49 PM
AP, what VAC did you get at the output with that setting?? I had my PR setting at 50. was getting about 15-18 db reduction.
Title: All things LA2a related
Post by: AnalogPackrat on September 18, 2005, 12:05:11 AM
I get 0.225VAC out with gain at 15 and PR at 80.  Look, we're not going to be able to easily match up if we don't simplify.  Our pots aren't going to be the same.  Let's set PR to 0 and try match our voltages at the input and go from there, OK?

A P
Title: All things LA2a related
Post by: Paul B on September 18, 2005, 12:29:01 AM
With Gain at 15 and PR at 0, I got 2.91VAC at R5-R6 and 1.14 at R6-R7.  The VU meter was a hair away from pegging, +3db (at +4 setting)
Title: All things LA2a related
Post by: AnalogPackrat on September 18, 2005, 12:36:26 AM
OK, I just traced mine at gain 15, PR 0.

750mV, 1kHz input
2.89VAC at input transformer secondary
2.86VAC at R5/R6
1.24VAC at R6/R7
0.015VAC at V1 pin 2 (grid)
0.1VAC at V1 pin 1 (plate)
3.55VAC at V1 pin 6 (second plate)
0.345VAC at A14 (V2 pin 6)
3.22VAC at C5
1.08V at output jack

Try to set your gain so that your V1 pin 2 voltage matches mine.  We'll SWAG the AX vs AY gain differences...
Title: All things LA2a related
Post by: Paul B on September 18, 2005, 01:11:06 AM
Quote from: "AnalogPackrat"
OK, I just traced mine at gain 15, PR 0.

750mV, 1kHz input
2.89VAC at input transformer secondary
2.86VAC at R5/R6
1.24VAC at R6/R7
0.015VAC at V1 pin 2 (grid)
0.1VAC at V1 pin 1 (plate)
3.55VAC at V1 pin 6 (second plate)
0.345VAC at A14 (V2 pin 6)
3.22VAC at C5
1.08V at output jack

Try to set your gain so that your V1 pin 2 voltage matches mine.  We'll SWAG the AX vs AY gain differences...


Here's what I got:
750mV, 1k input
2.91 at R5-R6
1.14 at R6/R7
.033 at V1 pin 2
turned gain down to 12 to match your .015 at V1 pin 2
.1VAC at V1 pin 1
2.83VAC at V1 pin 6
1145mV at A14 (!!)
2.57 at C5
.84VAC at output w/ gain at 12 , gain at 15, output = 1.28VAC

where exactly is the input transformer secondary? isnt that the same as R5-R6??
Title: All things LA2a related
Post by: AnalogPackrat on September 18, 2005, 01:18:26 AM
I think your 12BH7 may be oscillating.  Can you describe how you wired your grid stoppers?  They're 1k's right?  Let me go check one other voltage on the 12BH7.  Can you measure ACV on Pin 6 with the current setup?  Be right back...

Yeah, R5/R6 = input trafo secondary.  I somehow managed to take that measurement twice  :oops:
Title: All things LA2a related
Post by: AnalogPackrat on September 18, 2005, 01:24:35 AM
OK, I get 0.318VAC at V2 pin 6.  Watcha got?
Title: All things LA2a related
Post by: Paul B on September 18, 2005, 01:29:35 AM
the 1k gridstoppers are between V2pin2 and A9, and between V2pin7 and B8.  

The ACV on pin 6 (thats the same as A14, right?) is 1118mV
Title: All things LA2a related
Post by: AnalogPackrat on September 18, 2005, 01:39:05 AM
Am I getting tired and foggy or what?  This will be my last post for tonight, I think.

Yeah, I think you've got something weird on the BH7.    Here are some things to try:

1) if you gradually turn down your gain while watching A14, does it suddenly drop off or gradually diminish?  Doea the sound of the output also change at the same point?

2) My Bloo never had the oscillation problem so I'm not sure how it looks with just a meter.  If you had a scope it would be cake to see it.  Have you tried using a wooden pencil or chopstick to move some of your grid and plate wires around while you listen to the thing and/or watch the troublesome point (A14, for instance)?

3) Do you have another BH7 to swap in?  Or have you done that already?

4) Recheck all that R21, C4, R22, R11 wiring (and pretty much everything at the BH7).

I really think you are close.  It's probably one silly thing somewhere obvious (it's always obvious once you find it).

A P
Title: All things LA2a related
Post by: AnalogPackrat on September 18, 2005, 01:41:46 AM
BTW, it sounds like your grid stoppers are correct.  DId you shrink wrap the bare wires?  They aren't touching anything else like the little meter turret board, right?
Title: All things LA2a related
Post by: Paul B on September 18, 2005, 01:43:09 AM
I appreciate all your help, AP.  I'll get on those things in the morning.  Yes, I dont care how silly the problem will end up being, just want to nail it.

see ya later
Paul
Title: All things LA2a related
Post by: Paul B on September 18, 2005, 01:43:59 AM
yeah, I did shrink wrap the bare wires.
Title: All things LA2a related
Post by: AnalogPackrat on September 18, 2005, 01:46:26 AM
No problem, Paul.  Hope we can kill that bug soon...
Title: All things LA2a related
Post by: CJ on September 20, 2005, 09:08:13 PM
Paul, what are you using for C4? If adjustable, try tightening it down all the way. This might kill any osc. that you have.

Everything looks halfway normal in the GR circuit, which is good news.
It looks like the trouble starts at the BH7 top plate. Before when you had 7 volts here, that would definately cause distortion because you are exeeding the 4 volt bias on the bottom cathode.

Try a 0.01 uF cap at 200 volts if you have one. Put it from ground to the bottom plate of V2 (pin 1) and see if that helps get that pin 6 ac volts reading down.

Also, what are you using for R16? Make sure it is 68K.

Do you have a Fender guitar amp around by any chance? If so, pull the 12AT7 phase inverter tube out and try it in the place of V1 and see what you get.

I am with AP here, since we have it narrowed down to the output stage, and your GR seems to be working good, I will do a new set of voltages with no GR. I will take home the signal generator to get a steady tone instead of music.

cj
Title: All things LA2a related
Post by: Paul B on September 20, 2005, 09:36:07 PM
C4 is the adjustable cap that Steve includes with the kits. I believe its all the way tightened down I will check again.  R16 is a 68k.  

Im thinking about replacing caps 1-3.  The dc voltages are whacked off of C1 and 2 and the AC voltages are way off on C2 and 3.  I think with all the tweaking (not me, the caps) between the 1st and 2nd builds they might have gotten damaged.

I have an 12AY7 and and replacements for V2 & 3 on the way.
Title: All things LA2a related
Post by: CJ on September 20, 2005, 09:45:42 PM
Cool on the 12AY7!
If you can find a 6.8K 2 watt resistor, try it in place of the 4.7K R29 to bring your B+ down a bit. Sometimes dropping the voltage will cool oscillations.
Title: All things LA2a related
Post by: Scenaria on September 20, 2005, 10:15:31 PM
hey paul...

you might have answered this before but how would you rate your wiring? is it pretty neat and tidy? I had some oscillations in one that was on my bench that was caused by some untidy wiring..

I also had problems with sowters before I put the grid stoppers in...but I think I remember you saying that you have the grid stoppers in?
Title: All things LA2a related
Post by: AnalogPackrat on September 20, 2005, 10:33:32 PM
Scenaria,

Paul's got the grid stoppers in on the WCF and from his description it sounds like they wired correctly.  I'm curious how you would rate my wiring  :roll:  

Analog Packrat's Bloo LA2A Build (http://photobucket.com/albums/a173/AnalogPackrat/Bloo%20LA2A/)

password is "Bloo"

A P
Title: All things LA2a related
Post by: Paul B on September 20, 2005, 11:20:07 PM
Well honestly some of the turret board, while all solid and without shorts,  could be more tidy. I've just thought that the root of my problem lies in something more obvious, like a wrong connection or component failure.  It has always been in the back of my mind to redo a lot of it.

AP, I think your work looks pretty effing pro, for whatever THAT'S worth.
 :roll:
Title: All things LA2a related
Post by: Scenaria on September 20, 2005, 11:21:28 PM
Quote from: "AnalogPackrat"
Scenaria,

 I'm curious how you would rate my wiring  :roll:  


does it work?

if so then I would rate it to be excellent haha

im staying away from that question..  :green:
Title: All things LA2a related
Post by: AnalogPackrat on September 20, 2005, 11:47:26 PM
Smartass!  Yes, it works.  Quite well.  If you thought it was better than good you would have said so, I guess I got my answer (C, maybe).  Hey, I can take it.  At least I used different colors for the wire and components! :razz:

A P
Title: All things LA2a related
Post by: AnalogPackrat on September 20, 2005, 11:51:51 PM
Quote from: "Paul B"

AP, I think your work looks pretty effing pro, for whatever THAT'S worth.
 :roll:


Hey, no need to butter me up!  Look at some of the other photos posted around here.  There are some really neat wiring jobs being done by this crowd.  I'm always loathe to neatly bundle up a bunch of wires with tie wraps for fear of crosstalk/coupling/oscillations/bad dreams.  That's my excuse and I'm sticking to it!

A P
Title: All things LA2a related
Post by: Scenaria on September 21, 2005, 12:02:51 AM
seriously... if it works then I would avoid waking the "sleeping dog" cause you never know what can happen once you start poking around in there..

BUT

whenever someone tells me they are having induced hum or oscillation issues I always ask how tidy their work is.. if they say its kinda messy I usualy suggest cleaning it up a bit...

if anything it atleast makes it easier to troubleshoot and spot potential problems
Title: All things LA2a related
Post by: AnalogPackrat on September 21, 2005, 11:17:31 AM
Don't get me wrong--it works and I'm not changing it.  I was just looking for feedback for improving future builds.  Fire away!
Title: All things LA2a related
Post by: Scenaria on September 21, 2005, 12:10:55 PM
im not sure how you went about wiring the turret...

but what I found easier is to actually put 12 inch leads on every turret where needed before putting the board into the chassis...

then I would simply run the wire to correct tube or whatever and trim to length... it was easier than trying to work in the box with the turret board in there
Title: All things LA2a related
Post by: AnalogPackrat on September 21, 2005, 12:23:15 PM
Yeah, that's what I did.  You had that table of approximate wire lengths in the manual, too, so I just added an inch to that length and tacked them on before bolting the turret board in.  That chassis looks big until you get near the end and then it's hard to get a couple of fingers AND a soldering iron tip anywhere near some parts.  I think my wiring inside the chassis (the REAL point to point stuff) is a little messy.  I tried to keep grid wires away from, or at least perpendicular to, plate wires.  But then I ended up having longish loops of wire instead of short and neat wiring.  Oh well.
Title: All things LA2a related
Post by: CJ on September 21, 2005, 02:57:30 PM
Here is the voltage chart for no Gain Reduction and no T4 plugged in.
I used 500 hz instead of 1000 because 1 khz was too iritating to listen to.

I also did a resistance chart. Unplug the unit and T4 for the resistance chart comparrison with your LA2. Infinite ohms on the V1 plates and top V2 plate.

cj

http://vacuumbrain.com/The_Lab/TA/LA2/la2_resistance_1.jpg
Title: All things LA2a related
Post by: dissonantstring on September 29, 2005, 04:21:16 PM
hello.
i am building the bloo LA-2A and i'm on my final leg!!
powered up and no smoke and neon lights!
did the basic testing and wanted to check if these voltages looked o.k.
(without tubes of T4B):

AC Volts between A24 and A25 = 7.27VAC
B21 = 396VDC
B20 = 392VDC
A1 = 54.9VDC

both of the voltages on the B+ looked a little high so i thought i'd better
check first.  do these voltages look within safe levels to put in the tubes
and T4?
Thank you for your help and patience (in advance, in case something
blows up later and i need mucho help!)  
-grant
Title: All things LA2a related
Post by: SSLtech on September 29, 2005, 05:05:56 PM
All of them look a tiny bit (though not disastrously) high. One two-part question: Which power transformer do you have, and did you connect it for 110V or 120V?

My gut feeling is that you either have high AC mains on your area, or maybe you've got it rigged for 110V and you 're feeding it something closer to 120? (which would elevate your voltages all round a little... That 6.3V is usually about 6.8-7.0 unloaded... 7.2 suggests that it too is a little high.)

Keith
Title: All things LA2a related
Post by: dissonantstring on September 29, 2005, 05:42:10 PM
hi keith,
thanks for such a quick response and any help.
i have the power transformer which came with the kit (allied 6K88VG).
i didn't know this power tx could be wired to anything other than the 120V on the primary.
 :shock:
have i wired this incorrectly?  :sad:
-grant
Title: All things LA2a related
Post by: dissonantstring on September 29, 2005, 05:49:08 PM
oops!  forgot...
if the allied tx is wired correctly for 120V and i am getting 120V from my
AC lines (according to my furman AC rack power led read-out, however accurate that is :roll: ) is there something else i should be checking?
-grant
Title: All things LA2a related
Post by: Siegfried Meier on September 30, 2005, 12:33:39 PM
Does anyone have a simple diagram of how to connect the Sowter input and output transformers?  I've seen the schematics on Sowter's site, but they don't really help me much as I have a hard time reading these.  Does Cayacosta have a simple diagram for this?

Thanks.
Sig
Title: All things LA2a related
Post by: SSLtech on September 30, 2005, 12:51:03 PM
grant,

Like I mentioned, the coltages do not look all that worryingly high. -If you could select a slightly higher primary voltage, that would knock about 8% off them and put you right in the usual range, but you're still plenty within the acceptable range... And if the voltage sags a bit, you'll stay there longer than most people! :grin:

Sid,

As regards the Sowters, I wired one up with Scenaria once: He's out of town, so I can't sneek a quick peep, or I'd post a redux for you.

Also, I think that Ron (Cayocosta) has left the building, taking down his posters as he left, so there's no diagrams any more. -He also only did the UTC layout, no Sowters

Keith
Title: All things LA2a related
Post by: AnalogPackrat on September 30, 2005, 01:48:00 PM
Sig,

You can check my Bloo build using Sowters by following the link I posted in this message (about half way down the page):

http://www.groupdiy.com/index.php?topic=95160

I used a 3 position terminal strips mounted near each transformer to connect components which were wired across the transformer terminals in the original.

A P

p.s.  SSLTech--didn't realize Ron had also bugged out.  Bummer.
Title: All things LA2a related
Post by: Siegfried Meier on September 30, 2005, 02:48:50 PM
Okay.  Got the pics.  But I still don't understand where exactly the leads of the transformers are supposed to wire to.  The Bloo manual only refers to UTC's.  Any idea?

Thanks.
Sig
Title: All things LA2a related
Post by: Siegfried Meier on September 30, 2005, 05:11:35 PM
Got the data sheets for the Sowter's.

Thanks.
Sig
Title: All things LA2a related
Post by: Siegfried Meier on September 30, 2005, 07:30:09 PM
Data sheets aren't helping me much.  I need specifics on where to connect  all of these leads of the transformers.  Can't see a thing on the pics that anyone has either.

Sorry to be a pain.

Thanks,
Sig
Title: All things LA2a related
Post by: Scenaria on October 01, 2005, 10:16:23 AM
Sig,

sowter input

brown and green go from the input xlr

dont worry about the pink (just shrink wrap the end)

blue and grey goto the circuit

and black gets tied to chassis ground

for the output

yellow and green come from the circuit

grey and pink to the output XLR

black gets tied to chassis ground.

cant get much easier than that :)
Title: All things LA2a related
Post by: dissonantstring on October 01, 2005, 10:41:24 AM
hi keith.
thank you very much for calming my fears.  i'll do a couple of more
checks on voltages at the T4 module and 12AX7 sockets, as well
as the voltage drop across R33 per some of the other comments earlier
on in this thread.  if all looks well i'll drop in the tubes and check voltages
per the bloo manual and then hopefully do the T4 module checks and calibration.  
thanks for your assurance and wealth of knowledge keith!  :thumb:
cheers!
-grant
Title: All things LA2a related
Post by: Siegfried Meier on October 01, 2005, 10:43:57 AM
Making sense now.  I just found the schematic as well.  Thanks so much!

Sig
Title: 470pf across NE2?
Post by: yodermr on October 02, 2005, 12:06:24 PM
What is the significance of the 470pf cap across NE2? I built the LA2a off of the Jahnsen book. He did not list it but I noticed it on the original schematic and on Caycosta's layout diagram.

Additionally, I was wondering if the wires coming off of SW3 (compress/limit) should be twisted or shielded. I mounted the switch on the front panel.

I'm trying to track down some noise and meter problems ( currently researching how to build the VU rectifier) and came across these differences.
Title: Re: 470pf across NE2?
Post by: AnalogPackrat on October 02, 2005, 01:38:32 PM
Quote from: "yodermr"
What is the significance of the 470pf cap across NE2? I built the LA2a off of the Jahnsen book. He did not list it but I noticed it on the original schematic and on Caycosta's layout diagram.


It's probably just providing a path for HF crud to ground.  That neon tube is just providing a DC reference to offset the meter needle when it's switched to GR.

Quote
Additionally, I was wondering if the wires coming off of SW3 (compress/limit) should be twisted or shielded. I mounted the switch on the front panel.


Yes!  That's your input signal path and any noise/hum that gets injected is going to get amplified.

Analog Packrat
Title: All things LA2a related
Post by: AnalogPackrat on October 02, 2005, 03:00:33 PM
What happened to Paul B?  Vacation?  Hangover?
Title: All things LA2a related
Post by: yodermr on October 02, 2005, 03:53:17 PM
Thanks for the help Packrat

That didn't seem to help reduce the noise. I tried moving wires around with a chop stick but I could find anything that helped


When the output and G/R pots are at 1 o'clock this thing is pretty quite, move them away from that position and it gets quite noisy

Any thoughts on why that would be or where to look?

Guidance much appreciated :grin:
Title: All things LA2a related
Post by: Paul B on October 02, 2005, 10:16:33 PM
The saga continues....

Before I throw up my voltage and resistance next to CJ's (with the T4B removed, PR at 0) let me see if there's anything to say about the following:

C7a-d are axial, and I have all the [-] terminals on one side across the tag board nearest the power socket

C5 is a 22uf

In place of C8-1 and 2 I have a single .033uf

I've re-done the turret board and replaced most all of the components.  The basic problem still remains:  the box is blasting loud and distorted if I turn the gain past 10, and barely registering on the VU, although better than before.  It seems to be compressing just fine, just have to nail this amp problem...
Title: All things LA2a related
Post by: AnalogPackrat on October 02, 2005, 10:37:18 PM
Paul,

I assume C5 is an electrolytic with at least a 250V rating, right?  Is it wired with the positive end toward the tube and the neg toward the otuput transformer?  Everything else seems fine.  Did you replace the BH7?  Did you recheck the wiring on the BH7 socket?

A P

p.s. Is there any way you can post some pics with closeups so we can have a gander at your build?
Title: All things LA2a related
Post by: AnalogPackrat on October 02, 2005, 10:41:14 PM
Mark,

Can you bring us up to speed on what you're doing?  Do you have all the tubes and the T4B installed?  Have you checked DC voltages?  Does the thing pass audio?  Does it seem to compress?  When you say "noisy" do you mean hum/buzz or hiss, pop, spit, crackle, scratch?  Remember, no one here can see your unit or hear it, so you have to give gory details and measurements :wink:

A P
Title: All things LA2a related
Post by: Siegfried Meier on October 02, 2005, 11:48:00 PM
Ok.  I have the schematic and the Sowter Input Tranny document from Cayacosta and I'm good with wiring the input of the LA2A.  However, I'm not entirely sure where the yellow and green wires come from for the output, or where I'm supposed to put this 10uF cap across.  Any ideas?  Is there a similar pic like the input?

Much appreciated.
Siegfried
Title: All things LA2a related
Post by: SSLtech on October 03, 2005, 07:45:34 AM
Look at the schematic.

The 10µF cap goes IN SERIES with the primary of the output transformer.

There's really nothing to wirring the Sowters, a transformer's just two coils of wire when you think about it. The Schematic reflects this aspect so beautifully well that you really can work it out quite easily...

Just do it! -The Cap is there to block DC from the primary. That's it!

Keith
Title: All things LA2a related
Post by: Siegfried Meier on October 03, 2005, 10:11:13 AM
Haha.  Thanks.  I'm not so hot with reading schematics, although you're right.  There's really nothing there and it's quite easy.

Thanks again!
Sig
Title: All things LA2a related
Post by: yodermr on October 03, 2005, 05:46:39 PM
Sorry, I was posting in the wrong place before

Tubes and the TB4 installed. DC voltages...

C7B+ 257
12AX7 #1
1 - 110
3- 1.1
6 - 108
8- 1.11

12BH7
1- 117
3- 3.3
6- 250
7- 81
8 - 121

12AX7 #2
1- 82
3- .79
6- 82
8- .79

6AQ5a
2- 5.8
5- 143
6- 106

It passes audio (sounds very nice) but noisey. Seems to be unity gain with the output at 10 o'clock (no compression). With Output and G/R at 1 o'clock its the least noisey. From there going counter clock wise  with the Output control its noise like an open line (like a guitar cable not connected to an amp) going clockwise its more brighter crackly noise, hard to describe brighter would be my best decription. Similiar change in noise character change with the G/R control going clock wise or counter clock wise from the 1 o'clock position.

I have some work to do on the meter, apparently I have a DC movement type and I need to build a rectifier (waiting on geranimun diodes) to get it to work with AC signal (I'm learning here) Anyway it does compress as I can see the range calm down on my AD converter down stream.   :oops:
Title: All things LA2a related
Post by: CJ on October 03, 2005, 05:48:33 PM
Paul, do you have a mic and a mic pre laying around?
If so, plug that in and see what happens.
It could be that your LA2 has been working as it should this whole time. I have seen that happen before. Maybe you are just over driving it,
cj
Title: All things LA2a related
Post by: cannikin on October 03, 2005, 10:12:26 PM
Hey Scenaria, CJ or anyone

Where have you guys purchased terminal posts? the ones that you can attach directly to chassis? Are they still manufactured?

thanks :thumb:
Title: All things LA2a related
Post by: Paul B on October 03, 2005, 10:19:33 PM
David and everyone else,

theres a lot of cool stuff including tag boards at

www.turretboards.com

Paul
Title: All things LA2a related
Post by: Paul B on October 03, 2005, 10:22:01 PM
Quote from: "AnalogPackrat"
Paul,

I assume C5 is an electrolytic with at least a 250V rating, right?  Is it wired with the positive end toward the tube and the neg toward the otuput transformer?  Everything else seems fine.  Did you replace the BH7?  Did you recheck the wiring on the BH7 socket?

A P

p.s. Is there any way you can post some pics with closeups so we can have a gander at your build?


C5 is a 450V electrolytic w/ the pos end toward the tube.  Replaced BH7 and re-checked the wiring there (and everywhere else)

Ill try to get some pics up in a few days

Paul
Title: All things LA2a related
Post by: Paul B on October 03, 2005, 10:28:34 PM
Quote from: "CJ"
Paul, do you have a mic and a mic pre laying around?
If so, plug that in and see what happens.
It could be that your LA2 has been working as it should this whole time. I have seen that happen before. Maybe you are just over driving it,
cj


Hi CJ,

I will give it a try. I have been feeding a signal straight in from a CD player per your suggestion, and the ol 440 and 1k sine wave.  The hum is gone now, but if I feed a weaker signal into the box I wont get the t4b to fire up and compress, and the VU meter wont register a signal.

gonna finish that chart now...
Title: All things LA2a related
Post by: CJ on October 03, 2005, 10:33:26 PM
Cool. I think I slipped a decimal point on the upper cathode of the bh7, IF YOU GET SOMETHING DIFFERENT, IT MIGHT BE OK.
CAPS OFF.
 :razz:
Title: All things LA2a related
Post by: Paul B on October 03, 2005, 11:17:55 PM
OK, here's the chart.

R9 and 13 are 220k again.

I also am running the 4.7k 10k and 470 for R's 29, 34 and 36.

I put back C13 and R38

V1 is a 12AY7

http://photobucket.com/albums/a5/PaulB_/?sc=3
Title: All things LA2a related
Post by: CJ on October 04, 2005, 12:04:37 AM
Is your gain pot an audio taper?
Put it at half way on and measure the resistance from wiper to both ends.
Title: All things LA2a related
Post by: SSLtech on October 04, 2005, 10:10:02 AM
Quote from: "CJ"
Is your gain pot an audio taper?
Put it at half way on and measure the resistance from wiper to both ends.

...but ONLY after disconnecting it. -The parallel pots and the 1k resistor and the input TX secondary etc all make readings useless otherwise.

Keith
Title: All things LA2a related
Post by: dissonantstring on October 04, 2005, 10:27:53 AM
hello.
a couple of days ago i did my first check of voltages after completing
my wiring on my bloo and some seemed a little high, but keith said it wasn't too high, so i put in the tubes.  my voltages still look high and now i'm concerned with the some of the voltages.  can someone tell me if these voltages are o.k.   something looks wrong...

A24 and A25= 6.83VAC
B21 = 368 VDC
B20 = 286.3VDC
A1 = 55.5VDC
A20 = 83.2VDC  
A14 = 232VDC
A17 = 106VDC
A19 = 123.8VDC
pin 5 on T4 socket = 33VDC (too high?)

please help!   :sad:
thanks,
-grant
Title: All things LA2a related
Post by: CJ on October 04, 2005, 01:13:36 PM
tubes circuits can have a 20% tolerance on voltages and not be affected.
tweak that 4.7k pwr resistor between the two filter caps if you have a thorn in you Levis about it.
otherwise, wait for Emron to get back in business. :grin:
Title: All things LA2a related
Post by: dissonantstring on October 04, 2005, 01:50:44 PM
coolio!!
thanks CJ!  your knowledge is greatly appreciated and much respected (so is your sarcasm )  :razz:  
this is my first tube project so all the high voltages scare the be-jezus out of me.  i know i'm being paranoid and you'll give me a lashing for this, but i read earlier on in this post that pin 5 of the T4 socket should be around 8.5VDC, but mine is 33.2VDC!  :shock:
i haven't put in my T4 module yet, though i also read it shouldn't fry the T4 since the ELs can handle more voltage than that.
Should I put in the T4? :?
(i keep hearing a chant in the back of my head saying, "do it! do it!")
-grant
Title: All things LA2a related
Post by: Scenaria on October 04, 2005, 02:13:04 PM
heck...

you can shove 120V AC into the EL panels... and they will light up just fine   :green:
Title: All things LA2a related
Post by: CJ on October 04, 2005, 02:51:37 PM
Do any surfing in Beumont?  :razz:
Title: All things LA2a related
Post by: dissonantstring on October 04, 2005, 06:28:16 PM
thanks a bunch CJ and Steve!
i will be doing some tests with the T4 module in.
i can't wait!!   :green:
-grant
Title: All things LA2a related
Post by: kruz on October 05, 2005, 06:47:27 AM
I'm finishing to compile my shopping list for the bloo, and i'm searching around for a pot like the ones used by UA for R4, R3 and R37 (1Mohm LIN) with that short shaft, slotted to be adjusted with a screwdriver.
I can't find any..
UA seems to use PEC K/RV4 ones, but the shaft seems to be a special order.
Orginial LA2's seemto use AB ones..
I've already browsed a lot of surplus places with no luck.
Do you pals have any source for these?
I know i can cut a longer shaft and do the slot by myself, but it will not stand the professional bloo quality :)
Title: hi there...
Post by: keithcamilleri on October 05, 2005, 06:52:48 AM
hi there,

regarding the 510pf newar the 6aq5a does that cap has to be of a type like mica or something i cannot find any thing on the cap....

regards

keith
Title: All things LA2a related
Post by: Scenaria on October 05, 2005, 09:13:06 AM
kruz...

I have a number for that pot... I'll get it for ya just as soon as I can find it.

keith...

your thinking way too precise...

anything from 400pf to 600pf range will work just fine... mica is will be ok. I think a ceramic would be ok too..
Title: All things LA2a related
Post by: dissonantstring on October 05, 2005, 11:31:44 AM
hi kruz,
i got this PEC 1 Meg LIN pot from digikey for my bloo kit.
i believe this is the one you are looking for:
the digikey item number is RV4L105C-ND.
here is the catalog page with some specs.
[/url]http://rocky.digikey.com/scripts/ProductInfo.dll?Site=US&V=98&M=RV4LAYSA105A[url]
it has a short shaft that only sticks out a little from the front panel when installed.
hope this helps.
-grant
Title: All things LA2a related
Post by: Siegfried Meier on October 06, 2005, 10:34:31 AM
Just about to begin wiring up the main turret board.  I've got a roll of insulated Buss Bar wire that should work fine, but it's all the same color (white).  Any idea what guage and for what voltage I should be buying wire?  Thanks!

Siegfried
Title: All things LA2a related
Post by: Scenaria on October 06, 2005, 10:45:54 AM
24 gauge is fine

believe me its nothing special
Title: All things LA2a related
Post by: SiliconAudioLabs on October 08, 2005, 03:57:49 PM
OK,

Last question:

So both XLR's shielded grounds (XLR input AND output) go to terminal 7 GND?
Title: All things LA2a related
Post by: dissonantstring on October 08, 2005, 05:17:13 PM
hi.
i need help! :mad:
my voltages check out within tolerance as CJ suggested (within 20%) with the tubes in so i put in the T4 and my Bloo is passing audio, plenty of gain, peak reduction makes the volume go down with no significant compression that i can tell by just listening.
on top of that my meter doesn't even move (stays down to the left).   just for kicks i adjusted the zero adjust (eventhough my meter doesn't do anything) and noticed that my NE2 does dim and brighten with the zero adjust pot.  i checked the T4 with the cover off and the EL panel doesn't light to input signal.  now that i think about it, doesn't  peak reduction have to be up in order for the EL panel to start lighting up?  :oops:
anyway, i've checked and double checked my rotary switch wiring and all seems correct (i hope).  
so to sum up:
1) meter does not move in any of the meter settings (GR, +10, +4)
2) no audible compression.
3) T4 EL panels not lighting up (but need to check with the PR up) i guess
that's why there's no compression.
can someone point me into a direction to remedy my bloos!
i know i have to do some tests on the T4 as Keith and Analog Packrat suggested previously on this thread, but i was wondering if there is anything else i should possibly look at?
or maybe i'll just use my Bloo as a nice "audio tube warmer-upper" thingy.
 :?  any help would be greatly appreciated.
-grant
Title: All things LA2a related
Post by: Scenaria on October 08, 2005, 05:34:21 PM
well it sounds like you might have more than one problem... if in fact your EL is not lighting at all..

the first thing I would do

pull the T4b module.. this is only needed for compression but the unit will pass audio and add gain without it.

send +4dbu into the input of your XLR jack with a 1Khz osc

switch your rotary switch to +4 position

make sure your peak reduction is tured fully counter clockwise

bring your Gain knob up and look at the meter

does the VU meter move at all?

do you hear the level getting louder as you bring the gain up?

if you hear the level getting louder but the meter does not reflect this then you have something amiss in your meter and rotary switch wiring

if the meter does show the gain as you bring up the gain pot then we can move on...
Title: All things LA2a related
Post by: dissonantstring on October 08, 2005, 08:17:08 PM
thank you steve!
i'm at work, so i'll have to check when i get off (but that's not til 1am). :mad:
-grant
Title: All things LA2a related
Post by: SiliconAudioLabs on October 08, 2005, 10:37:49 PM
Question about this thread. I'm sorry if I seemed to hijack with my previous question. :oops:

Should I post any general question about LA2 HERE in this thread or in the general area in The Lab?

Thanks in advance and sorry (if I got in the way).

 :wink:
Title: All things LA2a related
Post by: Scenaria on October 08, 2005, 10:47:16 PM
yes!

this thread is for all questions related to la2a's

I know its a long thread but I think it makes it easier when searching just one thread instead of the entire forum


I was thinking of maybe setting up two threads..

one for la2a troubleshooting and the other for basic technical questions..
Title: All things LA2a related
Post by: SiliconAudioLabs on October 08, 2005, 11:31:53 PM
Yeah I think two might work best:

1) General day to day operational, conection (gnd loops/hum/buzz), meter funk, "mines gray maybe I want a silver one..." blah...blah....

2) Then a more detailed techical one like this thread which has already blown up my head three times.  :!:
Title: All things LA2a related
Post by: Siegfried Meier on October 09, 2005, 12:43:52 AM
Quote from: "cannikin"
steve;

I'm hooking up the Allied Power Trannie and just want to confirm my connections:

Primary:

1 Black to Live on power connector (Wire to B24 then to B25)
1 Black to Neutral  

Secondary:

Center Tap 6.3v (green w/yellow stripe) to ground Bus/TAG 3
Center Tap 250v (red w/yellow stripe) to ground bus/TAG 3

2 - 6.3v (green) to A24 & A25 with heater Wire.
2 - 250v (red) to A22 & A23

Ground:

Grey to Gound on the Power Connector.

Thanks.
David


The 2 center tap connections - what do you mean by ground bus/TAG 3?  What is TAG 3?

Thanks,
Siegfried
Title: All things LA2a related
Post by: Scenaria on October 09, 2005, 10:32:44 AM
the tag strips are the two strips that you screw onto the back of the chassis and attatch the power supply caps.

tag 3 would be the 3rd solder lug on that tag strip
Title: All things LA2a related
Post by: Siegfried Meier on October 09, 2005, 12:02:11 PM
Of course, hence "ground bus".

Thanks.
Sig
Title: All things LA2a related
Post by: Siegfried Meier on October 09, 2005, 04:53:02 PM
1 more question.  Does it matter which black goes to which?  Which green goes to which?  Or which red goes to which?  One color has writing, the other doesn't, which is why I ask.  Probably not, correct?

Thanks,
Sig
Title: All things LA2a related
Post by: Scenaria on October 09, 2005, 05:52:07 PM
it doesnt matter...

if you look on the side of the box that the tranny came in you would see a color code :)

red and red are your HV taps
Green and Green are your heater taps
Black and Black should be tied to the 120V IEC power socket
the two with yellow stripes need to be tied to ground along with the gray wire.

-s
Title: All things LA2a related
Post by: Siegfried Meier on October 09, 2005, 06:13:44 PM
Thanks.  The help is much appreciated.  

And, once again, another question.

SW3 - does it matter what I make Pa and Pb, as long as it conforms to the diagram as far as direction goes?  In other words, does it matter how the switch is oriented into the front panel?

Thanks,
Sig
Title: All things LA2a related
Post by: Scenaria on October 09, 2005, 06:16:30 PM
nope

just arrange the knob accordingly
Title: All things LA2a related
Post by: dissonantstring on October 10, 2005, 01:58:27 PM
hi steve,
o.k., i'm retarded! :oops:   for the benefit of others i'll post this...
i left the wire which is connected to the + and - lugs on the beede vu meter and that is why i couldn't get my vu to zero out.  took it off and i can now zero out my meter in GR mode with the zero adjust pot.   do they put that wire in for stablizing the posts on the meter during shipping?

anyway,  when i did the test you suggested  (sending +4 dBU 1KHz sine wave into the unit with +4 selected on the meter switch) there was no reflection or movement on the meter.  it just sits to the left at -20.  i triple checked my rotary switch/ mini turret board/ front panel pot wiring and all looks as it should be though as i stated before i'm retarded so i could definitely be wrong.  i tried to adjust my peak reduction and the needle on the vu does not reflect compression, though the signal level does become audibly softer when i increase PR.   oh, i have an extra T4 i got from JBL and tried to stick that in to see if my bloo T4 was wired wrong, but got the same results.
i'm at a lost now.  any suggestions?
thanks.
-grant
Title: All things LA2a related
Post by: Scenaria on October 10, 2005, 02:04:25 PM
I have no doubt that something is wrong in the rotary switch wiring...

sounds like the compressor is working... the only place you can really be having a problem is in that area..

remember sometimes when we look things over its easy to assume that something is right after having looked at it a billion times.
Title: All things LA2a related
Post by: dissonantstring on October 10, 2005, 02:12:35 PM
thanks for the quick response steve!
yes, as i stated before, i am retarded so i'll look into it more carefully and
if worse comes to worse, i'll redo all the wiring in that section and maybe something will stick out or hopefully i'll do it correctly this time.
one question though, i grounded A5 on the mini turret to the rivet point on the tabs which mount onto the meter, is this correct?
thanks.
-grant
Title: All things LA2a related
Post by: Scenaria on October 10, 2005, 02:36:37 PM
yep!

thats correct
Title: All things LA2a related
Post by: dissonantstring on October 10, 2005, 02:47:56 PM
cool!
thanks steve!
-grant
Title: Thank You
Post by: yodermr on October 11, 2005, 11:25:38 PM
I just wanted to thank everyone for putting all this experience down in one spot. Took me a while but I have sucessfully built an LA2A. If you have read any of my posts this should surprise you. I know I could handle making it pretty but pulling off the rest was a challenge.

Few minor things left to do but it works great and sounds great.

Finally got it together so I thought I'd throw a few pics out there.

http://www.demofactory.biz/Front.JPG
http://www.demofactory.biz/Back.JPG
http://www.demofactory.biz/Inside.JPG

While Jahansen's book was a great start, I couldn't have completed it without this site.

  :grin:
Title: All things LA2a related
Post by: AnalogPackrat on October 11, 2005, 11:53:25 PM
Nice one, Mark!  Real man's point to point and a chunky style front panel.  Looks good.  Sowters?

 :thumb:  :guinness:
Analog Packrat
Title: All things LA2a related
Post by: CJ on October 12, 2005, 01:56:33 AM
Coolsville!

You might be able to straighten out the T4 by seeing if there is a tension clip inside the octal case. If there is, sometimes you can rotate the octal plug within the base to make it come out right. This way you do not have to re drill the tube socket mounting holes. And don't feel bad, every single DIY LA2 I have seen here has had CTS. Crooked T4 Syndrome.
I cheated. I used a round can.
Title: All things LA2a related
Post by: Siegfried Meier on October 12, 2005, 09:39:58 AM
The meter in the Bloo LA2A has a small clip across the - terminal and the + terminal.  In the manual, it does not state to remove this, but I've been advised by a very knowledgable person to do so.  Anyone?

Thanks,
Sig
Title: All things LA2a related
Post by: Scenaria on October 12, 2005, 09:44:23 AM
yes you need to remove it...

think about it for a sec.. if you keep the clip inplace.. this would short out the two leads... your meter would not work  :wink:
Title: All things LA2a related
Post by: yodermr on October 12, 2005, 09:53:33 AM
Thanks for the complement. I was stealing the look from DW Fearn. The knobs and dial plates were from an antique radio shop in NY. They got lots of really cool stuff.

I used Jensen transformers. This was the only option that I knew at the time I was sourcing parts.

The T4 was from ADL - the bracket for the housing seems to be glued to the pin socket. Feel bad? Hell I'm so excited about just pulling this off it makes me horny. Tell me that's not messed up :wink:
Title: All things LA2a related
Post by: dissonantstring on October 12, 2005, 01:44:43 PM
wow mark! :thumb:
that is a very cool looking LA-2A and that meter has got to be the hugest one ever.   where did you get the chassis?  it almost looks like a guitar amp chassis.  great work!

i feel like i'm coming in last in this race, but i'm still in it.  i'm not giving up damn it!  with that i need help again steve!
i re-wired my rotary switch which brought up more problems which i won't get into.  anyway i re-did the +4dBu 1kHz sine wave input (no T4) with +4 meter selection and noticed that when i turn the gain all the way up the needle moves up ever so slightly (from rest at 0 to a smidge over -20).  then when i increase PR the needle moves down towards 0.  so i have hope...right?  :grin:
please help.
-grant
Title: All things LA2a related
Post by: Scenaria on October 12, 2005, 01:53:01 PM
im a little confused....

you said you have no T4b in it?

but your seeing the needle deflection downward?
Title: All things LA2a related
Post by: dissonantstring on October 12, 2005, 02:18:21 PM
Quote from: "Scenaria"
well it sounds like you might have more than one problem... if in fact your EL is not lighting at all..

the first thing I would do

pull the T4b module..
this is only needed for compression but the unit will pass audio and add gain without it.

send +4dbu into the input of your XLR jack with a 1Khz osc

switch your rotary switch to +4 position

make sure your peak reduction is tured fully counter clockwise

bring your Gain knob up and look at the meter

does the VU meter move at all?

do you hear the level getting louder as you bring the gain up?

if you hear the level getting louder but the meter does not reflect this then you have something amiss in your meter and rotary switch wiring

if the meter does show the gain as you bring up the gain pot then we can move on...


i actually did this test with both the T4 out and T4 in with the same results.
the gain does increase volume and PR does decrease it.
since it does the same thing with and without the T4 am i in worse shape?

 :shock:
-grant
Title: All things LA2a related
Post by: Siegfried Meier on October 12, 2005, 02:30:35 PM
Got one working.  Careful with the wiring description of the Power Transformer - mine was wired permanently on because of this.  Also, watch the grounds on the last few steps of the twin shielded wiring - this was making the Gain Reduction not happen.  Things get really messy near the end.  All in all, it seems okay on the bench.  Next test is the studio.  Hopefully it's okay.  BTW, all my voltages were 99% accurate to what was posted in the manual.

Working on seeing if the second one makes it now.  Will keep you posted.

Ignore the meter thing I said before - it obviously needs to be removed, even though not mentioned in the manual.

One last question - what does C4 do and how what do I adjust this to??  

Sig
Title: All things LA2a related
Post by: CJ on October 12, 2005, 02:59:40 PM
What happened to Paul B? We either fixed him, or he jumped off a bridge somewhere.
 :?:
Title: All things LA2a related
Post by: dissonantstring on October 12, 2005, 04:15:51 PM
Quote from: "Scenaria"
im a little confused....

you said you have no T4b in it?

but your seeing the needle deflection downward?

yeah, what the hell?  if my T4 isn't in how can my unit be compressing?
 :? is my rotary switch wiring still messed up?  what could be causing
this?
-grant
Title: All things LA2a related
Post by: Siegfried Meier on October 12, 2005, 05:33:10 PM
And the second one is working now too.  Again, a few shorts in the tube sockets - I find that popping in a tube at a time, firing it ensures that each tube socket is properly wired.  This time, no ground problems and the unit appears to test out great.  Voltages are a little different from the last one, but still in the ballpark.  

Thanks to everyone that helped out.  I'm sure I'll have more questions as time goes on.

Siegfried
Title: All things LA2a related
Post by: yodermr on October 12, 2005, 05:47:17 PM
Grant,

The chassis was from par metal. My next door neighbor is an Mold Maker. He has some machine (EDM) that uses a wire and some plasma arch for cutting metal. It was quick, easy and left perfect cut outs.

I searched for sometime on Ebay for a meter that would stand out. Nice focal point I thought :thumb:
Title: All things LA2a related
Post by: dissonantstring on October 12, 2005, 05:57:21 PM
Quote from: "yodermr"


I searched for sometime on Ebay for a meter that would stand out. Nice focal point I thought :thumb:


focal point?!  i'd say.  your unit is very unique looking (even if it is colored red like D*W*Frn).  IMO it says, "here i am, use me!"  great job mark!

hey steve,
sorry for being a nag, but could you point me an a direction here.. my LA wants to make sweet, sweet music. :wink:
-grant
Title: All things LA2a related
Post by: yodermr on October 12, 2005, 06:34:12 PM
Half the fun is making it look like a very expensive unit and knowing you can do it fairly cheap. Then I can sell the cost of doing it as an investment to my wife.

Here is another example I have. (excuse the braggin for a minute) Just some stainless steal, oak and new knobs, oh yea and my next door neighbor's EDM machine. Looks expensive but it wasn't.
http://www.demofactory.biz/Wedge.JPG
Title: All things LA2a related
Post by: Siegfried Meier on October 12, 2005, 07:31:11 PM
Couple of questions.  Testing the units out and they both sound very good.  However, 1 is much louder on the output, even though all the front panel pots are equal (including output gain and gain reduction and meter cal).  Does the C4 adjustment have anything to do with this?  What exactly does C4 do?  Are there any other adjustments besides the 2 calibrations mentioned in the manual?

Thanks.
Sig
Title: All things LA2a related
Post by: Paul B on October 12, 2005, 08:53:47 PM
Quote from: "CJ"
What happened to Paul B? We either fixed him, or he jumped off a bridge somewhere.
 :?:


Yeah, you fixed me, for the most part  :grin:  :grin:  :grin:

Now comes the hard to admit part.  The crux of it is right here:

Quote from: "CJ"
Paul, do you have a mic and a mic pre laying around?
If so, plug that in and see what happens.
It could be that your LA2 has been working as it should this whole time. I have seen that happen before. Maybe you are just over driving it,
cj


 :oops:  :oops:  :oops:  :oops:  :oops:

Ive been feeding the wrong kind of signal into the box.  I have been taking the CD output signal from the monitor/speaker outputs.  I had in fact run tests with a preamp but it was with the same signal.   I used a mic into the preamp and then into the box.  The signal has to be really hot  for the box to compress to the levels I've seen posted by others.  The signal coming out is still very hot but it is usable.  The gain is working much smoother.  The unit  is VERY quiet.

So I would bet my box has been working for the last 6 weeks or so.  :oops:  :oops:

Now I could use a cooler input level, and just crank up the PR (with lower input levels I need to turn PR to 70-80 to get about 10dB reduction) because i probably wont be needing 20-30 dB PR, to tell you the truth!  I've not messed with it much so Im heading out to go twiddle knobs and flip switches here in a minute.

CJ, do you think it would help to put the 100k's back in for R9 and R13?  Im still running the AY7.
Title: All things LA2a related
Post by: CJ on October 12, 2005, 09:22:04 PM
If you clip a bit, the 100K's will make the clipping more mellow. You will not notice much benifit form the 100K's until clipping, but they do reduce gain a bit, and make the distortion more symetrical.

Don't  stress the overdriving. Others will benifit, including you and me, from the troubleshooting extravaganza we did.
Title: All things LA2a related
Post by: SSLtech on October 12, 2005, 09:34:29 PM
C4... that's the trimmer cap -right?

If so, sweep a sine wave until you find the rolloff at the top end (about 18kHz... look at the output with the peak reduction turned all the way off, (...and the T4 removed if you want to make double sure!)

Just before the response takes a dive at about 20kHz, there's probably a peak, that's where the trimmer lets you set it for flattest response, or add a touch of brightness just before the dive, if you like.

Keith
Title: All things LA2a related
Post by: AnalogPackrat on October 12, 2005, 09:57:59 PM
Great news, Paul!  Don't sweat the running around in circles debugging stuff.  Every time I look at the circuit I learn something, even if it's small.  And all the discussions are good mental exercise.  I think I'm going to dig out a 12AY7 from my stash and give it a go, too.

Anywho, belated congrats on getting it working  :grin:  Now post some pics of your wiring for everybody to see!

A P
Title: All things LA2a related
Post by: on October 12, 2005, 11:25:10 PM
hi there ,
 
  built an la-2 all the voltages check out
  except 12bh7a pin 6
  mine is showing 230v

  reissue is 222/schematic 220/ la-2 195 /la-2a 200

  you think the 10v difference is ok ?

 all the othervoltages check out , but im hung up on this one.

   :?:

thanks for any input.
Title: All things LA2a related
Post by: AnalogPackrat on October 12, 2005, 11:48:42 PM
Driller,

If your other voltages are ok you seem good to go.  10V on 230V is like 5%.  No biggie.  Is this a Bloo kit or from scratch?  Are the tubes in yet?  T4B installed?

A P
Title: All things LA2a related
Post by: on October 13, 2005, 12:09:13 PM
ah thanks ,  i was worried and reviewed my wiring a million times ,
seems like every thing is correct.

its a home build point to point.
with all tubes and t4b plugged in.
 
i was looking at a reissue , noticing they have added 1k resistors
here and there.
on the 12bh7a pin 2 they added a 1k .

 my voltages are more or less matching that of the reissue
 except that pin 1  being 10v higher
 
ill post all the reissue voltages latter today just for refrence for others.
Title: All things LA2a related
Post by: Siegfried Meier on October 13, 2005, 05:17:58 PM
Had the 2 of my Bloo LA2A's running for 6 hours last night and they worked great.  Fired them up today and one of them blew a fuse immediately.  Then, after 5 minutes the fuse blew in the second one.  Any idea what the deal is?

The fuses I'm using are F-GMA-00.500
GMA-500 MA

Should I be using something else?  Am I under the correct assumption that if they worked for this long and haven't been touched or moved that nothing else could be wrong with them?

Thanks,
Sig
Title: All things LA2a related
Post by: dissonantstring on October 14, 2005, 11:07:09 AM
i just wanted to give a big thank you to steve for pointing me in the right direction.  :thumb:   :guinness:  :guinness:
i redid my rotary switch wiring and also relocated my C5 cap (meter turret squishing it) and also redid my wiring from R2 to A15.  i now have beautiful compression which is reflected in my VU meter when i adjust my PR.  awesome sounding!  still have no meter movement at +4 or +10?   i'm convinced, as steve has stated, that it's in my rotary switch so i'll have to re-re-re-check against the schematics, instructions and cayocosta's layout.  still more to do.
one really puzzling question for anybody who has done the bloo kit, did you link turrets A11 to A14?  i saw it in the cayocosta layout and verified it in the schematic in the bloo manual, but when i checked pg. 14 in the manual it doesn't list it as one of the main turret board links?  can anyone verify this link for me?  my unit sounds pretty awesome without it so i'm wondering if it was removed from the original schematics for some reason.
thanks again to steve for his generous help and his awesome bloo kit!
thanks to Keith and CJ for their sharing their wealth of knowledge and calming my fears.
cheers,
grant
Title: All things LA2a related
Post by: SSLtech on October 14, 2005, 11:19:41 AM
Yeah A11 andA 14 were linked in the LA-2a. The unit works without the link or with. The output drive clipping level is changed, IIRC. For fully-authentic LA2a behaviour, link A11 and A14. For 'warm-n-dirty' without having to blow the front end out of the following device, try it without.

Keith
Title: All things LA2a related
Post by: dissonantstring on October 14, 2005, 11:42:15 AM
thanks for the info keith!
i was a bit surprised when i found that i hadn't linked A11 to A14.
i think i'll leave it out for now as it sounds great without it, but maybe
do the link in the future if i find i want a little more grit.
cheers!
-grant
Title: All things LA2a related
Post by: Bluzzi on October 14, 2005, 03:37:26 PM
Quote from: "SSLtech"
Yeah A11 andA 14 were linked in the LA-2a. The unit works without the link or with. The output drive clipping level is changed, IIRC. For fully-authentic LA2a behaviour, link A11 and A14. For 'warm-n-dirty' without having to blow the front end out of the following device, try it without.

Keith


Thats great info Keith! Thanks.

Do you think it would be OK then to install a switch to be able to have both choices? Or would that cause a huge spike in the output when switching?

Jim
Title: All things LA2a related
Post by: Siegfried Meier on October 30, 2005, 10:07:01 PM
I can't ever seem to get the Search Topic function to work.  Looking for info on the Grid Stoppers for the 12BH7A - I'm using Sowters.  What pins exactly do I place the 1k in series with?  Thanks so much.

Siegfried
Title: All things LA2a related
Post by: AnalogPackrat on October 30, 2005, 10:17:55 PM
Siegfried,

Pins 2 and 7 are the grids of the 12BH7 triodes.  You know how to do this or do you need some pointers?

A P
Title: All things LA2a related
Post by: Siegfried Meier on October 30, 2005, 10:29:14 PM
Thanks.  I believe I need a 1k resistor wired in series to each pin.  So, lift the wires going to pin 2 and 7, solder the 1k to the pin, and solder the wire to the other side of the pin.  Correct?
Title: All things LA2a related
Post by: SSLtech on October 30, 2005, 10:53:43 PM
Quote from: "Bluzzi"
Do you think it would be OK then to install a switch to be able to have both choices? Or would that cause a huge spike in the output when switching?

Actually I'm not sure... -there's only one way to find out though, and it can't hurt to try it out!

-First person to wire a switch and test it wind a bag of jelly-babies!

Keith
Title: All things LA2a related
Post by: AnalogPackrat on October 30, 2005, 11:24:13 PM
Quote from: "Siegfried Meier"
Thanks.  I believe I need a 1k resistor wired in series to each pin.  So, lift the wires going to pin 2 and 7, solder the 1k to the pin, and solder the wire to the other side of the pin.  Correct?


That's correct.  Go for it.
Title: All things LA2a related
Post by: Siegfried Meier on October 31, 2005, 12:15:53 AM
Ok.  Grid stoppers are in, but unfortunately not stopping anything.  1 unit is fairly quiet, with a pretty high buzz (almost sounds like a dimmer switch with flourescent lights through a guitar pickup kind of noise) but the other unit has a really loud hum - reminds me of when a power supply fails and has that weird kind of hum.  It kind of fades when I wiggle the output XLR around.  Could there be something not making proper contact there?

Man, it's really tough sorting out these noise problems.  I think I may need to hand these over to a qualified tech.  It's just too difficult to sort out on my own - and not enough time.

Thanks for any help.
Siegfried
Title: transformer
Post by: keithcamilleri on October 31, 2005, 04:52:59 PM
hey guys i just finished wiring the bloo one...one problem it seems that i don't have at lease a sign of any voltages coming out of the hammond transformer... i am using the 369jx... can someone guide me out on the wiring for this transformer i feel i've missed something...

my wiring is

greens = a24 and a25 = 6.3v
reds = a22 and a23 = high voltage
purple = idle

black brown = linked
blue going to the power inlet
white = b25

is this right?

thanks loads
Title: LA2A problems
Post by: Dino on November 19, 2005, 09:53:21 PM
Dear all,

A couple of days ago I finished building my LA2A and (of course) I've encountered some problems.  Here's a list of things I found:

- audio output is way too high - I'm feeding it into my mixer and as soon as the gain pot is higher than about 20% I get a clipping indicator in the mixer. Is that normal?

- meter seems to work in "output+4" or "+10" mode.

- it does compress when I turn the gain-reduction pot clockwise, maybe up to 10 dB.

- I can't get the meter to 0 dB in meter-compress-mode:
 the highest I can get it is to - 3dB with R4 set to 0. However, then the neon lamp is off... is that normal??? Furthermore, the neon is only glowing for R4 between 30% and 70%.... is that supposed to be like that? I've tried the "matching resistor" protocol in Jahnsen's appendix (replaced R25 with a 100K pot etc), but it doesn't help....

- I've used Jensen transformers (JT-11P-1 input) and JT-10K61-1M (output) with the corresponding changes in the schematics (as indicated by the Jensen datasheet - basically: R5=11K, R17=680 Ohm (!!), R18 and R20 are both 330 Ohm, C5=22u).

- For all other component values I sticked to the ones in the original (?) United Industries LA2A schematics (1968)... I'm beginning to wonder whether that was a mistake? Should I have used D. Jahnsen's values instead?

Some of the voltages just don't seem to be correct, but I can't find what's wrong... here's some of my measurements:

point measured: without tubes ; with tubes (should be)

- main voltage (at R29): 370V; 242V (275V)
- pin6 (6AQ5A): 146V; 48 V   (100-137 V)
- pin2 (6AQ5A):  n.d.; 1.9 V   (7-7.5 V)
- pin5 (6AQ5A):  146V; 149 V   (114-138 V)
- pins 1,6 (V3): n.d.; 80 V   (107 V)


I'd be extremely grateful for any help!

Best,
Dino
Title: All things LA2a related
Post by: lanxe on November 24, 2005, 01:09:22 PM
man,
i just finished my bloo kit..............BUT of course there are problems.

The voltages seem to be close, the neon glows as well as the tubes

it seems to compress fine, but i am getting very little gain out of it. even with the bloo turned all the way up, compression all the way down, i get very little signal (signal is about as loud as the noise!).

i have gone through the wiring already....but i am going to go back over it. any pointers of where to start?

Thanks,
Ryan
Title: All things LA2a related
Post by: Dino on November 25, 2005, 07:20:01 PM
I'm still having trouble figuring out what's wrong with my LA2A.

Is it normal that the neon goes off when changing the setting of the R4 pot? I'm unable to get the meter to 0dB in meter compress mode.

Any ideas?
Many thanks in advance

Dino
Title: All things LA2a related
Post by: audioforge on November 26, 2005, 06:26:45 AM
dino
i ve got the same issue in my diy la2a (not a bloo one). what is your vu meter? it appears that mine had not a rectifier inside so i can adjust the 0db in gr mode but neon fails light for this setting and the meter do not work in +4 and +10...
i ve had 4 diode (1n4148) and everythings now ok and the neon shine again within the same setting.
hope it help
Title: All things LA2a related
Post by: Dino on November 26, 2005, 01:48:35 PM
Hey Audioforge,
Thanks for the hints. I'm using a Shurite 7801Z vu-meter. And the thing does work in both +4 and +10 mode. It's only the damn "meter compress mode" in which I'm unable to get it to 0dB.
How did you connect the 4 diodes?

Best,
Dino
Title: All things LA2a related
Post by: audioforge on November 26, 2005, 05:08:23 PM
dino
unfortunately i have nothing to post a drawing... but i will try to help.
download this schematic even it s not the la2a...
http://www.montagar.com/~patj/am-864u-sch.gif
at the upper right corner you ll find the vu diodes rectifier. connect the + and - to your + and - of your vu. the 2 other leads will go:one to R24(3,9k) ,the other to sw2 .
hope it help.....
Title: All things LA2a related
Post by: on November 26, 2005, 05:59:17 PM
on the shurite meter , i had no luck and was not able to get the meter to
zero .

 i was using the shurite 8801z rectifier type.

 it would work on audio in , but when i finally bought a siafam meter
 it worked perfectly.

 i dunno , but i think shurite might be lame. :roll:

 i thiink i was expirienceing the same problem
 so before you rip your wiring apart you might try another
 vu meter.

 hope this helps
Title: All things LA2a related
Post by: audioforge on November 26, 2005, 06:39:44 PM
oops  i forget to read :"And the thing does work in both +4 and +10 mode. It's only the damn "meter compress mode" in which I'm unable to get it to 0dB. "
my " rip off vu meter did not have rectifier and after add one everythings was fine.... but if driller says that the shurite allready have a rectifier inside (and it have one if it works on +4 +10) adding diodes rect. will not help you. maybe the sensitivity is too hight (much more that 100ua) try adjust R24 (put a pot and tweak till you ll can adjust your 0db in gr mode without the neon fails lighting or as driller suggest, buy a 100ua vu as the sifam)
good luck
Title: All things LA2a related
Post by: Dino on November 26, 2005, 08:33:04 PM
Thanks so much for your help fellas!
Driller, I got the same impression: the shurite is lame/weird. I'll buy a Sifam and give it a try.  Thanks again!!

Dino
Title: voltages
Post by: keithcamilleri on November 29, 2005, 04:35:47 AM
my volages are a bit high is there anyone that could point me in the right directon...

a20 is way too high its 240v
b20 is around 300v
and pin 5 on the t4 is aroun 52v

definetely something wrong...all other voltages seem right or within limits i think...

these voltages are wit tubes in and no t4

regards
Title: All things LA2a related
Post by: kruz on November 29, 2005, 05:01:32 PM
ok dudes.. i would like to introduce you my just finished bloo series two

(http://www.beatbazar.com/personal/diy/bloo/bloo-01.jpg)

NOW, the problems:

1- There is something wrong around the meter.. in GR mode the meter tilt to full scale and stays there even if compression is applied (yes, seems that the compression is working). In +4 mode it reads more then in +10 mode..

2- I scoped the output, and the phases haven't got equal amplitude

3- When i open the front panel, the difference between the two phases flips :cool: (hopefully this is a simple solving problem, maybe something is shorting when closing the door)
Title: All things LA2a related
Post by: kruz on November 29, 2005, 05:15:37 PM
ah, few things i'd need to add..

I did the basic voltages tests described in teh manual and they are all in the ballpark..
I'm using utc transformers
Title: All things LA2a related
Post by: kruz on November 30, 2005, 02:59:34 PM
I did my homeworks today and found the solutions to all these problems.
Now the unit is 100% perfect, dead silent too.

I had miswired the wiper of R2 to A16 instead of A15
Also, the meter wasn't working cause i found that i missed to bridge 1b and 3b on SW3

but beside this, i found that the problems i had on the output phases was caused by a not correct wirings of both input and output transformes to their XLR
Bloo's wiring guide is not correct here. TAKE NOTE!
At page 10 where it talks about the ground connections, the ground path DO NOT have to be connected to the pins 1 of both xlr but JUST to the ground tags.. pins 1 of both xlr NEEDS to be tied to pins 3 (or 4, shorted) of each transformer (xlr in pin1 to pin3 of the input transformer, out xlr pin1 to pin3 of the out transformer) as in the original schematics, transformers needs that!!

I still have to test it in the studio, checked just with a cd and it's perfect but i can't yet judge its sound. I'll let you know.
I'm really proud of MY diy :green:

Now.. i want to truely thank Scenaria for his amazing kit anbd to all the peeps here that keeps helping people like me :wink:
Title: All things LA2a related
Post by: kruz on November 30, 2005, 04:15:55 PM
this is how the + and - on the output looked like before the correction on both the in e out transformers and xlr's wirings
(http://www.beatbazar.com/personal/diy/bloo/bad-out.jpg)
notice that the two waves weren't fully out of phase and that the top one is much lower then the other one
Title: All things LA2a related
Post by: CJ on December 01, 2005, 12:30:10 AM
Good deal, Kruz Mon!
Title: All things LA2a related
Post by: kruz on December 01, 2005, 03:11:00 PM
today i listened to it quite a bit..
and.. what can i say?..
it's imrpessive.. i really love this kind of compression on vocal tracks
but also noticed that i had a serious roll off on the HF..
so i sweeped the out and i confirmed my ears
(http://www.beatbazar.com/personal/diy/bloo/bloo-spectrum.png)
Now, i set C4 fully CW, this is supposed to be the position with less roll off, am i right?
I don't know what to think now.. but i'll take a look at it tomorrow again
Title: All things LA2a related
Post by: Bluzzi on December 01, 2005, 08:42:33 PM
Maybe that's why you like it!

It would be interesting to see if others are getting wider frequency responses from their LA2A.

Jim
Title: All things LA2a related
Post by: CJ on December 01, 2005, 09:08:22 PM
Clockwise tightens up the plates, shunts more hf feedback to ground= less roll off.

That looks like an A-24 curve to me.

Which vintage?

Black, drk gry, lt gry or TRW blu/grn?
Title: All things LA2a related
Post by: kruz on December 02, 2005, 03:47:30 AM
Quote from: "CJ"
Clockwise tightens up the plates, shunts more hf feedback to ground= less roll off.


done

Quote
That looks like an A-24 curve to me.


are you sure? have you seen the starting freq of my roll off? it seems way to much imho

Quote
Which vintage?


Light grey A24
Title: All things LA2a related
Post by: kruz on December 02, 2005, 12:19:22 PM
not much time today to find the solution.. but i did another freq test comparing both C4 fully CW and fully CCW, i can't see any dramatic difference, is this normal??
(http://www.beatbazar.com/personal/diy/bloo/bloo-test.png)

Also, i've checked the freq rensponse of the A24 as noted in an UTC catalogue, it doesn't look like my bloo output imho
(http://www.beatbazar.com/personal/diy/bloo/a24.gif)
Title: All things LA2a related
Post by: AnalogPackrat on December 02, 2005, 12:38:21 PM
Something definitely looks wrong.  What cap did you use for output (C5)?  Can you do another sweep and try to look at the node between C5 and the output iron (and then work your way backwards until you find out where the HF loss happens)?

A P
Title: All things LA2a related
Post by: kruz on December 02, 2005, 12:46:06 PM
Quote from: "AnalogPackrat"
Something definitely looks wrong

 :cry:
Quote
What cap did you use for output (C5)?

Sprague Atom 10uF 600V (TVA1963)
Quote
Can you do another sweep and try to look at the node between C5 and the output iron (and then work your way backwards until you find out where the HF loss happens)?

Will try to do.. but probably on monday

Thank you!
Title: All things LA2a related
Post by: on December 02, 2005, 09:57:44 PM
heres the sweep i did of an la-2a reissue
a bit low on the volume .

hope this helps

(http://driprecords.com/pcb/test-la2-reissue-sweep101.jpg)
Title: All things LA2a related
Post by: CJ on December 03, 2005, 12:43:39 AM
Kruz, that cap curve looks about right.

You can put ther probe on the V1 output cap, grid side of V1b and sweep it.

Try it with the neg feedback disconnected also.

Lower your gain pot first.
Title: All things LA2a related
Post by: on December 03, 2005, 06:56:00 AM
the c4 vari cap seems to do magic on the frequency response

here are some curves of two la-2s

top curve is using arco 465(open aprox 3/4) (universial audio reissue factory setting)

bottom curve is using 390 pf cap


(http://driprecords.com/pcb/using-390pf-cap.jpg)
Title: All things LA2a related
Post by: on December 03, 2005, 07:03:05 AM
and both using arco 465 (3/4 aprox open(eye balled))

frq response snaps back into place  :shock:

(http://driprecords.com/pcb/using-arco.jpg)
Title: All things LA2a related
Post by: Scenaria on December 03, 2005, 08:03:01 AM
you need to mess with that trim cap

take it completely out and see if the curve goes back up..
Title: la2a voltages
Post by: pahstah on December 17, 2005, 01:37:27 PM
i just finished my la2a bloo ( for the meter i used sifal al29 but i'm waiting for the beade meter to arrive), adn i took some readigs of voltages, there's some difference with those recommended by scenaria:

with tubes on

b21  318v  (sugg. 360v)
b20  252v  ( 275v)
a1    48v    ( 60v)
a20  77v    (131v)
a14  206v   (216v)
a17  80v    (90v)
a19  109v   (105v)

can i put 4b in or should i try to change voltages?

thank you all

paolo
Title: All things LA2a related
Post by: CJ on December 17, 2005, 04:42:30 PM
Everything looks cool but A20. Might be the tube, might be a component value, might be the schematic, might be reading error, might be wiring error,

you don't need the T4 to listen to the line amp. Put some music threw there.
then, if it sounds fine, plug in the t4.
Title: All things LA2a related
Post by: pahstah on December 19, 2005, 07:03:43 AM
thank you cj, i did as you counselled and everything is okay: it passes music and it compress as an la2a should, i think; looking at different schematics i've found that r34 is 22k 2w in some and 10k 2w in others so maybe that's the reason for a difference in voltage at A20?
however the bloo kit is really really nice! i'm only waiting for the meter to arrive and then it will be completed!
THANKS!
Title: All things LA2a related
Post by: CJ on December 19, 2005, 03:25:58 PM
Congrats!

Rock over London.
Rock on Chicago.
Weston-Better Meters For Better Reading

 :thumb:
Title: All things LA2a related
Post by: khstudio on January 02, 2006, 08:15:52 AM
I just finished  building an LA-2A with the Jenson mods & transformers & NEED some help.
I'm following the Cayocosta layout & noticed that pin 1 of the 6aq5 isn't connected(to pin 7, like on the scematic). I didn't think too much of it until tonight when I took it to a local tech that helps me out & he checked it for noise, oscilation, etc...
Anywho, the scope kept showing a spike that didn't look normal. We checked the schematic & found the missing connection (6aq5), fixed it and the spike went away. :shock:

Is this a missed error?

Also, could everyone/anyone PLEASE post what they use, or don't use for caps:
C4 & C14?

I also asked if there were any boards left?
& what caps were used (C4 & C14) in the one that "MATCHED THE REISSUE".

??????????????????????


Just to add, I have recieved some feedback on the caps...just trying to gather a little more.
Title: All things LA2a related
Post by: Bluzzi on January 02, 2006, 02:16:17 PM
Looks like you are right about the 1 an 7 pin on the 6AQ5! I started my first wiring (turret boards) last night 'till 5:30AM. It felt so good to again be constructing instead of buying parts. The turret boards are so simple to do! The circuit boards were looking good but I think I really like wiring point to point. There is something about stripping wire, wrapping it on posts and soldering that I enjoy (man I better get a life!)

I was planning on going point by point comparing schematic to Cayacosta layout. Mine has some parts that are not marked (C5 which I now marked) and there is a 470pf cap going from pin 10 of A10 (Ground) to R28 on the main turret board which is not named on the schematic or the Cayacosta layout (so I think I didn't order damn!).

Can someone else comment on these discrepancies. Once I have finished my LA2A's I'll try and create a new layout and parts list from scratch.

Jim
Title: All things LA2a related
Post by: kruz on January 02, 2006, 02:25:30 PM
i haven't noticed this 6AQ5 pin 1-7 discrepancy...
mine works without the link.. i've also checked the hi-res photoes at the UA website.. and i can clearly see that pin1 is not connected to anything.
now.. dont' ask me why cause i don't know :green:
Title: All things LA2a related
Post by: Bluzzi on January 02, 2006, 02:35:40 PM
I just verified and all of the schematics I have which is about 3 variations all show pins 1 and 7 linked going to R3.

Does anyone have an LA2A working with these pins linked?

Anyone making only from Cayacosta's layout would not link but others using schematics may have caught this.

It may be that both work! Would be nice to have a definitive schematic/layout though.


Jim
Title: All things LA2a related
Post by: CJ on January 02, 2006, 05:02:24 PM
Look up the tube diagram for a 6AQ5.

Pin 1 and 7 are both tied to the same element.

Probably done for mechanical reasons since they had a pin open.

Or maybe to give the P to P guy more layout options.
Or both. People in the old days usually had two or three good reasons for everything. Now were down to one or two.

So use either/or pin 1 and 7. I would use both. Provides more integrity to the tube socket/tube system. Seems to thghten things up, which is much needed for the weaker and smaller 7 pin socket.

Some old Joe's, probably with RF burns on their hands, like to tie down all loose metal so it dosen't float in space and pick up stuff.

But that does not apply in audio land.
Title: All things LA2a related
Post by: Bluzzi on January 02, 2006, 05:19:37 PM
Thanks CJ, that clears that one up.

Back to finishing my turrets tonight. I can't go further until I get some metal work done and I can't until mid Jan so I will probably solder all wires coming from turrets now and just cut to correct length when I have the metal for mounting the trannies, tubes and T4B.

Any wiring tips? I mean things like keep certain wires away from certain components to minimize hum?

I already know about the twisting the filament feed wires together.

Thanks.

Jim
Title: All things LA2a related
Post by: kruz on January 02, 2006, 05:37:28 PM
thank you CJ.. as always :thumb:
 :guinness:
Title: All things LA2a related
Post by: CJ on January 02, 2006, 05:38:26 PM
Mechanical tip:

Get all your transformers and tube sockets first. Then lay them on the chassis for a realty check before drilling and punching. And do not drill holes for the T4 octal socket until you have the T4. It is in a square can and looks real silly when sitting at a 45 degree angle. Line the socket up with the can in it, then mark your mounting holes.

IF you use A10 and A24 type transformers, be prepared for some close tolerance aggrevation, the mounting holes are very close to the bolt circle.

I have some templates floating around  here for the UTC stuff if interested.
Title: All things LA2a related
Post by: Bluzzi on January 02, 2006, 05:56:28 PM
Thanks again CJ. I was planning doing exactly as you say CJ, except the orientation of the T4B. I had not thought of that one! I guess its an esthetic thing though and will not affect the audio either way.

I'll be mounting components as per tradition. Why mess with decades of experience.

I'll be using non traditional VU meters though. I have some older Simpson meters (huge black bakelite ones and some more modern looking ones marked 3M. These are even bigger!).

I am not sure which (R24, R25,R26 or R27) resistor I will have to match. My guess is R24?

Jim
Title: All things LA2a related
Post by: khstudio on January 02, 2006, 10:04:59 PM
The insert jacks on my console are unbalanced, tip & sleve. How do you guys hook up your transformer balanced gear to stuff like this?

My new LA-2A is my main concern. Trying to find the best way to deal with it in my (TRS) patch bay.
Title: All things LA2a related
Post by: CJ on January 02, 2006, 10:15:17 PM
just get the phase right and you should be fine.
one secondary lead to ground, one to the tip.

now as far as chassis grounding is concerned, you might have to play with the A/B thing, an extra  wire connecting board and compressor chassis' together, or no wire.
Title: All things LA2a related
Post by: khstudio on January 02, 2006, 11:28:46 PM
CJ, your pic kills me every time you post.
:green:

It might be the best one yet.

Thanks for helpin man.



Hey, I'd like to post some pics of my LA-2A, How... & what's a good size?
Title: All things LA2a related
Post by: CJ on January 02, 2006, 11:48:30 PM
go by the file size, as some of us are at the mercy of 56k modems.
25 kb per pic is friendly but hard to meet.
400 by 600.

i have a ton of space if you want me to throw them up.

mail to

[email protected]

or use the Lab photobucket whic I know nothing about.
Title: Bloo kit
Post by: Coco on January 03, 2006, 02:43:10 PM
I am about to order a Bloo kit and have read all the posts here. (Whew! Long read) I do have some questions. Some guys are asking about what pots and VU meter to use. I thought all the parts are included in the kit, or am I wrong to assume this? Is the T4B included as well or do I have to buy this too? This is my first DIY and am biting off a mouthful instead of a small piece but I figure you just have to sh*t your pants and dive right in. The only thing I have done before is change filter capacitors in my Hiwatt.
Title: All things LA2a related
Post by: Greg on January 03, 2006, 03:10:56 PM
Originally Steve offered partial kits, but recently he's been offering complete kits. Hope this answers your question.
Title: All things LA2a related
Post by: Coco on January 03, 2006, 03:18:44 PM
Yes, thanks. I was reading through the old posts on the thread and started to think (big mistake). I have never built or assembled a kit and I am "electronically challenged" so I want to cover my bases. I assume I have to buy wire and heat shrink or is that included too? Is there any type of drilling to the circuit boards? I read about drilling holes for tube sockets and the T4B?
Title: All things LA2a related
Post by: Greg on January 03, 2006, 04:04:58 PM
Coco: I'm nearing the end of my Bloo LA-2A, and will admit it's not the easiest build out there. Being point-to-point, it's very easy to make mistakes. I've built many compressors and several preamps so I have some DIY under my belt, and this kit is by far the most challenging; however, it's definitely the most fun. If you have never built or assembled a kit, or even worked with tubes, I'd recommend you start with something else. However, Steve does provide detailed instructions, so if you are methodical and precise and make sure to follow them, I'm sure you can get the LA-2A up and running. Just wanted to throw that out there.

What type of lab material do you have? ...multimeter, scope, etc... ?

To answer your questions, Steve includes wire and some clear tubing, and I only had to drill holes to mount the Sowter transformers. I didn't have to drill any holes for the T4B.
Title: All things LA2a related
Post by: Coco on January 03, 2006, 05:07:29 PM
Don't have many tools. Just the basics like a weller soldering iron, digital multi meter ect. No ocilliscope but a tech I know has one and he could check things out for me. Actually a friend has 2 he is not using so I should probably snag one. It is a big old beast and has a huge Lambda power supply that goes with it. I am patient and methodological and am confident I can do it, I will need a bit of help checking it over ect. but think I can do it over some time. I have decent soldering skills and have fixed some of my tube gear but that was simply connecting the dots like replacing parts that were alredy there and reconnecting the wire or replacing wire ect. Nothing too complicated. Maybe I like to punish myself since I always seem to go about things the hard way.
Title: All things LA2a related
Post by: Greg on January 04, 2006, 03:16:39 PM
I was just worried that you hadn't ever really soldered or done anything with electronics. In that case I might say you're getting in over your head. But I will definitely suggest taking your time with this build. I haven't worked on this one for over 2 hours without taking at least an hour break, then coming back to re-check my work. As I go through I've also been following the schematic to get an understanding of how this thing works. Also, try and be as tidy as possible when wiring. If not you'll get a huge clusterfudge of wire in there.

But I'm almost complete... I just have to wire up the front panel, then I'll be ready for testing... I'll just give the it smoke test !!! (kidding)
Title: All things LA2a related
Post by: khstudio on January 05, 2006, 02:03:00 AM
:!: Just a note to ALL builders who are unaware of the variable caps:

I've done tons of research on these: Listening, visually analyzing with a scope & RTA computer sweeps. I've also called UA & checked ALL the schematics trying to find out what?s going on. You will find MANY opinions about these caps & from my personal experience can have a dramatic effect on the sound & response of the unit. I will say if you build one right, all these variations can be cool for something.

C4 controls the frequency response of the entire unit (high end) & can cause a roll off that starts off slow from 2-3k to around 10k, then faster from approximately 10k up to 20k.
I've found that setting C4 to its highest point or slightly less yields little to no roll off on the high end, closest to flat. This seems to be in the 330pf ? 380pf range. When the cap was backed off, counter clockwise, the roll off could be down as much as 10 to 15db @ 20k. I was at about -12db with less than ½ turn counterclockwise from being full on.  :shock: So be aware. Don?t think you can set it where you ?think? its right. These things are sensitive.

Now, UA told me they put a 1k test tone thru the unit and set the volume to 0, while checking on a scope or RTA (real time analyzer), then they send a 20k signal thru & set that to be as close to flat with the 1k tone as possible. I was very shocked when they told me this, especially after looking at some of the pics from the builders on this sight. Most of them sure don?t look flat to me. This, in my opinion isn?t bad. A ?little roll off on the high end is pleasant. It seems to be more HI-FI sounding when flat.
I will note that I?m using the Jensen transformers & their mods.

UA also told me that they DO NOT USE C14 on the reissue.
It wasn?t in the original LA2 units either, but was in one of the versions.
What does it do? I?ll tell you.
C14 is in the detector part of the circuit & causes the unit to react to the higher frequencies more. The sound to me is somewhat like a D-esser. The caps value affects the frequency itself. Nobody seems to know where it should be set stock so you?ll have to listen and see for yourself. Some guys don?t use it at all. Without the cap, the detector is un-weighted & reacts more evenly to all frequencies.

 :idea: In my LA-2A, I?m going to try putting a multi switch on the front so I can dial in the D-essing function (C14) & the frequency of.

Something like ?
OFF, 100pf, 150pf, 220pf, 330pf

That should give some nice options.

BTW, I could use some help wiring the switch to the front panel for my D-esser control. My concerns are with the style of wire to use - shielded or unshieled. A very experienced tech told me today on the phone that all shielded cable has some capacitance. :shock:
Me not know that.
Anywho, would it be safe to run single wires from the output terminal strip,(where the cap belongs) acrossed the middle of the chassis to the front panel... maybe right under the meter? Would there be any problems with noise, crosstalk, oscillation, etc...?

I think I'm going to use a 3 position switch, center off.
Also, any idea what the best 2 cap sizes should be???
150pf & 330pf?

Or, could one of you brainiacs that know more than me help with the math & how to figure out the frequency affected by these caps in this part of the circuit???
That would be killer! :thumb:


*If there is any known or false information in this post please let me know so I can change it. I wish I had this info when starting my build. It took days of research & testing to find it.

Good luck with your builds. :wink:
Kevin
www.khstudio.net
Title: All things LA2a related
Post by: guavatone on January 05, 2006, 09:01:56 AM
I'm thinking that it would be cool to have a C4 switch too.  One for "vintage" sound and one for more "hi-fi"

maybe someone can comment about stray capacitance of wires 6" long or so.  a relay would seem to be the best but sounds like too much work.
Title: All things LA2a related
Post by: khstudio on January 05, 2006, 09:13:19 AM
Don't get confussed.

C4 = Hi freq roll off (or not if set flat)
C14 = Hi freq weighted compression/D-essing - located in the detector circuit. This is what I was reffering to about the switch. Thats not saying you can't try it with C4 too.

I'm also waiting for some inteligent replies concerning the wires: style & lenth.
Title: All things LA2a related
Post by: khstudio on January 05, 2006, 09:15:03 AM
dude,
sorry, I just noticed you did say too. (as in also)
My bad.

O BTW, you asked me where I was from in you PM. I'm in South Jersey, Millville.
1 hour from Phily or AC.
Title: All things LA2a related
Post by: synthi on January 05, 2006, 09:22:53 AM
Great idea!

I think having a switched cap for C4 and C14 would be nice!:

C4: 3 positions: flat (hi-fi), [email protected], [email protected]
C14: choose your deesser freqs :)

Synthi
Title: All things LA2a related
Post by: CJ on January 05, 2006, 01:31:04 PM
UA might have tweaked the output iron for better high end response. So folks using the original UA iron may not get the same milieage if tweaking cap for 20 k.
Title: All things LA2a related
Post by: synthi on January 05, 2006, 01:36:45 PM
So what would be the better trafos for hi-end, UTCs? Sowters?

Thanks,

Synthi
Title: All things LA2a related
Post by: CJ on January 05, 2006, 01:48:12 PM
I say whats the point of building vintage gear if it does not sound vintage?
Always go for as original as you can get on the first copy, listen to it, say "OK, I know what an early LA2 sounds like" (or close enuff), then build another one and "modernize" it if you want something different. (like more high end for guitar instead of a rolled off mellow vocal sound) Or mod the one you have. But if you do not have the original standard to go by, then whatever you build will leave you wondering "what it I had built the original."
Can you dig my trip, man?

If you want your girlfriend to sound like Billy Holiday, go for the original.

Too much coffee. Sorry!
Title: All things LA2a related
Post by: synthi on January 05, 2006, 01:51:43 PM
:roll:  Err.... What!?  :grin:

Synthi
Title: All things LA2a related
Post by: khstudio on January 05, 2006, 06:49:24 PM
Obviously we would all like an original to compare to.

CJ kind of backs up what I said in my post. All the variations are cool for something. Mellow vocal & bass was definitely what I got WITH C4 set with a -10 db roll off at 20k & C4 installed with a value of aprox. 330pf. In some ways I liked it better...On other material it was too dark.

But I think this is where the switch idea comes in.

CJ, for those of us who want to step outside and add a mod switch to these caps, what would be the best way? What you said was cool but you really didn't answer the big question. Come on man...I know you got it in ya
:green:
Title: All things LA2a related
Post by: CJ on January 05, 2006, 07:17:18 PM
If you really wanna have some fun, put a pot in place of R11 68 k feedback resistor.
Title: All things LA2a related
Post by: khstudio on January 06, 2006, 01:50:01 AM
Out of the 10 guitar amps I've built, I put a negative feedback control in 4 of them. I like having control of the neg feedback, especially in my deluxe reverb. Its more touch sensitive & responsive but also noisier & overdrives faster...but nicer & smoother.

With that said, are you pulling my leg or is it something cool in the LA-2A?

Will it respond like my Guitar amps did?

Also, could you please help with the wiring type for my D-esser (type) switch. I'm not having any noise problems & don't want to.

The bottom line is I like it both ways and would love to have a switch on it.
I really what to make sure before I drill into the front of this thing.
Title: hey guys
Post by: keithcamilleri on January 07, 2006, 12:15:16 PM
hi there,

I tried passing audio trough the la2a ...sounds really good did not put in the the t4b still since i m not very happy with my voltages is there anyone that could point me in the right directon...

a20 is way too high its 240v
b20 is around 300v
and pin 5 on the t4 is around 52v

is there something wrong?

regards
Title: All things LA2a related
Post by: dissonantstring on January 07, 2006, 04:47:42 PM
hi keith,
i'm a newbie so these are just suggestions of what i would check.
first, IIRC CJ suggested in this thread that tube cirucuit voltages can be within +/-20%.  i'm not sure if you are using 10K/2W for R34, but if so, maybe use 22K/2W as suggested early on in this thread.  but before that maybe you should check to see if your diodes (CR1 & 2) are in the correct orientation and that they are doing their job.  please remember to power off, unplug and discharge PS caps for safety's sake before doing anything.
sorry, like i stated i'm new at this so i don't know if this will actually lead you to the real problem.
hopefully our resident LA-2A geniuses (CJ, SSL TECH and Scenaria) will chime in.  let us know how it goes.
regards,
grant
Title: All things LA2a related
Post by: CJ on January 07, 2006, 05:49:03 PM
Keith, if no smoke, you are half way there.

I still do not have Steve's wiring diagram, so you will have to relate the A20 thing to an actual point on the schematic.

Pin 5 on the T4 will drop when you plug that thing in.
Title: All things LA2a related
Post by: keithcamilleri on January 07, 2006, 10:25:47 PM
thanks dissonantstring I 've checked the diodes and they were all ok I appreciate your help a lot.

The back row of the turret is A and the front row is B. The neon is connected    to A1.

Thanks CJ , A20 is where the 10k/2W resistor on the turret board is placed  the point I am measuring is the point where there is a wire connected with pin 5 of 6aq5a. the voltage is 240V. instead of 10k I am using 22k and before the resistor I am measuring 320V.

I've turned SW2 to Gain Reduction and the meter went into full scale output +10 and output +4 did nothing...

Do you think there is something wrong with unit?

regards

keith
Title: All things LA2a related
Post by: CJ on January 07, 2006, 10:28:49 PM
Looks OK.
Measure pin 2 on 6AQ5 which is bias which tells you the health of the circuit.
s/b around 7.5 dc.
Title: hey
Post by: keithcamilleri on January 07, 2006, 10:33:02 PM
thanks cj for yor reply what is the expected voltage for pin 2 ?
Title: All things LA2a related
Post by: CJ on January 07, 2006, 10:36:07 PM
7.5 vdc
Title: All things LA2a related
Post by: CJ on January 07, 2006, 10:38:36 PM
he is a link to a voltage chart - your mileage may vary 20 percent:

http://vacuumbrain.com/The_Lab/TA/LA2/la2_voltages.jpg
Title: All things LA2a related
Post by: keithcamilleri on January 07, 2006, 10:40:58 PM
hey cj not looking good the voltage is measuring 2.2v dc is there any test u might suggest so that i would not fry the t4b :)
Title: All things LA2a related
Post by: CJ on January 07, 2006, 11:01:34 PM
pull the 6aq5 out.
read the label to make sure it is a 6aq5.
is it new?

unplug unit.

set meter to ohms.
check ohms of  r34, r35 and r36


if ok, fire up unit without 6aq5

measure dc volts on pins 2,   5 and 6

report back if still alive
Title: All things LA2a related
Post by: khstudio on January 07, 2006, 11:44:29 PM
HELP.

Is it ok to use ceramic-wirewounds in the PS divider circuit?
I read that these can cause oscilation or other problems.

I originaly installed the 2w metal oxides recommended in the parts list but the tech I took it to said they aren't rated for that high of voltage & when he checked it it measured 5.5k instead of the 4.7 I measured when I installed it. He said it couldn't handle it and was caused by the heat.

Whats the story here.
2watts:
4.7k
10k

Also, should R38 - the 22k feeding the 6AQ5 wattege be increased a little above the recomended 1w metal film?
Title: All things LA2a related
Post by: keithcamilleri on January 07, 2006, 11:46:09 PM
still alive  :grin:

well the 6aq5a is brand new

pin 2 is 0v
pin 5 is 324v
pin 6 is somewhere around 48v

i am in pain i heard the a24 and a10 working now and i need this baby to start working :)
Title: All things LA2a related
Post by: khstudio on January 07, 2006, 11:46:26 PM
CJ,
What should I do about the C14 switch?
Title: All things LA2a related
Post by: CJ on January 08, 2006, 03:35:59 PM
Keith, your screen voltage is too low. This is why the tube is drawing so few ma.
Increase the screen voltage to 135 volts somehow. Which screen circuit are you using? There were several different versions over the years.

If you increase the screen, you lower the internal resistance of the tube, which will increase currrent which will bring your cathode bias point up, as well as bring the voltage measured dirrectly of of the 6AQ5 plate down to a more nominal value.


Sandblock resistors are fine for the power supply. Thats what they were designed for. If your caps are in good shape, what could those resistors possibly do to cause trouble?
Not enough inductance to resonate correctly with the caps and power transformer.

What is the C12 switch? Are you building some sort of eastern troublemaking device?


Hmmm, maybe I f'ed up. You should measure the screen voltage with the tube installed.
Otherwise, the internal resistance of your volt meter will just be forming a voltage divider with the plate and screen resistors.
So plug in the tube and see if it changes.
 :oops:
Title: VU lamp for Bloo?
Post by: rove on January 16, 2006, 01:38:46 PM
How would I go about adding a light to the beede meter on my bloo?
thanks.
Title: All things LA2a related
Post by: Rafita on January 21, 2006, 09:19:33 PM
Hi first post asking help.

im doing the basic testing on the bloo and A19 and A20 read diferent

im doing the test with tubes in

A24 -A25   =  6.72 V
B21           =  359 V
B20           =  281 V
A1             =  54 V
A20           =  92 V      * Should be around 131v *
A14           =  227 V
A17           =  88 V
A19           =  122 V


R34  = 22k /2w
R35  = 220 k

thank's

cheer's from mexico
Title: Re: VU lamp for Bloo?
Post by: Greg on January 24, 2006, 09:58:56 AM
Quote from: "rove"
How would I go about adding a light to the beede meter on my bloo?

What I've seen is to get a 6VAC lamp (or close to that) and just hook it up straight up to the 6.3VAC coming from the secondary of the power trafo. This is what I'm planning to do, but haven't found the lamp yet. Or the lamp holder as well if I can find one. Gonna have to search around.
Title: All things LA2a related
Post by: rove on January 24, 2006, 12:03:53 PM
Quote
What I've seen is to get a 6VAC lamp (or close to that) and just hook it up straight up to the 6.3VAC coming from the secondary of the power trafo. This is what I'm planning to do, but haven't found the lamp yet. Or the lamp holder as well if I can find one. Gonna have to search around.

That is what I thought, basically wire it up like the heater wires for the tubes... If you find a proper lamp/holder please post about it.  My guess is the lamp is pretty easy to source, but what about the holder?
thanks.
Title: All things LA2a related
Post by: CJ on January 24, 2006, 06:04:44 PM
Radio Shack has lamps and holders...
Title: All things LA2a related
Post by: AnalogPackrat on January 24, 2006, 06:39:02 PM
Rafita,

What did you use for R36?  With R34 of 22k, R36 should be 1k.  In case anyone else is wondering where A20 is, its the plate of the 6AQ5.  

A P

p.s.  I think it would be more useful for folks asking for help if they'd post voltages relative to nodes on the circuit diagram (e.g. plate of 6AQ5, between R6 and R7, etc.) rather than the Bloo manual labels since most people don't have the Bloo manual.
Title: All things LA2a related
Post by: bradb on January 26, 2006, 10:35:14 AM
Hi everyone,

I'm working on a build of a dual/stereo la2a based on the pcb (found here on the forum).

EDIT:  Just to clairfy, I'm building this dual LA2A into a single Rack case.

I'm designing the face plate and I have some operational questions.  I'd like to bring the maximum functions to the front but leave things that are used only once to the back.

On the front of the unit, I plan on putting the Lim Response (R37), gain and peak reduction, +4/GR, Lim/Compress controls.

On the back, I plan on having the Zero adjust pots mounted...

The question is what to do with the stereo adjust pots (R3), every time I use the units in stereo, do I need to Re-adjust them?  can I just do the stereo adjust procedure once, set it, forget it and then put a DPST switch on the front that connects the stereo parallel and grounds (terms 6 and 7)?  I would just flip the switch when i wanted to move between dual and stereo.


also.....

I will wire R37 such that turning it clockwise will increase resistance and label the Lim Response on the face place "HF Sensitivity"... .does this make sense?


Thank you everyone!
Title: All things LA2a related
Post by: Rafita on January 26, 2006, 12:06:50 PM
thank's analog

R34    22k
R35    220 k
R36    1k

so my readings are diferent on V4

my  B+    281 V

R34         92 V
R35         122 V
R36         9.07

also it doesn't change the readings after setting  R3 and R37  full clock wise or ccw

thanks :roll:
Title: All things LA2a related
Post by: CJ on January 26, 2006, 02:40:47 PM
set it and forget it. it won't move.
Title: All things LA2a related
Post by: bradb on January 26, 2006, 03:19:01 PM
Thanks CJ,

Ok, Zero Pots will be mounted on the back (or sides) as will the stereo adjust pots.  everything else will be on the face.

thar she blows:
(http://show.imagehosting.us/show/1112215/0/nouser_1112/T0_-1_1112215.jpg) (http://www.imagehosting.us/index.php?action=show&ident=1112215)
Title: All things LA2a related
Post by: AnalogPackrat on January 26, 2006, 06:18:40 PM
Rafita,

Looking back through some of the older posts in this thread may help.  I believe others have had the exact situation that you have, but their units worked fine.  If all your other voltages check out you should be OK.  Have you tried to pass audio through it without the T4b in?  Does the gain control work?  If that all seems OK, you should be able to power down, plug in the T4b, and go for the final test.

Good luck,
A P
Title: All things LA2a related
Post by: Rafita on January 26, 2006, 06:39:53 PM
Thank's  A P

And Yes i found someone that have the same situation on page 73 , i've just send it a message.

Same Readings on V4

i did pass audio and it's working , just had some hum , i haven't put the grid stopers.

i have on Pin 5  (t4b)  8.5 V
so i thnk from what i read that it's safe to put  the t4 module

Thank's     :roll:
Title: All things LA2a related
Post by: AnalogPackrat on January 26, 2006, 07:07:16 PM
Rafita,

You only need the grid stoppers if you used a Sowter output transformer.  Hum is from somewhere else.  Did you keep your heater wiring twisted tight and laid flat against the back of the chassis?  Do you have dimmers on the lights in your lab?  Did you get hum with the gain around 3 or 4 (which is where you'll end up setting it most of the time)?  Was the chassis closed up?  You're getting close!

A P
Title: All things LA2a related
Post by: dissonantstring on January 26, 2006, 07:20:32 PM
hi rafael,
i saw your pm, but i guess i should have looked at the posts first to see where you were at.  if your Bloo is passing audio and compressing o.k. then you are doing fine.   :thumb:
AP is correct, you only need the grid stoppers for the sowter tx to prevent oscillation IIRC/ i could be wrong, but your hum is from somewhere else.  i still have slight hum in mine, but i believe it's due to bad wiring layout which i have yet to correct.  
 
hey AP,  gotta find time to get together and get some grub in mt. view.  i've been swamped at work.  did you hook up cj's op tx on your api yet?
Title: All things LA2a related
Post by: CJ on January 26, 2006, 09:53:54 PM
yeah, he was here. and I'm missing a a 1 k resistor.
son of a....
Title: All things LA2a related
Post by: mnats on January 27, 2006, 01:26:22 AM
Quote
...get some grub in mt. view.

Go to La Costeña (http://www.costena.com/) for me and have a lengua taco. Very hard to find them here...
dissonantstring - sorry I had no time to try hooking up this time around sj but sounds like you've been busy too. My poor wife was meeting either a relative or friend of mine for the first time at the rate of 2.6 per day.

I haven't forgotten about posting pics of my homemade LA2 chassis either - just got to find some time...
Title: All things LA2a related
Post by: AnalogPackrat on January 27, 2006, 11:17:37 AM
Quote from: "CJ"
yeah, he was here. and I'm missing a a 1 k resistor.
son of a....


Excellent!  He didn't notice me pocketing the Langevin iron... :shock:

Mr. String:  We'll get it together sometime.  Life's hectic.  No, I haven't used my trifilar bastard son of Profile hand-wound job yet.  Still working on the rest of the API.  Stuffed a couple of 1731s and one works fine, the other not.  This weekend is not looking too good for DIY, either.  Oh well...carry on!

A P
Title: All things LA2a related
Post by: dissonantstring on January 27, 2006, 01:36:11 PM
CJ - somehow my mind freaked when reading your post and lemmy became angry...i need coffee.

mnats - no problem.  hopefully next time, i know how that can be,  not always fun for either of you i'm sure when you're in a rush (2.6 persons/day?  did you really calculate that?)
oh, did you get peets limited sulawesi stock?  i'll try to get some for you, but i think it ends january, so hopefully i'll have time to get you some to try.
wait...dual 1176 completed, webpage on 1176 rotary switch and an LA-2A?
you are a monster!

AP wrote:
Quote
Oh well...carry on!

i don't think we have a choice?  do we?  :?   i gotta work this weekend too - keep reminding myself it's for the kids.

Quote
my trifilar bastard son of Profile hand-wound job
:shock:  
sounds a bit "steamy" :grin:  -grant
Title: All things LA2a related
Post by: idylldon on January 29, 2006, 03:00:16 PM
I've finished up building the PCB version of the LA2A and have encountered a weird problem with the VU meter.  The meter only works correctly when I hook it up backwards; that is, I reverse the + and - leads.  When I do this, I can set the meter to zero and it tracks correctly during compression; however, then I use it in VU mode when the leads are hooked up either correctly or reversed, the meter bounces around and then slams back to rest.

When I hook up the meter correctly, I can't get it to set on 0--it only goes to -7.

I'm using Jensen transformers and have checked and double-checked that they are hooked up correctly.  I have verified the meter switch wiring, the values of the resistors on the board, the caps, etc., and I can't find anything that would explain this weird meter behavior.  I know I have to be overlooking something, but at this point I'm stumped and need some other perspectives and ideas.

Any ideas would be greatly appreciated.

Cheers,
--
Don
Title: All things LA2a related
Post by: idylldon on January 30, 2006, 07:09:15 PM
Well, it seems as though the compressor is working fine and the meter tracks the gain reduction IF I have the leads reversed to the meter.  In VU mode, it's all wonky and sure isn't tracking what's going in correctly.

There's also a post by dripelectronics on the LA2A PCB thread where he experienced the same problem with different VU meters.  I'm assuming that all VU meters aren't quite up to working in the LA2A, so what meters have folks found to work correctly in VU and GR mode?

Thanks,
--
Don
Title: All things LA2a related
Post by: bradb on January 30, 2006, 07:16:59 PM
and more so, why are they different?

could it be a rectification issue?
Title: All things LA2a related
Post by: idylldon on January 31, 2006, 01:44:26 AM
I figured out my meter needs a germanium bridge rectifier.  Once I hooked one up, the meter seems to work perfectly.

Cheers,
--
Don
Title: All things LA2a related
Post by: Greg on January 31, 2006, 06:21:55 PM
I just finished wiring up my Bloo LA-2A...

Fired her up no smoke with no tubes  :thumb:

Measured some voltages:

Heaters: 7.21VAC
B21: 388VDC
B20: 383VDC

I think this looks ok... time for the tubes !!!
Title: All things LA2a related
Post by: Greg on January 31, 2006, 06:40:08 PM
With tubes:

Heaters: 6.73VAC

Measured (Ideal reference):
B21: 362.9V (360V)
B20: 294V (275V)
A1: 52.6V (60V)  
A20: 93.5V  (131V)
A14: 275.8V  (216V)
A19: 128.5V  (105V)

What do you think? Are A20 and A14 too far off? Thanks.
Title: All things LA2a related
Post by: CJ on January 31, 2006, 06:42:57 PM
A lot of us don't have the Bloo schematic, so tube socket numbers are cool.
Title: All things LA2a related
Post by: Greg on January 31, 2006, 06:49:01 PM
Yea good point... I passed some audio through her with no T4B and the sine and square wave look good. This thing has a ton of gain !!!
Title: All things LA2a related
Post by: CJ on January 31, 2006, 07:04:58 PM
That's what they say.  :grin:

Distortion is not pretty though.
Title: All things LA2a related
Post by: Greg on January 31, 2006, 07:13:17 PM
There she blows (with tubes and no T4B):

If it's not listed, then 0V...

V1:
1: 138
3: 1.51
6: 117.3
8: 1.37

V2:
1: 227
3: 22.3
6: 275.5
7: 143.2
8: 228

V3:
1: 101.7
3: 0.875
6: 101.7
8: 0.875

V4:
2: 8.94
5: 90.2
6: 127.2
Title: All things LA2a related
Post by: CJ on January 31, 2006, 07:15:51 PM
Looks like you nailed it on the first try.

Plufg in that square can thingy with the brown amphenol base.
Title: All things LA2a related
Post by: Greg on January 31, 2006, 07:24:35 PM
V2 is screwy, but I took a quick look and I think I found my mistake already.
Title: All things LA2a related
Post by: CJ on January 31, 2006, 07:26:44 PM
V2 is busy, thats for sure. Especially with point to point.
Title: All things LA2a related
Post by: Greg on January 31, 2006, 07:40:25 PM
I think V2 looks better now:
1: 104
3: 4.13
6: 227
7: 80
8: 110

T4B time !!!  :thumb:
Title: All things LA2a related
Post by: Greg on January 31, 2006, 07:55:47 PM
I don't think I'll work on this anymore tonight, but it's passing signal but no compression yet (with T4B). When I increased peak reduction, I noticed the neon starting to flutter. I'll double check some wiring then post back.
Title: All things LA2a related
Post by: Greg on January 31, 2006, 09:43:44 PM
So decided to run some audio through the unit just to see how it sounds. I did this with and without the T4B. First off, the LA-2A is very quite and sounds great as a line amp. I hear no buzz or hum or anything like that, so that's good  :thumb: .

Unfortunately, I have no compression. My voltages still seem to be ok, so I'll concentrate on the sidechain and perhaps the gain reduction wiring. One thing I did notice, and this happens with or without the T4B. When I switch to the GR metering mode, the neon becomes less bright.

I don't know if I have a bad T4B... when I put it in, the unit behaves just as if it wasn't there. Any advise is appreciated. I'm gonna sleep on it.
Title: All things LA2a related
Post by: Greg on January 31, 2006, 10:56:55 PM
Sorry to keep posting, but I did a little more work on the LA-2A. I put the T4B back in without the lid in a dark room. Then I shined a little light on the T4B, and sure enough, the meter deflected and the signal was reduced... ie compression. So I guess that either means the EL panel is bad or the control voltage isn't getting to the EL panel.

From the schematic, it appears that the control voltage is at pin 3 of the T4B. Is this correect and if so, is it AC or DC? And what's the typical voltage range.

I'd like to test the EL panel, make sure it works, then I'll know the problem lies in my wiring.
Title: All things LA2a related
Post by: Rafita on February 01, 2006, 12:02:24 PM
Ok so after reading the whole thread  , i got mine working , the only differences were on V4 where i got

pin 5 ( r34 )    92 volts
pin 6 ( r35 )    122 volts

Still the same readings but its working

Hey Greg read this !  

on page 53 AnalogPackrat quote:

"""" This is the older config with R34 = 22k and R36 = 1k, right? You're not using C13 and R38 which were added in a later rev, right? Recheck R34 and R36 as well as R35. Something isn't right with the wiring around V4.

BTW, I built the newer config with the extra stuff on the screen grid of V4 and 10k R34, 470R R36 """"

On page 54

"""" He also accidentally had a combo of old and new circtuitry around the 6AQ5. Now he's back to the old version with no R38 or C13 (his choice).

Paul has also taken the cover off of his T4B with the unit in place and the power on. Shining a light into the thing made his meter (in GR mode) move much more than his input signal would even with max peak reduction setting. Maybe you're right and he's just got too weak of a signal.  """"  


if you using the bloo manual as I we are using the old circuitry where
C13 and R38 is'nt used .

.

So be sure also that youre feeding a hotter  signal , and it will move.
Also check your wiring on the rotary ( i've redone it like 4 times ) , and then discovered that my signal was weak so my needle wasn't moving

thank's  to  AP , Dissonant String , Paul B , CJ    :thumb:
Title: All things LA2a related
Post by: AnalogPackrat on February 01, 2006, 12:18:01 PM
Greg,

I don't have my Bloo manual with me (I wrote my voltages on the schemo in the back).  I'll try to bring it in tomorrow, but hopefully you'll have it solved by then.  Rafita is right--let us know which version of the V4 wiring you used.  There are two different ones with changes affecting R34, R36, R38?, and C13.

You can tell if your EL panel works by popping the cover off the T4 and then running some dynamic music throught the unit.  Turn off your room lights and crank up the Peak Reduction.  You should see the EL panel flickering with the music.  If it isn't, then either the T4 is screwy or the side chain is messed up somehow.  Since you saw meter deflection with the flashlight trick, your CdS photocell for metering is probably OK.  You can check the other one by pulling the T4, hooking up your meter between pin 8 and pin 7 and shining a light on the cells.

Good luck!
A P
Title: All things LA2a related
Post by: rove on February 01, 2006, 12:19:40 PM
I got my lamp in, supplied by Radio Shack (thanks CJ!), and it works fine.  A little reticent to drill a new hole in the case, so for now, just taped in with electrical tape, but I can see that meter!  I also went ahead and replaced R9 and R13 with 100k resistors (had a 12ay7 in for V1 for a while, just hadn't changed the resistors).
Question, though: a problem I had before I put in the bulb, it seems the meter has a screwy connection as sometimes it loses all power and requires a few gentle taps to get back to work--the sound isn't affected and it almost seems to have something to do with how well the case is closed...  I have checked and rechecked the wiring and re-did the wires to the zero-adjust pot as tapping at that seemed to make a difference.  Any tips on how else to troubleshoot this?  thanks.
Title: All things LA2a related
Post by: AnalogPackrat on February 01, 2006, 12:24:22 PM
Rove,

Was your meter problem just in GR mode or all modes?  Check for a cold solder joint on R4 or R24.

A P
Title: All things LA2a related
Post by: Greg on February 01, 2006, 01:06:54 PM
Thanks guys... I did some reading this morning and I will do what you have suggested... Checking to make sure my signal is hot enough and checking the components around V4. I'll post back.

Quote from: "AP"
Since you saw meter deflection with the flashlight trick, your CdS photocell for metering is probably OK. You can check the other one by pulling the T4, hooking up your meter between pin 8 and pin 7 and shining a light on the cells.

I think both CdS cells are working. Because as the GR meter showed gain reduction, the signal on the output reduced proportional to the meter. The unit was compressing with the flashilight trick.  :thumb:
Title: All things LA2a related
Post by: rove on February 01, 2006, 03:04:00 PM
The meter problem is in all modes.  I just went over those points and guess what?  the meter isn't working at all now.  Not really sure where to begin.  One thing that may be an issue is that I am finding it difficult to get the wire from the mini turret board to solder well to the tabs of the board (though it appears connected just seems it could be better), any tips there?
thanks.

edit:
it appears that was the problem all along, the wire from the last turret on the mini-turret board to the tabs that attach to the meter was not well connected.  I solved this by attaching a short wire with a lug on it that then was screwed onto the screw terminal, definitely attached now!  and the meter is working fine in all modes.
thanks again.
Title: All things LA2a related
Post by: AnalogPackrat on February 01, 2006, 05:27:31 PM
Rove,

I vaguely recall having trouble with that same solder joint.  I just socked the heat to it and made sure the flux got all over it and it finally relented.  Your solution is probably better.  I'll keep an eye on mine.

A P
Title: All things LA2a related
Post by: Greg on February 01, 2006, 05:35:55 PM
Just an update...

Just got home from work, popped in the T4B, put in a 1.3V sine wave, turned both R3 and R37 fully clockwise, and guess what...

MY LA-2A WORKS !!!

I guess it was a combination of too low of input, and the way I had the pots on the back set.

Thanks guys for making me realize that my LA-2A worked in the first place. Still need to do some calibrations and get this thing tweaked up. But first I'm going to get some audio through it !!!

 :thumb:  :guinness:  :sam:  :green:
Title: All things LA2a related
Post by: khstudio on February 01, 2006, 06:32:24 PM
Do you guys just use the schematic for a voltage reference chart?
I need to recheck mine -

The peak reduction on it seems to be happy around 1:00 / 60% while the gain never goes beyond 9:00 / 20%

... does the redution control seem too high :?

Where are your setting, mostly???
Please tell.
Title: All things LA2a related
Post by: Greg on February 01, 2006, 06:38:31 PM
There are all the DC voltages on the schematic. I did a google search for LA-2A manual and it came up.
Title: All things LA2a related
Post by: thirtyips on February 01, 2006, 10:01:45 PM
Quick question guys (or gals), I was wondering if it was possible to use 2 meters instead of one.  One of the meters showing gain reduction and the other showing output and eliminate the switch between the 2 altogether.  This isnt necessarily directed towards the la2a, but thats the project I am working on at the moment.  Just a thought.  Thanks

~Joe

P.S.  Sorry if this issue has come up already, searched but could not find.
Title: All things LA2a related
Post by: CJ on February 02, 2006, 04:43:18 PM
That would work.
Only issue I see is having the meter on the same winding as the output, which some folks around here say can degrade signal, depending on the meter used.

Once you set your output level, you can switch the meter to Gain Reduction, it is no longer across the output and you don't have to worry about negative effects.

If you want full time output metering, maybe a opamp buffer for the meter might be a good idea.
Title: All things LA2a related
Post by: thirtyips on February 02, 2006, 08:46:05 PM
Thanks CJ, I might just try that
Title: All things LA2a related
Post by: dukasound on February 06, 2006, 01:22:41 PM
Hi
I am confused about C4 because there are different opinion (value) from 100p to 390p.
I have three Silver Mica caps 47p, 100p and 220p and I can combine them. What are yours suggestions.
BTW I mean all Sifam users must drop R24 - 3,9K to 3,6K (Sifam suggestion)
Thanks
Duka
Title: Voltages
Post by: keithcamilleri on February 07, 2006, 02:06:24 PM
Hi guys maybe someone can help me,

I've been revising the circuitry but I cannot find any wiring problems till now, Voltage from A20 to ground is measuring 257V (shoudl be around(130V) and A19 is around 48V shiould be around 105V which is quite low... other voltages are within the limits and these are looking good, these numbers are with no T4B in...


hope someone can point me in the right direction...

thanks
Title: All things LA2a related
Post by: JerryPbury on February 07, 2006, 02:45:49 PM
Keith this is what I posted before for the solution to my problem.

JerryPbury



Joined: 06 Jun 2004
Posts: 24

 Posted: Sun May 15, 2005 11:56 pm    Post subject: Success!    

--------------------------------------------------------------------------------
 
Hey guys, I finally figured out what I did wrong on mine. R36 was a 470K instead of a 470 ohm. I don't know how many times I checked this area out but I decided to look at it one more time and it just jumped out at me.

Gain reduction works perfect. Output is great. I just tried it out with an SM57 into a Seventh Circle N72 into the LA2A. Wow!
Title: Voltages
Post by: keithcamilleri on February 08, 2006, 01:46:27 AM
Hi Jerry ,

thanks loads, I checked all the resistors and connection still nothing , the funny thing is the Screen Voltage ...why is it this low? cannot understand this I tried changing the 220K Res connectted with pin 6 of the 6aq5a I changed this from 220K to 180K and vokltages from 46V stepped up to 56V and pin2 (6aq5) instead of 2.7V now is ~3.6V ... 22K 2w resistor *a19) voltage still quite high 257V... any thoughts guys,


regards
Title: Re: Voltages
Post by: dukasound on February 09, 2006, 07:17:07 AM
Guys!
All of us who make Bloo or other type of LA-2A, how you adjusted C4 cap :?
Thanks
Duka
Title: Wierd Grounding/Interfacing on my La2a
Post by: thedug on February 21, 2006, 02:09:21 AM
Howdy,

I don't get this all the time but intermittently I am having a problem with my La2as, and they both do it, and it seems to only happen when I patch in certain chains, i.e. maybe an impedance/interfacing thing...

But the problem is a strange motorboating/glitchy noise thing that comes in bursts...

D-
Title: All things LA2a related
Post by: dripelectronics on February 21, 2006, 02:57:06 AM
hi duka , heres what some curves look like using fixed capacitors.
(the frequency response cuts off at 15k or so (due to the trial version
or something?) 150 pf seems more like the real deal


(http://driprecords.com/pcb/multicurveweb.jpg)
Title: All things LA2a related
Post by: synthi on February 21, 2006, 06:04:19 AM
no trial version for the rightmark audio analizer...
Also I`m pretty sure that the last vertical line is 20KHz, not 15Khz.

Synthi
Title: All things LA2a related
Post by: dukasound on February 21, 2006, 09:45:57 AM
Hi
Am I right but 330p have a flatest curve?
Thanks
Duka
Title: All things LA2a related
Post by: Bluzzi on February 21, 2006, 10:04:48 AM
Was there any consensus on putting a switch to select various pf values?

I was wondering if anyone had tried that and if they had gotten any "pops" or "clicks" in the output?

Maybe a shorting switch woudl be appropriate here?

Jim
Title: Re: Wierd Grounding/Interfacing on my La2a
Post by: AnalogPackrat on February 21, 2006, 11:18:22 AM
Quote from: "thedug"
Howdy,

I don't get this all the time but intermittently I am having a problem with my La2as, and they both do it, and it seems to only happen when I patch in certain chains, i.e. maybe an impedance/interfacing thing...

But the problem is a strange motorboating/glitchy noise thing that comes in bursts...

D-


Can you describe the patch up that causes it?  What's immediately connected to the LA-2a (in and out)?  What output transformer did you use?  Those of us with Sowter outputs have installed 1k grid stoppers on the WCF to kill oscillation.  I wonder if some load you're driving is causing a similar thing.

A P
Title: All things LA2a related
Post by: synthi on February 21, 2006, 11:26:34 AM
I`m still collecting parts and building process is still a bit far away, but I`ve choose the expensive way and I`ve got 3 of the variable capacitors for each channel (I`ll be making a stereo unit). I`ll be switching between them and I can trim them for adjusting each to a response to match the other channel exacltly.

Besty Regards,

Synthi.
Title: la2a oscillation/noise
Post by: thedug on March 01, 2006, 08:05:46 PM
So it appears to be an acoustical/feedback thing. It seems to happen when cars or whatever go by.

I think it is a lowend thing.

Any ideas? Is the La2a just to slow ot compress low end like that?

I could post a sound file?
Title: Re: Wierd Grounding/Interfacing on my La2a
Post by: thedug on March 01, 2006, 08:07:47 PM
Quote from: "AnalogPackrat"
Quote from: "thedug"
Howdy,

Can you describe the patch up that causes it?  What's immediately connected to the LA-2a (in and out)?  What output transformer did you use?  Those of us with Sowter outputs have installed 1k grid stoppers on the WCF to kill oscillation.  I wonder if some load you're driving is causing a similar thing.

A P


See the post above... Also, I used the lundahls.
Title: Bloo Kit
Post by: erland on March 21, 2006, 11:35:35 AM
The capacitors that came with my kit don't match up with the parts list.

Do I use...

20 µF 500 V instead of 10 µF 450 V

33 µF 450 V instead of 30 µF 450 V

47 µF 450 V instead of 40 µF 450 V ?


Can someone tell me what values the capacitors below have?

715P400V 223J 0505 - Small orange

715P400V 104J 0513 - Bigger orange

560G 100V - Small black

Thanks a lot,
Title: Re: Bloo Kit
Post by: AnalogPackrat on March 21, 2006, 11:55:09 AM
Quote from: "erland"
The capacitors that came with my kit don't match up with the parts list.

Do I use...

20 µF 500 V instead of 10 µF 450 V

33 µF 450 V instead of 30 µF 450 V

47 µF 450 V instead of 40 µF 450 V ?

 Yes on all.

Quote
715P400V 223J 0505 - Small orange


0.022uF

Quote
715P400V 104J 0513 - Bigger orange


0.1uF

Quote
560G 100V - Small black


Probably 560pF silver mica, but you should eMail Mr. Scenaria to find out where he intends you to use it.  Do you have more than one of these?

A P
Title: Re: Bloo Kit
Post by: erland on March 21, 2006, 12:24:16 PM
Thanks a lot.

Quote from: "AnalogPackrat"
Quote from: "erland"

Quote
560G 100V - Small black


Do you have more than one of these?

A P


There's only one.
Title: t4b issues
Post by: rove on March 23, 2006, 11:23:25 AM
I need to troubleshoot my bloo t4b. I am getting distortion on compression, really nasty.  I ran signal thru the comp without the t4b and there was no distortion, then tried another t4b (jbl tent sale) and have been getting what I consider to be proper reaction from the comp.  The questionable t4b also showed more compression on the meter ie going into compression faster, releasing more slowly.  I have taken it apart and looked it over for a bad solder joint, etc and plan to go over it with the iron as my soldering skills have gotten a bit better since I built it... Any suggestions on other ways to troubleshoot it or common things that could cause such a reaction?
Thanks
Title: All things LA2a related
Post by: Greg on March 23, 2006, 11:44:39 AM
hi erland:

I think 560G is 56p, not 560p. The first two digits are the value and the last is the multiplier.

56p X 10^0 = 56p

56 x 10^1 = 560p.

For my Bloo I used a 470p and a 510p Mica, exactly where they're stated in the schematic.

Here's the datasheet:
http://www.cde.com/catalogs/STD-DIPPED.pdf

And the Mouser part numbers:
470p - 5982-15-500V470
510p - 5982-15-500V510

IIRC, Steve sent me the same thing.
Title: All things LA2a related
Post by: erland on March 23, 2006, 03:24:04 PM
Hey Greg,

I e-mailed Scenario and he said it was 560p. I don't think there's a 56p in the kit. Thanks though.

Can someone tell me what either V1 or V2 is? I only saw mention of V3 and V4 in the manual.

Thank you
Title: All things LA2a related
Post by: CJ on March 23, 2006, 03:27:57 PM
rove, check ldr off resistance by leaving can on, measuring pins that correspond to the  ldr's.

tell me how many k ohms.

maybe he had just gotten back from SxSW that day.  :razz:
Title: All things LA2a related
Post by: AnalogPackrat on March 23, 2006, 06:23:50 PM
Quote from: "erland"
Hey Greg,

I e-mailed Scenario and he said it was 560p. I don't think there's a 56p in the kit. Thanks though.

Can someone tell me what either V1 or V2 is? I only saw mention of V3 and V4 in the manual.

Thank you


Greg may be right on that cap value.  You should measure it before installing it as an order of magnitude difference in value will be bad.  Which cap did Scenaria say was 560p?  

V1 is valve (tube) 1.  Look at the schematic and you'll see them labelled.  Three of the tubes are dual triodes so you'll see V1a and V1b for the two halves of V1.  The 6AQ5 is a single pentode.

A P
Title: All things LA2a related
Post by: erland on March 23, 2006, 06:52:42 PM
Quote from: "AnalogPackrat"

Which cap did Scenaria say was 560p?  


The small black one labeled "560G 100V"

Quote from: "AnalogPackrat"

V1 is valve (tube) 1.  Look at the schematic and you'll see them labelled.  Three of the tubes are dual triodes so you'll see V1a and V1b for the two halves of V1.  The 6AQ5 is a single pentode.

A P


Sorry. That's a little confusing for me. I'm pretty new at this. It said something like "hook up the component to pin 3 of V1".

 I don't think I got a schematic. Is it separate from the manual?

Thanks,
Title: All things LA2a related
Post by: Greg on March 23, 2006, 07:27:24 PM
Guess what...

I just checked my 510p and 470p and they're labeled 510 and 470 on the cap itself, respectively. So I'd assume the 560 is 560p, but I that doesn't seem like standard cap marking to me. Maybe I need to brush up on my cap reading skills.  :roll:
Title: All things LA2a related
Post by: AnalogPackrat on March 23, 2006, 08:56:26 PM
I've seen both ways and always measure before soldering.  Standards  :roll:
Title: All things LA2a related
Post by: erland on March 24, 2006, 11:55:16 AM
When you measure a cap do you have to worry about it being charged afterwards?
Title: t4b issues
Post by: rove on March 24, 2006, 01:11:14 PM
not getting any K between any pins right now.  still have to run over it with the iron, looks a little sketchy in spots.  Busy with work at the moment, hopefully have time to mess with it soon...
thanks.
Title: All things LA2a related
Post by: AnalogPackrat on March 24, 2006, 01:43:37 PM
Quote from: "erland"
When you measure a cap do you have to worry about it being charged afterwards?


Nope.  We're assuming it has not already been soldered in place.  If it has, lift one leg (to isolate it from the rest of the circuit) and then measure it.

On your questions about the tube sockets...I think you need to brush up on your schematic reading skills a bit.  Don't take this the wrong way--everyone has to start somewhere.  Read the metas and google around for some examples.  If you look carefully at the bottoms of the tube sockets you will see embossed numbers beside each pin (you may need a flashlight and/or magnifier to read them).

A P
Title: Re: t4b issues
Post by: AnalogPackrat on March 24, 2006, 01:45:55 PM
Quote from: "rove"
not getting any K between any pins right now.  still have to run over it with the iron, looks a little sketchy in spots.  Busy with work at the moment, hopefully have time to mess with it soon...
thanks.


You mean it's open (infinite resistance) or shorted (near 0 ohms on your meter)?  When you made the thing did you really soak the solder into the base pins?  I takes a fair amount (and a lot of heat) to get the buss wire bonded well to the hollow pin--easy to have a cold joint in there.

A P
Title: All things LA2a related
Post by: erland on March 24, 2006, 01:59:01 PM
Quote from: "AnalogPackrat"
Quote from: "erland"
When you measure a cap do you have to worry about it being charged afterwards?


Nope.  We're assuming it has not already been soldered in place.  If it has, lift one leg (to isolate it from the rest of the circuit) and then measure it.

On your questions about the tube sockets...I think you need to brush up on your schematic reading skills a bit.  Don't take this the wrong way--everyone has to start somewhere.  Read the metas and google around for some examples.  If you look carefully at the bottoms of the tube sockets you will see embossed numbers beside each pin (you may need a flashlight and/or magnifier to read them).

A P


I agree. I don't really know how to read schematics too well.

I just didn't know if the bloo used a different schematic. I downloaded the LA-2A 1968 version and think I got the info I needed.

Thanks for your help.
Title: Re: t4b issues
Post by: rove on March 24, 2006, 02:58:39 PM
Quote from: "AnalogPackrat"


You mean it's open (infinite resistance) or shorted (near 0 ohms on your meter)?  When you made the thing did you really soak the solder into the base pins?  I takes a fair amount (and a lot of heat) to get the buss wire bonded well to the hollow pin--easy to have a cold joint in there.

A P

It appears to be infinite.  I think that will be the first thing I try is really heating up those pins and soaking them. thanks.
Title: All things LA2a related
Post by: raysolinski on April 03, 2006, 06:16:02 PM
I am having a problem with my LA2 and this thread seems to be a little more active than the "new la2" thread...My unit stopped passing audio..I did all the usual checking..grounds all appear to be good (visual and meter tested for breaks)....all the tube voltages are in the ballpark except pin 5 on my 6aq which is around 130 volts tube in, I expect 100 or so) my neon  lamp lights  and I can zero my meter in gr mode..My voltage at pin 5 of the t4b is 8 volts...when I switch to gain mode with the meter switch the voltage at pin 5 jumps to 29 volts...I will check voltages again but I am stumped! With no input or output cables hooked up the meter will jump to the end when I turn the gain up..This guy was working great on Thursday night..compressing beautifully :), meter working fine in both modes..the 6aq tube seems to be running a little hot (way hotter than the other tubes) so I only turn it on briefly for testing purposes...any insights would be greatly appreciated!

Cheers,
Ray
Title: All things LA2a related
Post by: CJ on April 03, 2006, 06:36:01 PM
AP, you need to get out  that house Now!

I would book a Holiday in Gatos for a week and let things drain.
OK, call me a wimp, but if you wake up in cold mud, don't say I didn't warn ya!
cj


Did you ohm out R14?  :shock:
Title: All things LA2a related
Post by: AnalogPackrat on April 03, 2006, 06:48:48 PM
Naah.  Just 2.75" in the last 24 hours.  Not that big a deal.  Wind was pretty strong early this morning, though.  My house is on top of the mud, so we'll just ride it down to Santa Cuz, see?  We have had a good bit of rain this year--my records show 60.5" since October 1, 2005.  Juicy!  45-50" is about normal up here (Oct-May).  El Nino can bring 80+.

A P
Title: All things LA2a related
Post by: AnalogPackrat on April 03, 2006, 06:53:53 PM
Quote from: "raysolinski"
I am having a problem with my LA2 and this thread seems to be a little more active than the "new la2" thread...My unit stopped passing audio..I did all the usual checking..grounds all appear to be good (visual and meter tested for breaks)....all the tube voltages are in the ballpark except pin 5 on my 6aq which is around 130 volts tube in, I expect 100 or so) my neon  lamp lights  and I can zero my meter in gr mode..My voltage at pin 5 of the t4b is 8 volts...when I switch to gain mode with the meter switch the voltage at pin 5 jumps to 29 volts...I will check voltages again but I am stumped! With no input or output cables hooked up the meter will jump to the end when I turn the gain up..This guy was working great on Thursday night..compressing beautifully :), meter working fine in both modes..the 6aq tube seems to be running a little hot (way hotter than the other tubes) so I only turn it on briefly for testing purposes...any insights would be greatly appreciated!

Cheers,
Ray


Seems normal to me.  Sketch out the meter circuit (R24-28, Neon, meter, etc) for both conditions of meter reading GR and meter reading output.  It should become clear why the voltage at Pin 5 changes like that.  The 6aq5 is working a bit, so it will get warmer than the rest.  Now go smush some audio!

A P
Title: Quick Output transformer question
Post by: emx on April 06, 2006, 10:36:42 PM
Hi, I've got an output transformer question.  I'm building an la2a and thought I would try cinemag trans for both the in and output.  I got the Cinemag CMLI-15/15b input and the CM-9589 output(Cinemag recommended it).  Basically what I've found is that the output(the 9589) has no center tap.  Is there a way around having this?  It's supposed to have one going to the terminal strip so is there a way to rig a substitute?
Title: All things LA2a related
Post by: CJ on April 06, 2006, 11:01:31 PM
Just snip the darn thing. It's a tap left over from the fifties.

Go end to end for your output signal.
Title: All things LA2a related
Post by: AnalogPackrat on April 06, 2006, 11:17:42 PM
I think your output is ok--just leave the CT terminal unconnected.  You only need it for unbalanced connections anyway (right?).  The other connections go to + and - signal.  I'm not convinced your input iron is going to work well, though.  Should be at least a 1:4 ratio if not higher (the original was 1:10).  Hopefully one of the real experts will chime in here...

A P
Title: All things LA2a related
Post by: CJ on April 07, 2006, 01:17:48 PM
What experts?
Please tell me your not calling me an expert?  :shock:
Dude, I'm like  high school edikated.

Man, your house almost came a few days a go. Two people hear at work got woken up by the rain from 2 to 5 am , right when everybody is sleeping.
They said it looked like a waterfall outside.

Actually, I have never seen your property so i don't know what the f i am talkin about, eh?

what is the transformer issue here?
Title: All things LA2a related
Post by: emx on April 08, 2006, 04:54:56 PM
OK, I'm actually thinking about just re ordering trans(Probably sowters) for the job.  Someone recommended that input tran of one of the forums for this unit for some reason.  Now that I'm looking at the papers on it I'm not so sure(it's a 1:1).   I think the output tran would do the trick but not the input. The output is a 4:1 I believe or maybe a 1:4 depending upon wich direction your looking through it. I would still like to have that center tap just for any possible use it might have so I'll probably get the sowters.  I'm attempting to build a stereo pair from the Cayocosta layout and want something I might be able to run a full stereo mix through when I get both running so if anyone here has suggestions other than the sowters for this use please let me know.
Title: All things LA2a related
Post by: CJ on April 08, 2006, 06:50:18 PM
What's wrong with the cinemags?
Can you link some data sheets for those guys?
Title: All things LA2a related
Post by: AnalogPackrat on April 08, 2006, 11:02:32 PM
Quote from: "CJ"
What experts?
Please tell me your not calling me an expert?  :shock:
Dude, I'm like  high school edikated.


And Ye Olde School of Life (experience)--it counts for something, dood.

Quote
Man, your house almost came a few days a go. Two people hear at work got woken up by the rain from 2 to 5 am , right when everybody is sleeping.
They said it looked like a waterfall outside.


See, that's why you hire a geologist before buying property up here.  It cost me a few bills to get: 1) a nice two hour on-site geology lesson, 2) site walk-through, 3) check of all known geo hazards, and 4) a 10 page official report of findings.  Worth every damn penny.  Of course there are no guarantees, but it's better than the alternative...

We had a couple or three of inches of rain.  No biggie.  Last December we had 11.25 inches in two days.  December 2002 we had 15+ inches in four days (three big storms in a row).  The first one packed 80mph gusts which knocked the tops out of a bunch of big trees and uprooted a 130' Doug Fir 60 feet from the house.  It fell away from the house and parallel to the driveway and took out a bunch of oaks and laurels on the way down.  Our power was out for five days.  That was a scary storm.

Quote
what is the transformer issue here?


emx here has some Cinemags for his LA-2a.  The outie looks ok, but the input he's got is a 15k:15k thing which doesn't seem right.  Here's a link (http://www.cinemag.biz/line_input/line_input.html) to the Cinemag line input datasheets.  He's got a CMLI-15/15B.

A P
Title: All things LA2a related
Post by: CJ on April 08, 2006, 11:21:35 PM
That 15/15B is absolutely perfect.

You want a 1:1 ratio so you can reduce the UTC A-10 1:10 ratio to the point where the gain knob does something useful.

But when you say 1:1, most joe's, probably with lace on their panties,  think 600:600.

Well, the 600 on the input is perfect, but 600 on a grid, well, that might be a problem.

But this Cine mag is 15K both sides. Our source wants 600 or above, so the 15K will be luxerious as far as work done on the pri side. And the 15K sec side will be better for the high impedance grid circuit.

That 20db rating should be plenty if I know these Reichenbach characters.
Over-stackin fools, nobody told them the price of Ni these days!
Title: All things LA2a related
Post by: analag on April 08, 2006, 11:39:19 PM
Quote from: "CJ"
That 15/15B is absolutely perfect.


CJ, now that's something we can both agree on.

analag
Title: All things LA2a related
Post by: emx on April 09, 2006, 04:43:36 AM
So what your saying CJ is that they both might work?  I should probably point out I'm new at this.  Also I'm wondering what the center tap was used for originally?
Title: All things LA2a related
Post by: CJ on April 09, 2006, 03:36:11 PM
The input will work great.

I just looked at the output specs and the output will work great.

0.1 % THD at 20 hz and 29 db is incredible.
Title: All things LA2a related
Post by: HappyTom on April 09, 2006, 05:06:00 PM
A couple of days ago I finished my LA2a but waiting on my VU and frontplate. I use a sifam AL39 and a really ugly homemade faceplate at the moment.

I power up and get:
Without tubes:
A24-A25 (Heaters) = 6.84
A1 (neon) = 54.9
B21 (Between CR1 and R29) = 372
B20 (Between R29 and R34) = 366

That seems to be good, now with tubes in:

A24-A25 (Heaters) = 6.something.. Bad note.
B21 (Between CR1 and R29) = 349
B20 (Between R29 and R34) = 270
A1 (Neon) = 54
A20 (V4 pin 5) = 73
A14 (Between R17 and C3) =225
A17 (V3 pin 6) = 96
A19 (V4 pin 6) = 115

V4 pin 5 is really low, the Bloo-manual says 131V...
And pin 5 is the only thing connected so there might be the tube?

I didn't find the issue with the low voltage on V4 pin 5 so i run some music thru the unit. No low freq and alot of hi freq..
Here is a freqsweep on my unit. Left channel is my soundcard and right channel is my LA2a:

 http://www.rollingthunder.se/la.jpg

I got Sowter in/out.
R29 is 4K7 2W
R34 is 22K 2W

Any ideas?

HT
Title: All things LA2a related
Post by: AnalogPackrat on April 09, 2006, 06:11:03 PM
You haven't installed your T4b, right?  If not, then the HF rolloff is somewhere in the main audio path (V1 and V2).  What did you use for C5?  Double check C1, 2, and 3.  What voltages are you getting around V1 and V2?

As for the V4 pin 5 thing...did you use 1k for R36?  You didn't install R38 or C13, right?  Did you adjust R3 and R37 as it said in the manual?

A P
Title: All things LA2a related
Post by: HappyTom on April 10, 2006, 05:56:26 AM
No T4b.
C5 is 20uF 450V sprague atom.
R36 is 1K
R38 and C13 is installed as written in the manual, 22K between V4 pin 6 and center terminal of V4 then a link from center pin to C7d (+) and 510p between V4 pin 6 and r3 pin1.
R3 and R37 is fully clockwise. I read it in this thread, it is not mentioned in the manual.
As I don't have an internet connection in my "lab" I'll take some measurements today and post tonight.
I'll check all things around V1, V2 and V4.

Thanks!

HT
Title: All things LA2a related
Post by: HappyTom on April 10, 2006, 03:14:56 PM
OK, 1 down 1 to go.
As you (AP) said I should look at C1, C2, C3 and I had one fault there.
I had no connection between C3 and R17. I look in the manual again but I didn't find it mentioned. Now it's connected with two gatorclips and a wire.
Put some audio thru it again and no difference at all.

As suggested before in the thread posted by CJ I traced the circuit and marked all connections with a pen in the shematic.
I found one more sucker at the inputtransformer. No connection from the black wire to ground.
Put audio thru it again and there it is, now i got the lowfreq.
Here is a new sweep:
http://www.rollingthunder.se/saker/LA2a.htm
Left channel is soundcard, right channel is LA2a.

No change at V4 pin 5.
Is my 6AQ5 drawing to much current and it might be a failure in the tube?
Can i test it someway?

HT
Title: All things LA2a related
Post by: HappyTom on April 13, 2006, 05:15:48 PM
I've poked around in the unit and i don't get it, mostly because I don't know how a tube works.
I know that pin 5 on the 6aq5 is the plate, but i really don't know what the plate does or how it connect with the other pins at the tube, I'll think I have to do my homework.... Til' then I might need some help with this. I did these readings with no t4b, no signal and all tubes in.
Code: [Select]

Pin    V3        V4
1      97        NC    
2      0         8,7      
3      0,8       Heat              
4      Heat      Heat                  
5      Heat      73,2                    
6      97        115                
7      0         0            
8      0,8                                  
9      Heat                            



Do you guys see something funny?

Thanks
HT
Title: All things LA2a related
Post by: CJ on April 13, 2006, 06:43:16 PM
Looks reasonable.
Title: All things LA2a related
Post by: The Kid on April 13, 2006, 09:00:34 PM
Quote
R34 is 22K 2W


I don't know what schem you have, but on mine, it's supposed to be 10K; which would explain the low voltage.  Think about it; if you put a higher value resistor in, you'll have an increased voltage drop across the resistor, which would in turn lower the voltage at pin 5.  (Kirchoff's law.) Algebra in action!   :twisted:
Title: All things LA2a related
Post by: CJ on April 13, 2006, 09:29:53 PM
10 K and 22 K were both used depending on the year, which is listed in the back of Jahnsen's book.

. Probably becuase they ran out of resistors.

The cathode resistor will bias the tube correctly, no matter what size the plate resistor. Well, within reason. A 1 meg plate resitor would not be good.
Title: All things LA2a related
Post by: AnalogPackrat on April 13, 2006, 09:38:18 PM
Quote from: "HappyTom"
No T4b.
C5 is 20uF 450V sprague atom.
R36 is 1K
R38 and C13 is installed as written in the manual, 22K between V4 pin 6 and center terminal of V4 then a link from center pin to C7d (+) and 510p between V4 pin 6 and r3 pin1.


If you are using 22k for R34 and 1k for R36 on V4, you shouldn't have R38 and C13 installed.  Those two were added with a later rev that changed R34 to 10k and R36 to 470R.  These details are in this thread, so search it.  This may well be what is making your V4 voltages a bit wonky.  Someone posted a table of revisions to the LA2a somewhere--probably also in this thread.  Use Ohm's law to calc current through R34 and R36 and you'll a) learn something and b) determine whether your V4 is behaving within the realm of happiness.

As for your last table of voltages--check that you did not wire R3 and/or R37 (or any of the pots, for that matter) backwards.  Otherwise, it looks close.  I think you could safely install the T4b and give it a try.

A P
Title: All things LA2a related
Post by: The Kid on April 13, 2006, 09:44:00 PM
Like I said, that resistor is wrong, for the parts listed.  That 135 volts at the plate only holds for the 1968  schem.  Do the math!  :wink:  :sam:
Title: All things LA2a related
Post by: CJ on April 13, 2006, 10:15:40 PM
What math is this?

Just plug it in, make some noise, compress the noise, then bow to the west. :razz:
Title: All things LA2a related
Post by: The Kid on April 13, 2006, 10:20:28 PM
Hmmm, well, I like the first part, but I'm not Muslim, so not sure about the bowing.  Unless Led Zep is back in LA!   :grin:  :sam:
Title: All things LA2a related
Post by: CJ on April 13, 2006, 10:42:51 PM
I invented the T4, you idiot!
Now down on your dirty knees!  :razz:

And do it right. I don't want to have to call Shaggy. :grin:
Title: All things LA2a related
Post by: The Kid on April 13, 2006, 11:29:25 PM
1000 pardons, oh great one!  

Speaking of Shaggy, I saw some Scooby Snacks at the store the other day.  Seriously!  In the pet section, but that made me wonder if that was an accident waiting to happen, on account of Shaggy used to eat 'em too!  (impressionable young kids?)   :grin:  :sam:
Title: All things LA2a related
Post by: CJ on April 13, 2006, 11:35:58 PM
I was talkin bout the cypress hill shaggy, but that's alright.
Jus kiddin on the T4 raz, ya know.
It was really Lawrence.
It's a 3 day weekend for bonzo, so ....
Title: All things LA2a related
Post by: The Kid on April 13, 2006, 11:43:36 PM
Yeah, I know you were razzin'; (if you think I was actually bowing, well. . .!) As far as Shaggy, the only new rap I know of that I actually kinda liked was that 99 problems and a ***** ain't one.  And, of course, by that I mean the voltage on pin 5 is definitely too low.  (Trying to segue back into the topic!)   :grin:  :cool:  :green:  :sam:
Title: All things LA2a related
Post by: CJ on April 14, 2006, 12:24:13 AM
Dude, do you know how many people get freaked by that AQ5 screen?

Every dang one of them!
I would like to split the forhead of one Dr. Lawrence, the nut who messed that simple screen circuit up so bad that it looks like a monkey on a typewriter invented it.
If you don't believe me, read this whole thread.
Then head over to the LA2 pc bord thread and read that.
Then do a search on LA2 and read that.
There is more stuff on the LA2 in this forum than in the darn patent office and the UA files combined!

Then there's the stuff at the "other" place, but we don't  go there no mo becuase this is a Happy place, right?

Sing it, everybody:

"Evrything is beautiful, in it's own way..."

I sing that every time I pass a really knarly car wreck with body parts all over the place.
Title: All things LA2a related
Post by: The Kid on April 14, 2006, 12:31:29 AM
Dr. Lawrence messed it up, but, according to what I just read, the guy you wanna blame for the T4 appears to be RE Halsted!  (1958, 8 years before Lawrence!) Well, 4 before he filed.

 :shock:
Title: All things LA2a related
Post by: CJ on April 14, 2006, 12:37:08 AM
There's an old saying,

"The first guy dosen't have a chance..."
Title: All things LA2a related
Post by: The Kid on April 14, 2006, 12:42:37 AM
You got that right!  Halsted must not have been into audio!  Well, that's the first I've heard of him!
Title: All things LA2a related
Post by: CJ on April 14, 2006, 12:45:41 AM
Actually, if you read up, you will discover that most inventions we use in our modern society came from ideas meant to kill people.
The T4 is no exception.
It comes from proximity fuse type technology, Lawrence was using his brain to make weapons, like most of the bright minds were back inthe cold war days.
Title: All things LA2a related
Post by: The Kid on April 14, 2006, 12:51:42 AM
Yeah, readin' it right now.  There's a show on cable about that, how military intended stuff becomes civilian.  What the hell's it called?
Title: All things LA2a related
Post by: HappyTom on April 14, 2006, 06:13:57 AM
Thanks!
To clear things up, i built this unit using the Blootech-manual. The kit comes from Blootech. I followed the instructions and was not aware i had to follow a schematic aswell.
I read this thread before i found my problems, that was like 12h work...
The manual might be unclear or I might be Swedish and got a hard time understand what's written in the manual.
Steve: Is there anything in the manual that I don't understand?  :grin:

Quote from: "AnalogPackrat"


If you are using 22k for R34 and 1k for R36 on V4, you shouldn't have R38 and C13 installed.  Those two were added with a later rev that changed R34 to 10k and R36 to 470R.  These details are in this thread, so search it.  This may well be what is making your V4 voltages a bit wonky.  Someone posted a table of revisions to the LA2a somewhere--probably also in this thread.  Use Ohm's law to calc current through R34 and R36 and you'll a) learn something and b) determine whether your V4 is behaving within the realm of happiness.

As for your last table of voltages--check that you did not wire R3 and/or R37 (or any of the pots, for that matter) backwards.  Otherwise, it looks close.  I think you could safely install the T4b and give it a try.

A P


I'll take out R38 and C13 and link V4 pin6 to C7d and see what happens.

Thanks guys!

HT
Title: All things LA2a related
Post by: The Kid on April 14, 2006, 11:01:10 AM
Quote
not aware i had to follow a schematic aswell


Well, that's what's great about electronics; it has a universal language, so it doesn't matter what country you're from.  It's great to build by the numbers (kits), but schematics are an important part of the process, especially in one like this with various revisions.  

Schematics are not always correct, either, but a good understanding of electronics, (basic circuits, what each part does) will usually help you find the problem.  And, as you have learned here (hopefully) they come in really handy for troubleshooting!  (Because even if you get really good at this, you'll still make mistakes, like everybody!)  :thumb:  :sam:
Title: All things LA2a related
Post by: ytrehalf on May 02, 2006, 11:39:46 PM
Can anyone recommend a 2 pole 3 position switch for the LA2a?

I'm also looking for vari-caps.
Title: All things LA2a related
Post by: babyhead on May 16, 2006, 07:20:23 PM
I am just about to piece together my LA2As  was thinking of a mod and I wanted to bounce the idea off of the forum before I commit it.

I have a couple of Cinemag 150:10k mic inputs and was thinking of putting one in place of the HA100x and a JLM Go between to make a mic input for the LA.

Will the 40db of gain from the LA with the step up be enough?

Would there be problems with the signal being too weak at the T4 attenuation point or the 100k pot?

What do you guys think?

 :guinness:
Title: All things LA2a related
Post by: babyhead on May 19, 2006, 11:09:53 PM
Shameless, self-serving bump!
 :guinness:
Title: All things LA2a related
Post by: Bluzzi on June 05, 2006, 08:13:11 PM
I am starting to wire my LA2A (finally) and I am using Cayocosta's layout.

The T4B comes with a matching resistor that is supposed to replace R25 on small turret board near meter.

Right now R25 is 33k. Does anyone know if should I be replacing it with the one that comes with the T4B? Or do we ignore it?

Thanks.

Jim
Title: All things LA2a related
Post by: CJ on June 06, 2006, 01:37:27 PM
Use it.

Calibrating is the last thing you will do. Have fun first, then worry about your meter reading 1% accurate db.

I dial it by sound, so meters are usually fun to watch, but ignored.
Title: All things LA2a related
Post by: Bluzzi on June 07, 2006, 10:15:34 AM
Thanks CJ. For now I'll keep it and try it after finishing the project.

Jim
Title: All things LA2a related
Post by: Bluzzi on June 09, 2006, 01:46:45 PM
Moving on, I finally got some time to do some more wiring. My turret boards are done and I am onto the back part (tubes & transformers). I'll reserve the front panel and turret hookup for last.

My wiring is starting to get the spaghetti look and so I am worrying about hum problems because of my wire routing. I am trying to keep as close to Cayocosta's layout. (some unmarked components there by the way).

Does anyone have any hints/tips as to what wires to keep from each other (ie: ac power from input or output wires).

Also I am missing C13. Its a 510pf cap. Can I use a 470pf or 560pf in its place?
I am not sure if its part of the stereo part of the circuit (which I would do without if I was certain of which components to exclude)

jim
Title: All things LA2a related
Post by: punk on June 09, 2006, 06:55:24 PM
Bluzzi,

check the resistor and you will probably find that it's 33k. You should use it if it is different but CJ's advice is right on.

C13 wasn't used in earlier LA2's so I can't imagine it making a big difference between those values.

You have the right idea about the wiring, seperate signal and power wires. I'm not familiar with Cayocosta's layout but think it out and plan your routes before firing up the iron.

good luck,

byron
Title: All things LA2a related
Post by: Bluzzi on June 09, 2006, 11:01:01 PM
Thanks Byron. I'll put in a 470pf for the time being. Even for the variable capacitors I am using a fixed value.

I know to keep away from AC power but I am afraid of the tube heater feed wires. I am trying to keep them along the bottom corner and everything else higher up.

I guess you would have to be a super neat freak to avoid any kind of proximity. Also I believe crossing wires at 90 degrees diminishes hum.

I'll just have to finish one and take it from there. I may be a bit too timid as its my firs DIY since I built Dynaco and other kits. My first kit was a shortwave radio and was way beyond my abilities, plus it came with the worst instructions. I f...ed it so bad I couldn't even tune in a local station!

After that all my projects worked. Its just been so long I've lost some of my feel for wiring point to point. But my soldering skills are much better though.

I am almost there and the anticipation is killing me! I hope I don't muck this one up!

scared but excited.

jim
Title: All things LA2a related
Post by: sysexguy on June 11, 2006, 10:44:17 AM
Please forgive me if this had been discussed before (search only gave a partial mention).

Why does one have to ground the centre pins of the tube sockets? I have assured my sockets are properly secured and that the tube shields, once complete will be at chassis ground.

btw, side note, I found a great source for hardware: www.rtlfasteners.com
great value, great service...and ships to Canada!!!!!!!

TIA

Andy
Title: All things LA2a related
Post by: CJ on June 12, 2006, 02:52:59 PM
More mechanical than anything else.
In RF or high voltage work, you sometimes need to ground what they call "floating metal".

This is anything made of metal that is not grounded.
Title: All things LA2a related
Post by: Bluzzi on June 15, 2006, 10:10:14 PM
OK, I'm almost there on my first unit. Just have to wire up front panel and double check my connections.

Past threads advise on leaving the T4B out until all checks out.

So now the question is should I turn it on with the tubes in or out ?



jim
Title: All things LA2a related
Post by: Bluzzi on June 16, 2006, 09:33:05 AM
OK, I am re-reading this entire thread and the tubes can be in from the beginning.

I'll be turning it on as soon as I verify my wiring. So if you hear the entire East coast has had a power failure you'll know when I flipped the switch.

jim
Title: All things LA2a related
Post by: synthi on June 16, 2006, 10:52:28 AM
Good luck!!!

Synthi
Title: All things LA2a related
Post by: drpat on June 16, 2006, 12:40:29 PM
...
Title: All things LA2a related
Post by: drpat on June 16, 2006, 12:41:35 PM
...
Title: Hello
Post by: Pierrelegere on June 16, 2006, 05:17:56 PM
Hi, I was reading this forum for some months, and I really like the attitude here, so I decidet to join! I did some modification on old German broadcast units (V276, V372...), and now I will start my first DIY project, either the La2a or the ssl clone... I wonder if anybody here is located in Germany / Europe, and has some experience with using Haufe or Pikatron (often used in German modules) transformers. They are availible here for cheap money, so maybe someone has already made some experiments???
Title: All things LA2a related
Post by: The Kid on June 17, 2006, 03:11:33 AM
Pierrelegere, Welcome to the forum!

And, yes, there's lots of folks in your neighborhood here; they'll probably be along momentarily!  :grin:  :thumb:  :sam:
Title: All things LA2a related
Post by: fum on June 19, 2006, 03:51:24 PM
allright, need some troubleshooting wisdom.

One of my la2a's is doing this thing that I need to track down.  

At random time intervals, it will make a pop, and then go back to merrily workin fine.

It could be 5 minutes, 10 minutes, etc., between pops.  Sometimes two pops within 15 seconds, and then some time gap before it comes up again.

thoughts?

regards

ju
Title: All things LA2a related
Post by: fum on June 21, 2006, 08:25:50 PM
And the answer lies in the chopsticks, young grasshopper.....

thanks CJ  :thumb:
Title: All things LA2a related
Post by: CJ on June 21, 2006, 11:16:27 PM
Kung Pao!
 :thumb:
Title: All things LA2a related
Post by: khstudio on June 21, 2006, 11:27:01 PM
Quote from: "fum"
allright, need some troubleshooting wisdom.

One of my la2a's is doing this thing that I need to track down.  

At random time intervals, it will make a pop, and then go back to merrily workin fine.

It could be 5 minutes, 10 minutes, etc., between pops.  Sometimes two pops within 15 seconds, and then some time gap before it comes up again.

thoughts?

regards

ju


Fum,
Try the easy stuff first:

 - Change 1 tube at a time.

 - I think the LA-2A will work without the T4B installed so you can try that to eliminate it being the cause.

 - Take it out of the rack & open it up, turn on, pass signal & listen while "Carfully" poking around & wiggling tubes to see if you can find it.

I find over %50 of my problems with guitar amps this way... give it a little beating to make sure it's tight with good connections.

You probably knew all this... just trying to help.

Kevin (The guy you helped last year with my LA2A... it's been kickin ass ever since, Thanks again for the tips)
Title: All things LA2a related
Post by: fum on June 22, 2006, 12:11:20 AM
guess my post was kinda cryptic (but CJ reads minds).  I fixed it, with chopsticks.

Left it open while I was workin, whenever I'd hear a pop, I'd take the chopsticks, and move some of the wires that were close to each other, re-arranging, cleaning etc.

Leave it for some time, if I hear the pops again, repeat.

After the fourth iteration, no more pops.  Have no idea which wires were causing the problem, but have been contemplating the phrase "silent as the grave" for the last several  hours , so am feeling pretty good about it =)

Regards

ju
Title: All things LA2a related
Post by: Bluzzi on June 22, 2006, 03:35:07 PM
Wiring now checked & I caught 1 wire on the meter switch that was on pole 1 instead of 3 but I believe it owuld not have been a problem (only the switch would have worked backwards).
Corrected anyway. Also I had the Neutral wired into the fuse and switch! That I corrected right away and now my Hot is on fuse "& switch. All now as per Cayocosta's layout.

Only one thing is bothering me. On my VU meter there is no marking for the positive or negative. Its an old Weston meter. Is there a way to know which one is which? If its reversed can I blow the meter?

I'll be turning it on in tonight. Fingers crossed and chicken sacrificed to the God of DIY.

Thanks.

Jim
Title: All things LA2a related
Post by: Bluzzi on June 22, 2006, 10:08:24 PM
OK, turned it on without tubes and nothing blew up! Fuse still good (.5A SloBlo)!

Then tried with tubes and all seems OK, no explosions or parts burning up! I left it on for 5 minutes with same results.

Only thing is the VU meter needle is at -2 and wobbles back and forth, almost like a 60HZ AC is getting in there?!

I'd like to start measuring voltages soon before I plug in the T4B but this VU thing has me baffled. Could it be that it needs a diode like some meters?

By the way the meter adjust pot does nothing to change the location of the meter needle.

I'm so close now I want to get going but I don't want to make mistakes because of my excitement.

Any ideas about the meter? HELP!

Thanks

jim
Title: All things LA2a related
Post by: imo on June 23, 2006, 04:49:36 AM
I am using the allied power transformer supplied with the Bloo kit(6K88VG). I have the whole unit hooked up, have gone through the wiring a couple of times and I am pretty confident that everything is where it should go. Upon powering up I noticed a funny smell, neon didn't light, so I turned everything off and checked connections again. I noticed that I was getting continuity between ground and the B+, measuring between turrets a 22 or 23 and ground. I disconnected the transformer and found that the continuity was coming from the transformer itself, between both (250v)red's and the red/yellow center tap, as well as between the greens(6.3) and the green yellow center tap. I had the power transformer hooked up as I think it was supposed to be, one black to turret and switch, other to neutral on the power inlet. One red to turret a22, other red to turret a23, green to a24 and a25, center taps of both to ground terminal between the 2 33u power caps.
Once I took the power transformer out everything else tested right for contintuity throughout the unit. I hooked the transformer up, this time only to the inlet and switch, and tried measuring AC volts between center tap of the 250 and either of the red wires. I got a negligible reading. Also the smell that I had smelled the first time was there, definitely coming from the trannie. It was really hot to the touch. I am thinking that somehow the transformer fried. Does this sound copasetic? I have built quite a bit of equipment, but generally non tube, using small toroidal transformers. I am a bit psyched out by this and wanted to make sure that I proceeded carefully. Any ideas or directions would be taken quite gladly
Peace
Ian
Title: All things LA2a related
Post by: Bluzzi on June 23, 2006, 09:20:15 AM
I am not sure but I think continuety in a finished circuit doesn't indicate much other than ...well continuety. Great for simple wiring but not where current can flow all over.

Ian, its usually hard to fry a transformer unless you seriously short the outputs.

What are the voltages between the center tap of the 250v (red/yellow) and either red wire?

Also same between filament heater voltage (green/yellow) and each green?

Hope it checks out.

Also for better safety make sure you wire the HOT ac into the fuse. I made that mistake and had the neutral going to fuse. Meaning HOT was always there with potential (no pun) to cause problems with other gear.

I am also at the end as you can see. Hang in there.

jim
Title: All things LA2a related
Post by: imo on June 23, 2006, 01:12:34 PM
Jim,
Thanks for the response, i should have been more clear. I was trying to look at the power transformer, as i am pretty sure that is the problem. I was getting continuity between the 250 leads and the center tap, as well as between the 6.3 leads and the respective center tap, even out of circuit.
I am pretty sure that there is no polarity on the primary side of the Allied transformer i am using. If there is, there is no marking at all, simply two black wires. I pulled the transformer completely out of circuit, only running it through a switch and IEC, tried to measure voltage, and was only able to get about 1-2 V AC between the two hot 250v leads. I was about to measure between the hot and the center tap when the transformer started smoking again. I am pretty sure it has thermalled or something, just not sure how it happened? It gets really hot really quickly. I used the Allied  6K88VG, which has two black primaries, a grey shield, two red 250v, two green 6.3's, a red/yellow center tap, and a green/yellow center tap. I tied both center taps to the ground buss, the two red's to the back side of the diodes, and the two greens attached to the heater supply.
I tested for continuity between both the B+ power and ground and the heaters and ground. Everything seemed to be properly isolated
Ian
Title: All things LA2a related
Post by: Bluzzi on June 25, 2006, 11:51:30 PM
Ian, did you get your problem solved? I have the same transformer on my second LA2A that is not hooked up yet and I could test it to see if I get same readings.

jim
Title: All things LA2a related
Post by: imo on June 26, 2006, 01:46:51 PM
Jim,
I am waiting on another power transformer. Short of a component failure i think everything in the unit is wired correctly and hopefully will work with the new tranformer. I appreciate your help and will post, hopefully tonight with some actual working voltage.
Ian

BTw, lost $50 to Albert Collins playing blackjack when i was 17<g>
Title: All things LA2a related
Post by: Bluzzi on June 26, 2006, 02:25:10 PM
Ian,

I missed Albert Collins two years in a row thinking I'd catch him. He passed away the third. What a fool I was. One of the most underrated Blues guitar stylists that was. You are indeed a lucky man to have lost $50 to him. I would have lost a lot more just to be able to jam with him, or play cards.

Have you checked any of the components for failure? You are sure the diodes are on the right way? I'm sure they are but you never know as you suspect transformer failure which I still think is highly unlikely (unless you had a real solid short before the fuse).

I am going to try posting a seperate message for my problem as I'm not getting any responses (my meter needle is oscillating in -10 and +4 mode).

jim
Title: All things LA2a related
Post by: Bluzzi on June 27, 2006, 08:51:29 PM
Ok, here I go again, somebody p l e a s e.....HELP! :sad:

I'm totally baffled. Probably something totally idiotic on my part but I can't find it. Checked my wiring and it all looks good and solid. At least nothing blew up or burst into flames. Even .5A fuse never blew.

Problem -
Meter is still showing same symptoms (needle wobbles when in -10 or +4 mode).

I've switched to another meter and same thing occurs. I've taken some voltage readings as follows.

All tubes are in. T4B out.

Here are voltages so far.

Heaters: 6.4

Turret board:

A14 = 250
A19 = 123
A20 = 132
B20 = 265
B21 = 354

V1:
1 = 102
3 = 1
6 = 102.6
8 = 1.23

V2:
1 = 58
3 = 3.6
6 = 248
7 = 58
8 = 100

V3:
1 = 72.2
3 = .834

V4:
2 = 6.41
5 = 124.3
6 = 116.6

Pin 1 of V2 seems low and pin 6 of V2 looks high. The other voltages seem close enough to Dave Jahnsen's book. I used Cayocosta's layout.

Any help would really, really , really be appreciated.

Thanks.

jim
Title: All things LA2a related
Post by: AnalogPackrat on June 27, 2006, 11:08:10 PM
Maybe it's oscillating or motorboating.  Do you have a scope?  Can you take a look at the output?  Did you use a Sowter output transformer?  If so, you probably need 1k grid stoppers (in series) on the 12BH7 grids.

Good luck,
A P
Title: All things LA2a related
Post by: Bluzzi on June 27, 2006, 11:43:47 PM
Motorboating is what it looks like. I haven't tried to put a signal into it yet and that was my next step after trying another meter.

I don't have a scope yet but I can get hold of one as a last resort.

I am using A10 and A24 transformers.

Thanks for the good luck wishes. I know its not easy to help in this way and I appreciate you trying.

jim
Title: All things LA2a related
Post by: Bluzzi on June 29, 2006, 06:14:26 PM
I changed all the tubes to rule out any bad tubes and still the same effect.

I tried plugging in a microphone (I heard the LA2A has enough gain to drive one) and the meter is not affected at all whatever level I set.

I think at this point I will have to re-verify my wiring entirely again as well as parts values. I took it for granted that resistor and capacitor values were as per label.

From what I've read so far about motorboating it can be caused by insuficient power supply filtering. I'll start by verifying the 30uf and  40uf caps in the power supply were wired correctly, although voltages check out so I doubt they are bad.

My front panel is detached and the wires leading up to it a bit longer, could that be the problem? I've tried moving all the wires around without any effect.

Anyone have any ideas here? Would pictures help? Thanks.

jim
Title: All things LA2a related
Post by: AnalogPackrat on June 29, 2006, 06:48:19 PM
A couple of photos might help.  Some of the eagle eyed might spot something.  What is the frequency of the meter movement?  Does it just move evenly and smoothly (like a sinewave thing) or does it jump up and down (like a square wave)?  Does anything you do change the frequency of the movement?

I wonder if you have a node somewhere that's supposed to be grounded that is floating.  Did you use star washers under your ground lugs to the chassis?

A P
Title: All things LA2a related
Post by: Bluzzi on June 30, 2006, 02:01:38 PM
The meter moves about 2 or 3 swings per second mostly but...yes it does change when I start to increase or decrease the Gain.

I have a from panel that is seperate from the back panel. They are both aluninum and All ground points have star washers. But I will check my grounds again, you never know.

I am going to be re-checking all my wiring again when I have some uninterupted time. If that checks out then I will check out all component values just in case I received a wrong resistor.

Thanks AnalogPackrat your help is much appreciated.

There is a lesson here for me to learn. I just can't see it by myself.

jim
Title: All things LA2a related
Post by: John Peacock on July 05, 2006, 10:13:48 AM
Hello all.

I am a total noob.  I just completed my BLOO, and using this forum I have managed to change it from a fuse blowing machine into something that actually powers up and passes audio.  Still some issues though...

1.  I have a Hoyt meter installed and am trying to hook up the lamp.  It looks like I just take some wires off A24 and A25 (tube heaters).  But how do I mount the lamp for the Hoyt?  I also have a lot of other wires that pass on top of the VU.  Do I need to be concerned about noise from the VU bulb and its wires?

2.  With tubes in:
A24 A25 = 6.53V AC
B21 = 346V
B20 = 267V
A1 = 56V
A20 = 97V
A14 = 222V
A17 = 95V
A19 = 122V
 
The only one of major concern for me is 97V for A20 (back side of R34).  However with my intial testing I was getting 81V there, so I replaced the resistor I was using (it said it was 22K, but it was measuring almost 25, so I replaced it with a Radio Crack Resistor that measures around 21.7) and got A20 up to 97V.  However, this is still low.  Also, when I changed that resistor, A19 went from 112V to 122V.  This doesn't concern me too much, but A20 is still quite a bit off from the recommended 131V.  Thoughts?

3.  Since things seemed "close enough," I popped in the TB4 and ran some audio.  The good news is that audio passes.  And until I crank peak reduction up to about ninety, it sounds very clean.  Then around ninety it begins to distort just a bit.  The problem is that it is NOT compressing.  At all.  As I increase the peak reduction, nothing changes except for that distortion around 90.  TB4 problem?  Wiring?

4.  And finally, when I turn peak reduction up to 90, I get distortion when in compress mode but not in limit mode.  This happens whether I am in +10, +4 or Gain Reduction.  Could this possibly be normal?

Sorry for the long post, and thanks so much for any help.

-Jp
Title: All things LA2a related
Post by: John Peacock on July 05, 2006, 04:49:32 PM
Just to update, I did a check on my TB4 using a flashlight in a dark room, and I get tons of gain reduction that way.  So I am guessing I either have a bad electro-fluorescent panel, or I have some bad wiring going to (or inside) the TB4.

Thoughts on how to confirm this?  Other things I should look for?

-Jp
Title: All things LA2a related
Post by: John Peacock on July 05, 2006, 05:35:51 PM
Okay...  I'm feeling very close now.  So I found at least one of the problems.  The wire that eventually goes to pin three on the TB4 socket was connected to pin 5 instead of pin 6 of V4 (6AQ5A).

I think that should ideally fix the "lack of compression" problem, but now whenever I turn my gain reduction past 20 (or basically whenever the unit begins to compress the audio) I get this aweful sizzling distortion with lots of HF content.  This is true whether I am monitoring +10, +4 or in gain reduction mode.  This also pins my meters.  This isn't what the grid resistors are supposed to correct?  Seems more like it might be a bad tube...  But then it wouldn't only do this at higher gain reductions, right?  I really don't know...  This is all very new to me.

Did I possible fry something by having that wire from pin 3 of the TB4 connected to the heater wire?

Oh, and this correction did not correct my low voltage at A20.  It has actually now gone down to 95.5V instead of the recommended 131V.  I'm still not overly worried about this one, but maybe I should be.

Anyone?

-Jp
Title: All things LA2a related
Post by: John Peacock on July 06, 2006, 01:37:14 AM
I've added the grid stoppers (1K Resistors) to pins two and seven of the 12BH7A.  No difference.  Still getting aweful HF distortion when the unit starts to compress.

I also tried running audio while leaving the lid off of the TB4 unit.  Using a flashlight I was able to get the meter to show -20dB, yet the audio running through the unit did not drop at all.  That doesn't seem right...

I feel like I've done a pretty good job of trouble shooting on my own, but I'm running out of ideas.

Help.

Please.

-Jp
Title: All things LA2a related
Post by: John Peacock on July 08, 2006, 11:07:49 AM
It's been a couple days.  I did manage to find one other small mistake with the placement of R6 around the input transformer, but this correction has had no impact on the unit's performance.  Still lots of HF noise whenever I try to compress.  Also using a flashlight to trigger the TB4, I was able to detect gain reduction using an external metering source, though nowhere near the 15-20dB displayed on the LA2A meter.

My A20 voltage is still very low, and I suspect this may be part of my problem, but I don't know.  That's why I am asking for your help.

Please!

-Jp
Title: All things LA2a related
Post by: Bluzzi on July 08, 2006, 02:41:07 PM
John, I feel for you my friend. As you see I too have some problems so I am not in  a perfect position to help you but I can try as best I can.

I'll be borrowing an oscilloscope this coming week and I am sure I'll pin down the problem. So hopefully I'll be in a position to help you.

I built it acording to Cayocosta's layout which I believe is very close to your Blue kit.

Have you tried to reach Scenaria privately? Or CJ.

Its also summer and so maybe people are taking a break, or maybe they are just sick of answering questions. Not an easy thing to do this thing of trying to pinpoint problems through words.

In the meantime if there is a wy for me to help you let me know. I'll do my best.

Hang in there.

jim
Title: All things LA2a related
Post by: AnalogPackrat on July 10, 2006, 11:52:13 AM
Quote from: "Bluzzi"
The meter moves about 2 or 3 swings per second mostly but...yes it does change when I start to increase or decrease the Gain.


Does the meter movement change frequency or amplitude when you change the gain?  It sounds like your problem is in the audio amp--V1 and V2 somewhere.  Carefully check the feedback stuff: R11, R21, R22, and C4.  What did you use for C4?

Quote
I have a from panel that is seperate from the back panel. They are both aluninum and All ground points have star washers.
 

Is there continuity between the front panel and the rest of the chassis?

Quote
If that checks out then I will check out all component values just in case I received a wrong resistor.


I don't know if you did this, but try to get in the habit of checking component values as you build (just before soldering the thing, measure it).  This forces you to think one more time about what you are doing as well as giving you that sanity check in case of a bad component.

Quote
Thanks AnalogPackrat your help is much appreciated.


No problem, Jim.  I hope you can get your build up and running soon.

A P
Title: All things LA2a related
Post by: AnalogPackrat on July 10, 2006, 12:08:39 PM
Quote from: "John Peacock"
It's been a couple days.  I did manage to find one other small mistake with the placement of R6 around the input transformer, but this correction has had no impact on the unit's performance.  Still lots of HF noise whenever I try to compress.  Also using a flashlight to trigger the TB4, I was able to detect gain reduction using an external metering source, though nowhere near the 15-20dB displayed on the LA2A meter.

My A20 voltage is still very low, and I suspect this may be part of my problem, but I don't know.  That's why I am asking for your help.

Please!

-Jp


If you can pass audio without GR, it sounds like your problem is not related to V1 and V2 circuitry.

Pop the cover off the T4B, hook up a signal of some sort to the input, turn off the lights and look at the EL panel.  Does it change brightness with the input signal strength (it's not really all that bright, so make sure you're room is dark)?  If it's pulsing in brightness with the music/signal, then your sidechain is probably working.  Perhaps the wiring on your T4B or its socket has a small mistake.  Did you wire the "X" and solder the point where the two wires that form the "X" cross?  Is the X grounded at some point?

A P

A P
Title: All things LA2a related
Post by: Bluzzi on July 10, 2006, 12:16:24 PM
The meter moves in amplitude but frequency remains the same. I used a 330pf cap for C4. I'll be checking R11, R21 and R22.

Yes there is continuety and I also checked ohms and ground is not my problem I think as I checked and re-checked all my grounds.

I put a 1khz tone in and listened on output. The tone is there and undistorted along with motorboating. The motorboating disappears when the gain is at max.

I think (can't remember) I did check the resistor values before wiring. There is still possibility of ruining a resistor or cap when soldering because of heat but I always solder using an alligator clip as a heat sink between resistor/cap body and solder point. But you never know.

I'll check my part stock first and then I'll have to unsolder parts to get correct readings I guess.

Might be nothing but last time I measured voltage the motorboating stopped when I probed pin 6 of V1 and pins 2 and 8 of V2. Grounding?

Thanks again AnalogPackrat, I'll focus on the area you mention. Get back to you with findings. (by the way, my T4B is not in).

jim
Title: All things LA2a related
Post by: AnalogPackrat on July 10, 2006, 12:45:51 PM
Quote from: "Bluzzi"
The meter moves in amplitude but frequency remains the same. I used a 330pf cap for C4. I'll be checking R11, R21 and R22.

I put a 1khz tone in and listened on output. The tone is there and undistorted along with motorboating. The motorboating disappears when the gain is at max.

Might be nothing but last time I measured voltage the motorboating stopped when I probed pin 6 of V1 and pins 2 and 8 of V2. Grounding?


Those are good clues.  I'll have to ponder that for a bit.  You have it set for "compress," right (SW3 shorting R7)?  If you set the gain to max and stop the motorboating, does messing with the "peak reduction" cause the motorboating to start again?

Also, let's sanity check your DC measurements on V1 and V2.  Now that you can make the motorboating stop, do that (crank the gain up) first.  Now remeasure those DC voltages and let's see if anything pops out.  I think some of your previous measurements were polluted by the low frequency oscillation.

You might also recheck the wiring on the T4B socket--specifically the "X."  Check that it's soldered at the crossing point and that you have continuity to ground at all pins that should be grounded.

A P
Title: All things LA2a related
Post by: Bluzzi on July 13, 2006, 06:14:55 PM
OK, actually after trying more things the meter does change in frequency as well as amplitude!

I have not plugged in the T4B so I don't think Peak Reduction should (and doesn't) do anything right?

I keep checking my wiring and it always comes up OK! At this point I get the feeling its a bad component or bad solder joint (I checked most and only found one suspect. I re-applied solder but nothing changed so it was OK to begin with).

I am starting to think maybe pictures may be a way to see if anything is amiss. Its harder for me to see my own errors! A lot of wiring has shrink tubing on the joints (some shrunk some not) so it may be hard to actually see solder points.

I can post an album of close shots (I finally got a digital camera) on photobucket. What thinks you?

Thanks again A.P.

Anyone else have any ideas or clues? C.J.?

jim
Title: All things LA2a related
Post by: AnalogPackrat on July 13, 2006, 06:39:19 PM
Photos might be useful.  Did you try stopping the oscillation (by turning down the gain) and then taking new DC measurements on V1 and V2?  That might turn up a sneaky wiring error or bad joint (now where's CJ?).

A P
Title: All things LA2a related
Post by: CJ on July 13, 2006, 08:54:54 PM
Get it motorboating and move wires around with a chop stick. Start with the grid wires of V1 and work your way to the 12BH7.

You should be able to get it to stop, or at least change in frequency.

cj
Title: All things LA2a related
Post by: imo on July 14, 2006, 01:11:32 AM
I have completed setting up the Bloo version of the LA2A. It is very high end heavy. I have checked all of the connections, and leaning towards possible component failure. I did find that on the hot side of C4, the variable cap, i only have 2-3 volts, and according to one of the schematics i found it should be around 27 volts. I have tried adjusting the cap through its whole range, as well as switching both of the rear adjust pots without any change. I am needing to turn the gain all of the way up to have any gain attenuation on either compress or limit.
Any ideas?
Title: All things LA2a related
Post by: CJ on July 14, 2006, 02:55:20 PM
what kind of transformers?

check transformer wiring.

could be supersonic oscillation causing gain reduction.

scope, or no?
Title: All things LA2a related
Post by: imo on July 14, 2006, 04:15:25 PM
CJ,
scope on the way, not yet.
I am using the Sowter input and output transformers 4383 and 8940. I have them wired up per Sowter's schematic on the webpage.
I have found some funky voltages on the 12B7A. All of the other tubes and voltages look pretty good.
Here is what I have got: DC voltages
Pin 1: 15
Pin 2: 0
Pin 3: 0
Pin 6: 196
Pin 7: 15
Pin 8: 24
I used 1k resistors in series on pins 2 and 7 as grid stoppers, which i heard might be necesary with the Sowters
Thanks
Ian
Title: All things LA2a related
Post by: CJ on July 14, 2006, 06:33:20 PM
Your cathode is shorted out.

How? Bias is gone, grid and cathode look shorted possibly, those sockets are tight, check for a solder bride or small starnd of wire.

I did a voltage chart, but it is burried on what page I do not know.
Title: All things LA2a related
Post by: imo on July 14, 2006, 10:53:58 PM
CJ,
Thanks so much for the time.
I went in and checked for shorts between cathode and the grids. I had no continuity between any of them. The only continuity i found was between the heaters (pin 4 and 5 and 9) and the center of the tube, which is also part of the ground path. This seems according to the schematics though.
I do have grid stoppers on both pin 2 and pin 7..
Any other ideas on where to look? I have been trying to find another tube to check it with, don't think i have one. The unit seems to be running other than this one area, and i am so close it is a bit maddening/
Thanks
Ian
Title: All things LA2a related
Post by: Bluzzi on July 15, 2006, 02:18:46 AM
Quote from: "CJ"
Get it motorboating and move wires around with a chop stick. Start with the grid wires of V1 and work your way to the 12BH7.

You should be able to get it to stop, or at least change in frequency.

cj



See, I knew it! Once CJ bugs you to do something it works!

In my shame I guess it must be my wiring route!

At first I got it to oscillate as much as possible before I started to "chopstick" around. At first nothing but soon I discovered a few wires that changed things but only in a worse way. Others changed the type of oscillation from motorboating to warbles, clicks and squeals.

Eventually I found one that stopped the noise completely! I had to tuck it under C2 (don't ask how I found out). Its the wire that goes from R15 to pin 6 of V1.

Then I plugged in a 1khz tone and listened in case no audio was passing. Yep, my tone was there! Did the gain work? Yep! Time to plug in the T4B (after all this LA2A needs its heart!).
Fired it up, and both speaker and meter confirmed compression was working.

Now time to put music through to hear quality of the audio. The music comes through but I have to turn down the gain a lot! There is distortion but this is such an improvement I'm really happy for now.

I got new voltage readings as follows;


V1
pin    

1 = 103.9    
3 = 1.1      
6 = 108                
8 = 1.2


V2
pin

1 = 83.9
3 = 3.8
6 = 212.6
7 = 71.9
8 = 98.9


These seem to be almost there. Next is to tame distortion. I'll come back for that one.

Lesson learned, routing of wire is very important in this circuit and I still have to find out right way instead of above solution.

If anyone has ideas about distortion please speak freely!

I'm not finished yet but big thanks to CJ and AnalogPackrat. Also thanks to Ron for his help and advise as well as a great layout. I felt all alone but you guys came through big time.

Sorry for long message...I'm excited!

jim
Title: All things LA2a related
Post by: AnalogPackrat on July 15, 2006, 03:00:16 AM
Quote from: "Bluzzi"

See, I knew it! Once CJ bugs you to do something it works!

In my shame I guess it must be my wiring route!

At first I got it to oscillate as much as possible before I started to "chopstick" around. At first nothing but soon I discovered a few wires that changed things but only in a worse way. Others changed the type of oscillation from motorboating to warbles, clicks and squeals.

Eventually I found one that stopped the noise completely! I had to tuck it under C2 (don't ask how I found out). Its the wire that goes from R15 to pin 6 of V1.


Hey hey!  Kung Pao Master CJ strikes again!  Great news on the oscillation fix, but looking at the schemo, shouldn't R15 go to C2 (th other end of C2 then connecting on to pin6 of V1)?  Have you accidentally bypassed C2?  If so, big DC on V2's grid could be your source of distortion.  Keep at it--almost there!

A P
Title: All things LA2a related
Post by: Bluzzi on July 15, 2006, 10:09:34 AM
Ooops, sorry my bad. R15 goes to C2 yes but I meant pin 7 of V2. Thats what you get looking at a layout rather than schematic.

I'll be picking up a scope today (have to re-learn how to use it properly) but the probes are supposed to be funky on it so I may not have it functional until I get a set  of probes for it.

I'd like to maybe understand why the oscillation occured to get the wiring permanently fixed up. Having a wire tucked in under a component is OK and a last resort but nicer would be proper wire orientation. That may be more of an art than science? I also have a second LA2A ready to be wired up once I am totally happy with this one.

How do the voltages look to you guys? Possible source of distortion?


jim
Title: All things LA2a related
Post by: AnalogPackrat on July 15, 2006, 02:11:03 PM
Jim,

When you say you have to turn down the gain "a lot" what do you mean?  I usually run my Sowter equipped Bloo with the gain around 3/10 or so.  The LA-2a is no slouch on gain!  If you're using the original iron, you may have even more gain (due to higher turns ratio on the input iron).  So I'd bet you are OK if you are only getting distortion above 3/10 or so.  Also, what is your input signal source?

Having said all that, CJ has recommended a small mod in the past that you might try.  First, pop out V1 and try a 12AY7 which is a better sounding and lower mu dual triode.  That will knock your gain down some.  If you want to tame it even more, replace R9 and R13 with 100k.  This also has the effect of making clipping more symmetrical by biasing the tube closer to the mid-point.  I haven't tried these yet, but maybe one day...


:guinness:
A P
Title: All things LA2a related
Post by: Bluzzi on July 15, 2006, 02:28:04 PM
The max I turn is to 8 or 9 oclock but eeven there I hear some type of harmonic distortion. Not bad. I have been listening through crappy headphones up until now. I'll be doing another fixin' session on the LA2A tonight and trying out better headphones and then speakers (now that I know I won't blow anything up).

I did get the 12AY7 that CJ suggested but had not heard of the 100k mod for R9 and R13. I am using UTC A10 and A24 so that also explains large gain. I'll pop in the 12AY7 tonight and if that doesn't tame it enough then I 'll do the resistor changes.

I feel so close now I am more relaxed about it. Thanks again for your continued support. You guys have no idea how much it means to me to get any type of help. Even if anyone is way off (which you are not) just the fact of writing to try things gets my ass off the floor and into de-bug mode.

I'll be mounting on the case tonight as well hopefully.

jim
Title: All things LA2a related
Post by: imo on July 17, 2006, 12:41:40 AM
Cj, and anyone else who was watching. I figured out what it was. I had replaced the output cap, the 20u electro, after one of the leads broke while trying to find a good place to fit it in the enclosure. When i replaced it i put it backwards, classic..
Now voltages all look to be in order. I will run some tones through it tonight.
Again, thanks for all of the help. It is so valuable, not only in figuring out the problem, but also in learning about what is going on.
Ian
Title: All things LA2a related
Post by: Bluzzi on July 17, 2006, 01:44:01 AM
I didn't get a chance to try the 12AY7 sub yet but I have a double question about R7.

This is the resistor marked 1k to 2.7k. So what determines its value and what effect does it have?

jim

Hey, Ian, good to see you found your problem.
Title: All things LA2a related
Post by: imo on July 17, 2006, 02:41:34 AM
I was trying to search for the mod that CJ suggested somewhere in this thread that raises the threshold a bit. DOes anybody have the info written down? Couldn't find it using the search function I think i got to it at about hour 20 perusing the knowledge base, but at the time didn't write it down as it didn't seem pertinent
Title: All things LA2a related
Post by: John Peacock on July 17, 2006, 03:11:33 PM
Quote from: "AnalogPackrat"
Quote from: "John Peacock"
It's been a couple days.  I did manage to find one other small mistake with the placement of R6 around the input transformer, but this correction has had no impact on the unit's performance.  Still lots of HF noise whenever I try to compress.  Also using a flashlight to trigger the TB4, I was able to detect gain reduction using an external metering source, though nowhere near the 15-20dB displayed on the LA2A meter.

My A20 voltage is still very low, and I suspect this may be part of my problem, but I don't know.  That's why I am asking for your help.

Please!

-Jp


If you can pass audio without GR, it sounds like your problem is not related to V1 and V2 circuitry.

Pop the cover off the T4B, hook up a signal of some sort to the input, turn off the lights and look at the EL panel.  Does it change brightness with the input signal strength (it's not really all that bright, so make sure you're room is dark)?  If it's pulsing in brightness with the music/signal, then your sidechain is probably working.  Perhaps the wiring on your T4B or its socket has a small mistake.  Did you wire the "X" and solder the point where the two wires that form the "X" cross?  Is the X grounded at some point?

A P

A P


Thanks for the response AP (and Bluzzi for words of encouragement).

I popped the lid off the TB4 and found that the EL panel does light up (GOOD), but it does not change with the amount of audio I put in (BAD).  In fact, it does not even need to have something plugged into the input to light up.  All that matters is how much I turn up the GR.  GR = 0, no light.  GR turned up to 50, lots of light.

I feel like maybe there is a loop somewhere, or perhaps I am bypassing something, or a short?

I can hear a very high pitched wistle when I turn up the GR on the unit (and this comes from the compressor, not the speakers).

-Jp
Title: All things LA2a related
Post by: John Peacock on July 17, 2006, 03:14:16 PM
Here is an audio sample of what the unit sounds like.  This is with gain around 10.  I start applying Gain Reduction around 5 seconds in.  I sweep the GR until GR = 100 around 15 seconds into the clip and then sweep back down to 0.

http://www.johnpeacock.net/bloo/Bloo_Sample.wav

Thoughts?

Thank you.

-Jp
Title: All things LA2a related
Post by: AnalogPackrat on July 17, 2006, 06:17:47 PM
Jp,

Something in your sidechain is oscillating for sure.  The EL panel only responds to AC, so the sidechain circuit has to be making it.  I suggest you print out the circuit diagram and recheck your wiring on related to V3, V4, and the T4b.  Get a hilighter and mark every component and wire as you check it in your circuit.  That way you'll know you checked it all and it will be obvious if anything is missing.

If you want a quick sanity check, turn the GR all the way down and post your DC voltages on V3 and V4.  Maybe something obvious will pop out, but it may be that it's a wire placement issue that's generating positive feedback.  You could try CJ's technique of using a wooden chopstick to physically move wires around while the unit is on (and oscillating) to see if you can change the oscillation or make it stop.  If you move something and it changes the oscillation, you've found part of the problem.

Is your wiring pretty neat?  Did you use shielded wire (or twisted pairs) for the front panel controls that carry audio?  Got an o'scope?

Good luck,
A P
Title: All things LA2a related
Post by: AnalogPackrat on July 17, 2006, 06:22:43 PM
Quote from: "Bluzzi"
I didn't get a chance to try the 12AY7 sub yet but I have a double question about R7.

This is the resistor marked 1k to 2.7k. So what determines its value and what effect does it have?


Read the note at the bottom left on the schematic.  It was selected to help match the CdS cell response.  I can't recall what I used there.  maybe pick something in the middle 1k5 or 2k or something.  If you're not trying to make a matched pair, it probably doesn't matter much.

A P
Title: All things LA2a related
Post by: John Peacock on July 17, 2006, 07:37:58 PM
Quote from: "AnalogPackrat"
Jp,

Is your wiring pretty neat?  Did you use shielded wire (or twisted pairs) for the front panel controls that carry audio?  Got an o'scope?

Good luck,
A P


Thanks for the reply and suggestions.  My wiring is pretty neat, and I did use sheilded wire for the front panel controls.  No scope currently, though I might be able to track one down if it comes to that.

Meanwhile, I'll retrace the wiring once again and try the chopsticks.

Scenaria thought perhaps a bad tube.  I might have 12ax7 lying around, but no extra 12bh7a.  Any thoughts on that idea?

Thanks.
Title: All things LA2a related
Post by: AnalogPackrat on July 17, 2006, 07:54:35 PM
Maybe on the bad tube.  It would be the second 12AX7 or the 6AQ5, though.  I think we can safely rule out the 12BH7 since it passes nice audio when the sidechain is out of the picture (GR = 0).  Got another 6AQ5?
Title: All things LA2a related
Post by: John Peacock on July 17, 2006, 08:48:45 PM
Quote from: "AnalogPackrat"
Maybe on the bad tube.  It would be the second 12AX7 or the 6AQ5, though.  I think we can safely rule out the 12BH7 since it passes nice audio when the sidechain is out of the picture (GR = 0).  Got another 6AQ5?


I can do you one better.  I've got a working LA2A compressor!  Chalk another one up on the board, Packrat.  Your solution was actually last page.  I had the "cross" on the TB4 circuit, all soldered correctly, but no ground.  I went back and looked all over for that in the BLOO manual.  Couldn't find it...  Checked it on the schematic, and saw it there.  Thought what the hell, and I tried it.  Fixed.  (Though not before miss wiring the unit it into my patchbay and thinking the output was now no longer working...  Argh!)

THANK YOU!!

I still am measuring LOW voltage coming from the back side of R34, but it's working (I'm getting around 81VDC and it should be 131VDC).  If all of the other voltages are fine and the unit is working, should I be concerned?
Title: All things LA2a related
Post by: CJ on July 17, 2006, 10:19:59 PM
Good going!

Just make R7 2.7 K and don't worry about it.

whats your B+ like on the other side of the 81 volt plate resistor?
Title: All things LA2a related
Post by: AnalogPackrat on July 17, 2006, 11:23:36 PM
Quote from: "John Peacock"
Quote from: "AnalogPackrat"
Maybe on the bad tube.  It would be the second 12AX7 or the 6AQ5, though.  I think we can safely rule out the 12BH7 since it passes nice audio when the sidechain is out of the picture (GR = 0).  Got another 6AQ5?


I can do you one better.  I've got a working LA2A compressor!  Chalk another one up on the board, Packrat.  Your solution was actually last page.  I had the "cross" on the TB4 circuit, all soldered correctly, but no ground.  I went back and looked all over for that in the BLOO manual.  Couldn't find it...  Checked it on the schematic, and saw it there.  Thought what the hell, and I tried it.  Fixed.  (Though not before miss wiring the unit it into my patchbay and thinking the output was now no longer working...  Argh!)

THANK YOU!!


Great!  You're welcome!  :thumb: :thumb:

Quote
I still am measuring LOW voltage coming from the back side of R34, but it's working (I'm getting around 81VDC and it should be 131VDC).  If all of the other voltages are fine and the unit is working, should I be concerned?


So, did you build the early version or the late version of the V4 circuit? I think the Bloo manual is a little vague on this part.  The schematic in the manual is the early version, but parts of the manual test refer to the later one.  It gets confusing.  Basically, do you have an R38 (22k) in your build?  And a C13 (510pF)?  If so, that's the later version and you should have R34 of 10k and R36 of 470R.  The early version has no R38 and no C13 and uses 22k for R34 and 1k for R36.  Check that...

And if you get the voltage CJ requested we can figure the current draw of V4 and see if it makes sense.
 
A P
Title: All things LA2a related
Post by: John Peacock on July 17, 2006, 11:35:12 PM
Quote from: "AnalogPackrat"
The early version has no R38 and no C13 and uses 22k for R34 and 1k for R36.  Check that...


I'm an early version with R34 as a 22k.  But I do have a C13 in there...  Hmmm.

Quote from: "AnalogPackrat"
And if you get the voltage CJ requested we can figure the current draw of V4 and see if it makes sense.
 
A P


Which pins do I measure to get that?  Is B+ the front side of R34 (or Turret Board B20 in "BLOO speak")?  I get 267VDC there.

Thanks.

-Jp
Title: All things LA2a related
Post by: AnalogPackrat on July 17, 2006, 11:43:02 PM
Just measure the other side of R34.  I can't remember where it is on the Bloo.
Title: All things LA2a related
Post by: John Peacock on July 17, 2006, 11:46:09 PM
Quote from: "AnalogPackrat"
Just measure the other side of R34.  I can't remember where it is on the Bloo.


267VDC
Title: All things LA2a related
Post by: AnalogPackrat on July 17, 2006, 11:47:00 PM
Also, be sure you've got R3 and R37 wired right--easy to get them backwards.  You've got them turned all the way, uh, which ever way the manual says, correct?
Title: All things LA2a related
Post by: John Peacock on July 17, 2006, 11:51:31 PM
Quote from: "CJ"
Good going!

Just make R7 2.7 K and don't worry about it.


Cool.  That's what I've got.

Quote from: "CJ"
whats your B+ like on the other side of the 81 volt plate resistor?


267VDC.

Thanks, CJ.  Your previous posts in this thread were a big help with the trouble shooting I did before I started posting.

-Jp
Title: All things LA2a related
Post by: John Peacock on July 18, 2006, 12:01:03 AM
Quote from: "AnalogPackrat"
Also, be sure you've got R3 and R37 wired right--easy to get them backwards.  You've got them turned all the way, uh, which ever way the manual says, correct?


No mix up with the wiring of R3 and R37.

I originally had them both set where ever it was recommended (I think counter clockwise).  Then I read earlier in this thread that someone was having similar problems with their R34 voltage and I think Scenaria recommended that R3 be moved more towards the middle.  I tried this and it maybe gave me a volt or two.  

My R34 measures rather high (it's supposed to be 22 but is pretty close to 25k), and when I swapped it out with a better 22k (only a .5 watt resistor), it got me up to about 95VDC.  But that is still no 131VDC.  I eventually went back to the original high resistor because it was the only one I had that was rated for 2W.

Also my C13 is actually 560pf instead of 510.  Why?  Because it is what came in the box.  I figured close enough.  

So should I get online and get these values corrected?  I probably will anyway, but I can't figure that they are making that much of a difference in the operation as is.

-Jp
Title: All things LA2a related
Post by: AnalogPackrat on July 18, 2006, 12:10:37 AM
Well, technically, if you're making the early type you shouldn't have C13 and R38 at all.  But I think CJ has said that it doesn't really matter.

Here's the deal on V4.  You're supposed to get about 135V at the plate.  Recalling Mr. Ohm's law:

V = IR or I = V/R

We know the voltage across R34 is 267-81 = 186V and it's resistance is, you say, 25k, so I = 186V/25000 = 7.44mA.

You're supposed to get something closer to 275-135 = 140V / 22000 = 6.4mA.

So yours is drawing a little more current than it should.  Is R36 1k?  If so, I would close that thing up and use it.  I think you're done.

A P
Title: All things LA2a related
Post by: John Peacock on July 18, 2006, 12:23:29 AM
Quote from: "AnalogPackrat"

So yours is drawing a little more current than it should.  Is R36 1k?  If so, I would close that thing up and use it.  I think you're done.

A P


Now that is the best advice I've gotten yet!

R36 is 1k.  So...

Done and done.

Thanks so much.

Pictures soon.  Same as everyone else's, except this time it will be mine.

-Jp
Title: All things LA2a related
Post by: The Kid on July 18, 2006, 02:35:25 AM
Quote
it got me up to about 95k. But that is still no 131k. I eventually went back to the original high resistor because it was the only one I had that was rated for 2W.



Umm, you mean 95 VOLTS and 131 VOLTS, right?   :wink:  

Quote
You're supposed to get something closer to 275-135 = 140V / 22000 = 6.4mA.


You have the same problem as Happy Tom (starting on page 88 of this thread).  As I said before, that 131~135 volts only applies to the 10K/470, not the 22K/1K.  Basic electronics.  Bigger resistor = lower current = bigger voltage drop.  Sounds like that kit might need a revision!  (Not a good idea to combine schemos!)  :cool:
Title: All things LA2a related
Post by: John Peacock on July 18, 2006, 09:38:02 AM
Quote from: "The Kid"

Umm, you mean 95 VOLTS and 131 VOLTS, right?   :wink:  


Yeah...  It was getting late.
Title: All things LA2a related
Post by: John Peacock on July 18, 2006, 12:41:30 PM
I've created sort of a photo journal and put it online:

http://www.flickr.com/photos/[email protected]/192632697/in/set-72157594203518828/

Let me know if the link doesn't work for everyone.  

NOTE TO ANY FUTURE NEWBIES: THERE ARE MISTAKES IN SOME OF THESE PICTURES!!!  :shock:  If you scroll through the pictures one by one, it will give you a caption for each (this is not there if you select slide show), and I tried to point out any mistakes that are in each photo (there are only a few).  Extra credit for the old timers who can catch them all. :thumb:

Also, for those who are only interested in mocking my wiring, jump right to the last six photos.  :wink:

-Jp
Title: All things LA2a related
Post by: AnalogPackrat on July 18, 2006, 12:50:34 PM
Looks good, John.  The only thing I saw that made me nervous was that you didn't use any grommets or heatshrink to protect your audio transformer wires where they pass through the mounting plates into the chassis.  I would consider going back and protecting those somehow.  The Sowter wire is small and has very soft insulation if I recall correctly.  Other than that--very nice job!

A P

p.s.  Your bench is way too neat, though.  How can you get anything done on such a clean work surface?   :razz:
Title: All things LA2a related
Post by: John Peacock on July 18, 2006, 01:26:54 PM
Quote from: "AnalogPackrat"
Looks good, John.  The only thing I saw that made me nervous was that you didn't use any grommets or heatshrink to protect your audio transformer wires where they pass through the mounting plates into the chassis.:


I actually did that, but not until after the "solo" pictures of the transformers were taken.  And it's clear shrink tube that I used, so it may not even show up well in the later pictures.

Quote from: "AnalogPackrat"
Your bench is way too neat, though.  How can you get anything done on such a clean work surface?   :razz:


Yeah...  I'm still new to all of this.  Give me some time.   :wink:   Actually, it was a mess before each shot, and I would just push everything over to the side right before each shot.  So not actually neat, just very anal.

Thanks for the comments (and getting it working!).

-Jp
Title: All things LA2a related
Post by: CJ on July 18, 2006, 07:48:17 PM
The low volts on V4 plate might just be the screen circuit.

If you use a different circuit, that voltage could move around, but even 86 volts is semi normal, so there is nothing major wrong, just maybe a circuit difference and a tube that has drifted a bit.
Title: All things LA2a related
Post by: Bluzzi on July 20, 2006, 12:16:51 AM
Hey guys! I finally got my LA2A to sound good! I plugged in a 12AY7 as per CJ's original advise and reminded again by AnalogPackrat! That makes it happen!

So far I only listened with a headphone but I went from a AT4047 into a John Hardy M1 right into the LA2A. My gain at about 10 oclock seems good enough and reduction at about 12 and 1 oclock moves needle enough and voice remains nice and present without loss of highs. Silky smooth so far. Real sweeeeeet....Pushing reduction all the way really sucks the juice out and I didn't try increasing gain to see what would happen.

I'm just so damn happy this thing works. All it was was a wire I moved! For now I am not touching it. I'll start on my second one in august. Love it!

I took a picture of it. Not pretty without paint but she just had to pose so proud is she!

http://i22.photobucket.com/albums/b304/Bluzzi/LA2A.jpg

Again thanks to CJ, AnalogPackrat. Also to Duka who probably doesn't remember me but gave me words of encouragement a while back.

A huge thanks to Ron (Cayocosta) for all his help and encouragement as well as making the layout available to us mere mortals like me.

Man, I'm bitten by DIY and there is no cure!

jim
Title: All things LA2a related
Post by: AnalogPackrat on July 20, 2006, 12:57:31 AM
Nice job, Jim!  Glad you tamed it with CJ's 12AY7 swap.  Man, I need to try that on mine.  I think your build looks fine, but you are going to have to show us the inside!  What's next on your build list? :grin:

A P
Title: All things LA2a related
Post by: Bluzzi on July 20, 2006, 01:12:32 AM
Thanks A P. It was your mention of the 12AY7 that made me put one on. I had bought them just in case as I had read that some were finding the gain a bit extreme.

I may even try your resistors swap later if need arises for more attenuation. I do still have to try it in real recording situation. But already it was a major improvement in the tests I performed so for me its the right direction. Whatever works.

I'll post more picts after I get back from vacation and give it a real world test. I will be painting the front panel as well. And then you'll see the wiring too! The naked truth! I promise!

I have another LA2A ready to be done and after that I have 2 GSSLs with sidechain to do. After that I am planning a Supro s6424 build.

But I would also like to do 2 Pultecs and then a bunch of preamps.

But I really need to get back to my music as well.

What about you? Anything in the works?

jim
Title: All things LA2a related
Post by: AnalogPackrat on July 20, 2006, 01:24:06 AM
Jim,
Hey, those 100k plate resistors with the 12AY7 were all CJ's idea!  I see you've definitely got the DIY bug.  I've got a pile of stuff to work on, too.  I'm currently stuffing some of Fabio's 1272 boards and some Neve Outs, too.  I've got a dual What compressor (Fabio style with DOAs and iron) ready to build, V72, NYDave passive EQs, passive mixer, CJ one bottle, NYDave one bottle, Firefly guitar amp, Vox AC-4 amp, another pair of 1272s using Marinair iron, lots of analog synth stuff...it's a disease!  If I ever build a second LA-2a thing it will be CJ's all octal version.

Looking forward to more pics of your build.  Enjoy your vacation!

A P
Title: All things LA2a related
Post by: imo on July 20, 2006, 05:21:05 AM
AP and Jim,
First, congrats on getting everything going Jim! It is a beautiful sounding compressor. I have mine up and running, paired with an Elux 251, and the combination is pretty phenomenal. I was wondering if you guys had any info on the tube sub (the 12Ay7 for the 12Ax7) I couldn't find it, and i vaguely remember reading about it when i originally scoured the thread. Where are the 100k resitors in the circuit? I would love to have a bit more headroom with the compressor.
Thanks
Ian
Title: All things LA2a related
Post by: emx on July 20, 2006, 08:42:59 AM
Just Curious about something.   I picked out an output transformer for my unit with 50%High nickel and 50% Iron  as the other option was all high nickel and I had read somewhere that the bass response in iron heavy cores was better.   Is nickel known for being more high end heavy?   I write bass heavy music so I thought the 50/50 would be better even though it was cheaper.  Just wanted to get more opinoins on this matter.
Title: All things LA2a related
Post by: Bluzzi on July 20, 2006, 10:26:55 AM
Quote from: "imo"
AP and Jim,
First, congrats on getting everything going Jim! It is a beautiful sounding compressor. I have mine up and running, paired with an Elux 251, and the combination is pretty phenomenal. I was wondering if you guys had any info on the tube sub (the 12Ay7 for the 12Ax7) I couldn't find it, and i vaguely remember reading about it when i originally scoured the thread. Where are the 100k resitors in the circuit? I would love to have a bit more headroom with the compressor.
Thanks
Ian


Thanks Ian,

The resistors to replace are R9 and R13. I haven't tried it yet and may do so in future.

I have another LA2A to do with A10 and A24 but if I had any suggestions for others it would be to use an input transformer with less gain. Even I may just sell my UTC iron and buy Jensen or Sowter.

OK, I found the thread with the tube talk. Go to page 4 of ...

http://www.groupdiy.com/index.php?topic=231&postdays=0&postorder=asc&highlight=12ay7&start=0

jim
Title: All things LA2a related
Post by: imo on July 22, 2006, 08:50:41 PM
Thanks Jim. I found the thread. I have put in the 12Ay7 and the 2 100k plate resistors. Will report back on sound soon. The AY7 does seem to be getting really hot, though all of the DC voltages look to be pretty close to CJ's from his schematic.
Also I have noticed no roll of on my unit up until past 25k. I am using the variable cap for C4. The best results i had was running the variable, turned to its lowest value in series with a 150p with the + leg floating. Then i was able to get a mild (1/2 db) drop around 16k.

Anyone have any ideas on this?
Ian
Title: Having problems!
Post by: API on July 24, 2006, 01:25:02 PM
Hello,

I am having a number of problems with my Bloo LA2A!
I have completed my build and double checked all my connections, tubes light up and all voltages seems to be right.
But, there is almost no volume on the output, what ever i put in i get maybe a tenth of it on the output with full gain.
Also, there is no meter movement (the adjust works though) from the audio.
There is a large hum from the +4 and +10 settings, and also the switch pops when i turn it to these positions.
What else, only one half of the neon is lighting up, is that normal??
It tried it with the T4B out and then there is no signal at all on the output.

Another thing i noticed from Coyastas layout was that i do not have C6, C14 or R30 in my circuit!
Do i need these??
They were not mentioned in the Bloo manual.

Does any of these symptoms ring a bell with anyone???
Any help appriciated!!!

For the record i use original UTC A-10 and A-24 iron, and a original UREI T4B.

Regards,
Dennis
Title: All things LA2a related
Post by: AnalogPackrat on July 24, 2006, 03:41:29 PM
API,

As far as the missing components go--those are all related to the stereo link stuff, so you don't really need them unless you plan to run a linked pair.  If you do want that option, you'll need to add them per the schematic.

On your other problems, it is hard to say, but since you say you get no output at all without the T4b, something is definitely wired incorrectly.  The T4b operates as a variable resistor (actually two--one for metering and one to actually modulate the signal) that shunts some of your input signal to ground.  The amount of shunting is controlled by the sidechain amp (V3 and V4).  With no T4b unit in, you should be able to control the output level with the gain pot and view the output level on the meter (+4 or +10).

Also, since your metering switch changes the output (the hum and pops which should not be there), you should carefully recheck the wiring there.  I messed mine up the first time (lots of people have).

Have you tried messing with R3 and R37?  It's easy to get those wired backwards and then have them turned all the way the wrong way.

Can you post your DC voltages on V1 and V2 with no input signal and no T4b present?  Can you also describe the connections you've made on the A-10 including the way it connects to R5 and R6?

Cheers,
A P
Title: All things LA2a related
Post by: API on July 25, 2006, 07:45:46 AM
Hi again.

Update:
I was wrong on the level thing, i had connected the signal in the wrong way (out of phase!) so now i have stronger signal coming out of the input transformer than what i put in.
Also, have checked the output transformer and its also working.

I was also wrong about not having any output without the T4B, i have the same output level with or without it.
The output pot works, but i dont get any meter reading.
Also, the switch still pops when i change from GR to +4 to +10.

Where should R3 and R37 be set??
Is there a standard position??
The wiring of these are correct as well.

Checked the wiring of he switch and i cant find any faults.

Here are my DC readings for V1 and V2.

V1:
1: 119v
2: 0v
3: 1.23v
4: 6.3 heater
5: 6.3 heater
6: 230v
7: 0v
8: 3.86v
9: 6.3 heater

V2:
1: 103v
2: 0v
3: 3.9v
4: 6.3 heater
5: 6.3 heater
6: 230v
7: 79v
8: 109v
9: 6.3 heater

Does this make any sense to you??

Also again, it looks like only "half" of my neon lamp lights up.
Is that normal???

Thanks,
Dennis
Title: All things LA2a related
Post by: AnalogPackrat on July 25, 2006, 11:02:30 AM
API,

Glad you got the A-10 wiring sorted...now on to the other issues...

Quote from: "API"

The output pot works, but i dont get any meter reading.
Also, the switch still pops when i change from GR to +4 to +10.


What output pot?  You mean gain?  You don't get meter reading for +4 or +10?

Quote

Where should R3 and R37 be set??
Is there a standard position??
The wiring of these are correct as well.


RTFM.  Somewhere in there it tells you which way to turn them before you start testing it.  I want to say its all the way CCW, but there's a 50% chance I'm wrong.  Don't have the manual with me at work.

Quote
Here are my DC readings for V1 and V2.

V1:
1: 119v
2: 0v
3: 1.23v
4: 6.3 heater
5: 6.3 heater
6: 230v
7: 0v
8: 3.86v
9: 6.3 heater

V2:
1: 103v
2: 0v
3: 3.9v
4: 6.3 heater
5: 6.3 heater
6: 230v
7: 79v
8: 109v
9: 6.3 heater

Does this make any sense to you??


Are you sure your reading on V1 pin 6 is 230V?  That's about double what you should see.  Check that R13 and C2 are wired correctly and that R13 is really 220k.  Pin 8 is too high, too, but it's probably because of the pin 6 problem.  Just for fun check R14, too.  V2 looks OK.

Quote
Also again, it looks like only "half" of my neon lamp lights up.
Is that normal???  


Mine acted funny because I had wired R4 backwards.  Which way does the needle move relative to R4 when the meter is in GR mode?  What's your voltage on the neon (between the neon and R28)?  Does the brightness of the neon change when you sweep R4 around?

A P
Title: All things LA2a related
Post by: API on July 25, 2006, 05:05:39 PM
Hi again.

Yes, i am glad i sorted out the A-10!!

Quote

API wrote:

The output pot works, but i dont get any meter reading.
Also, the switch still pops when i change from GR to +4 to +10.


What output pot? You mean gain? You don't get meter reading for +4 or +10?


Yes, sorry, i am talking about the gain pot, R1.
When i turn it the output level changes, but ther meter just sits still at the far left with no mevement, bith on +4 and +10.


Quote
Quote:

Where should R3 and R37 be set??
Is there a standard position??
The wiring of these are correct as well.


RTFM. Somewhere in there it tells you which way to turn them before you start testing it. I want to say its all the way CCW, but there's a 50% chance I'm wrong. Don't have the manual with me at work.


RTMF=Read The Fu***ng Manual ??
That was a new one for me!
I found nothing in my Bloo manual about these to, not even what they were used for!
Does anyone know which way to turn for basic testing??


Quote
Are you sure your reading on V1 pin 6 is 230V? That's about double what you should see. Check that R13 and C2 are wired correctly and that R13 is really 220k. Pin 8 is too high, too, but it's probably because of the pin 6 problem. Just for fun check R14, too. V2 looks OK.


Yes, i got a 230v reading here, bad i guess?
Will check the wiring and value of R13 tomorrow and let you know.
Will check R14 to.

Quote
Mine acted funny because I had wired R4 backwards. Which way does the needle move relative to R4 when the meter is in GR mode? What's your voltage on the neon (between the neon and R28)? Does the brightness of the neon change when you sweep R4 around?


From memory the meter moves in the same direction as i turn R4 in GR mode.
Is that right??
The voltage between the neon and R28 is about 60 volts, which i think is correct.
The neon shuts down when i am at either end of R4 (at least when its fully CCW), will double check tomorrow as well.
And again, only of the "wires" in the neon lights up at all.

Regards,
Dennis
Title: All things LA2a related
Post by: AnalogPackrat on July 26, 2006, 12:09:20 AM
Quote from: "API"

Yes, sorry, i am talking about the gain pot, R1.
When i turn it the output level changes, but ther meter just sits still at the far left with no mevement, bith on +4 and +10.


So you can hear gain reduction happening but it's not showing on the meter?

Quote

RTMF=Read The Fu***ng Manual ??
That was a new one for me!
I found nothing in my Bloo manual about these to, not even what they were used for!
Does anyone know which way to turn for basic testing??


See, I was wrong, twice!  It's full CW and it's not in the manual!  I think it's somewhere in the first 50 pages of this thread...sorry.


Quote

Yes, i got a 230v reading here, bad i guess?
Will check the wiring and value of R13 tomorrow and let you know.
Will check R14 to.


Yeah, something is definitely wrong around there.  Hope you can find it.

Quote
From memory the meter moves in the same direction as i turn R4 in GR mode.
Is that right??
The voltage between the neon and R28 is about 60 volts, which i think is correct.
The neon shuts down when i am at either end of R4 (at least when its fully CCW), will double check tomorrow as well.
And again, only of the "wires" in the neon lights up at all.


Sounds like your neon is fine.  They aren't bright.  If you've got 60V and can adjust the GR offset, you're fine.  I think you may have something wired wrong on your switch, though.  Or maybe you have a shorting switch and it's putting that DC offset on your output  :shock: .  Did the switch come with your kit?

I think you're close.  Just get that V1 thing fixed and check the meter switch again.

A P
Title: All things LA2a related
Post by: API on July 26, 2006, 05:14:02 AM
Hi again.

Quote
So you can hear gain reduction happening but it's not showing on the meter?


NO, there is no gain reduction.
What I can do is adjust the volume from zero to full (which is not much) with the gain pot.

Quote
See, I was wrong, twice! It's full CW and it's not in the manual! I think it's somewhere in the first 50 pages of this thread...sorry.


He, he!
No problem, have found the Bloo manual quite dull, lots of missing info.
I guess its full CW when you look from the back??

Quote
Yeah, something is definitely wrong around there. Hope you can find it.


Will check it out later today.

Quote
Sounds like your neon is fine. They aren't bright. If you've got 60V and can adjust the GR offset, you're fine. I think you may have something wired wrong on your switch, though. Or maybe you have a shorting switch and it's putting that DC offset on your output Shocked . Did the switch come with your kit?


Ok, actually my neon is quite bright (because only half of it is working??).
Yes, the switch came with the kit.
Will double check that it´s switching as it should.

Is there any known wiring faults in the Bloo manual that i could have done??

Later,
Dennis
Title: All things LA2a related
Post by: API on July 28, 2006, 06:47:50 AM
Hi again.

I am on tour for a few days so i wont be able to check things out.
But i do found something wrong.
The position of R12 and R14 are switched!
This fault is in the Bloo manual, and i had not double checked this to the schematic (which was right of course!).
Will chenge the position of these when i get back home and report what i find out.
Hopefully i will get some better voltages.

Thanks,
Dennis
Title: All things LA2a related
Post by: AnalogPackrat on July 28, 2006, 11:43:56 AM
Dennis,

That would be a problem!  If you fix the resistor swap I bet you will have normal operation of the audio path (V1 & V2).  You may still have something wrong that is preventing compression from occurring, but at least it should pass audio.

So, if you can get normal operation of gain, but without compression, try this next.  Hook up a signal to the input, pop the cover off of the T4b, turn off the lights and watch the EL panel.  Does it pulse with the input signal?

Keep working on it, you'll get there soon.  I will check my Bloo manual again tonight because I vaguely recall that I found an error in it when I was building mine--I think one step referred to the wrong capacitor.  I'll post what I find

A P
Title: All things LA2a related
Post by: AnalogPackrat on July 28, 2006, 11:24:44 PM
Dennis,

I checked my manual when I got home.  I had marked the R12/R14 corrections, so either I noticed it or someone posted on this thread about it.  The cap error I remember is on page 13, last sentence.  It refers to C3 isntead of C5, but it's pretty obvious since it specifies the 10uF cap.  I don't see any other corrections marked in my manual...

Cheers,
A P
Title: All things LA2a related
Post by: API on July 31, 2006, 05:33:43 PM
Hello again.

Now i am back home at the tech desk!!!
So, i switched R12 and R14 and that helped alot!!
I now have the correct voltages on V1.

But there are still problems.
The Gain pot now works better, but when i turn it up everything works good up to 70 (on the Bloo), about 10 past, then it all gets muffled and every time i go past the 10 past mark there is a small pop.
Also, the meter follows the gain pot, even if i do not have a signal applied.
And when i go past the 10 past mark the meter hits the full mark.

Another thing, when i am in +4 and +10 mode, if i tap the meter switch the meter jumps back and forth!
Also, the same thing happens when move the cable that goes from the switch to the output xlr.
Do i have a grounding problem???
Or???

Another thing that was left out in the Bloo manual was the linking between A11 and A14.
This helped the gain pot issue a bit but it still pops at 10 past and gets muffled, but opens up a bit again at 15 past but dies again at full on.

One strange thing is that when i set the gain reduction pot at full CW it also effects the gain pot.

I still have no meter reading in GR, and meter does not show the output volume, it just goes up according to where the gain pot is set.

Maybe i have to low input volume??
I read something about this in the early pages of this thread.

Any ideas????

Regards,
Dennis
Title: All things LA2a related
Post by: AnalogPackrat on July 31, 2006, 05:41:58 PM
Dennis,

What audio transformers are you using?  Sowter?  If you're using the Sowter output transformer (8940) you may be experiencing oscillation in the WCF stage (12BH7).  With Sowter output, use 1k grid stoppers on the 12BH7 (read back through this thread--pages around 30-50?).  If you're using some other kind of transformers, then ignore this.

Check for cold solder joints around your meter switch, output XLR, output transformer, etc.  Also, did you look at your T4b with the cover off and the lights out?  Feed in a line level input of music and tell us if you see the EL panel flashing or not.

A P
Title: All things LA2a related
Post by: CJ on July 31, 2006, 06:31:16 PM
I had the same problem with mine at first. Strange, but after a little burn in, it went away.

You can reduce the osc threshold by reducing the valure of the neg feedback resistor , R11, which is 68 K stock ( blue-grey-orange-gold).

Try a 56 K (green-blue-orange-gold) in there and see what happens.

Or, better yet, wire a 1 meg pot across the 68 k and turn it down til the osc stops.

Then pull the pot, measure it's resistance.

Now, you can either wire the same resistance across the 68 K in parallel, or calculate the parallel resistance and buy a new resistor at Radio Shack.

Just for the heck of it, open the door when the thing oscilates, and move it around. If the osc stops or changes, you have a door grounding problem.
Make sure there is no paint under your ground connections.



cj
Title: All things LA2a related
Post by: API on August 01, 2006, 06:50:23 PM
Ok boys, i only have one word for you:  SUCCESS !!!!

Finally got this thing going, phew!

So, i fixed the input tranny, had to put shrink tube on all the ends before i wired the thing back together, the cloth on the wires had become very brittle, and i think that is what caused a short before.
Cleaned the thing up and wired it back in.

Fixed the R12/R14 mix up from the Bloo manual.

Also connected A11 and A14 which was also missing in the manual.

Sorted my grounding to all pots on the frontpanel.

But i still had some bad oscilating going on.
But i took CJ´s advice and started to poke around with a stick.
Bingo, got lucky around the turret by the input tranny where all input cables meet.

What i had done was to put in a giant polypropylene 8uF cap in for the output, and to be able to close the frontpanel i had fixed it right next door to that input turret.
Thought it be cool to put that in there to see if it would help in any way (can get some pics tomorrow, its giant!) and it fitted so i left it there.

Anyway, cut the strap and moved it away from the input turret about an inch, and all of a sudden had a completely working compressor!!!
Which sounds great by the way!!!

Yes, i am now one lucky guy!!!

Thanks so much for the help CJ and AP !!!!!!
Your help has been invaluable!!

I was so close, finished this thing up last year, but was so disapointed when it did not work so i just put it on the shelf.
But who cares now, its working!!
Send me another kit, i need more!!!

Thanks,
Dennis
Title: All things LA2a related
Post by: AnalogPackrat on August 01, 2006, 06:56:10 PM
Quote from: "API"

But who cares now, its working!!
Send me another kit, i need more!!!


Another addict is born!  Congrats, Dennis.  Glad you got it going, now go have a few  :guinness:  :guinness:  and make some music.

:thumb:
A P
Title: All things LA2a related
Post by: CJ on August 01, 2006, 09:29:44 PM
You da man!
 :thumb:
Title: La2a blows fuses and meter light
Post by: emx on August 06, 2006, 06:47:44 PM
Ok, I finished the LA2a Clone and the unit blows it's fuse and burns out the two meter lights.   Hmmmmm?    I've got both the green lines From my Allied power supply going to the heaters and Meter bulb  and the meter is disconected so I don't fry it without checking everything else first.   I've got the fuse and the Power switch on the same black wire going into the Power tran and the line with the fuse and power switch are going to hot on the ac cord socket.  I don't have any tubes or the t4b installed yet,  I wanted to test it out first without so I don't ruin them.  I also have all the center taps and grey wire to chassis ground.
Title: All things LA2a related
Post by: CJ on August 07, 2006, 09:57:28 PM
gonna need a schemo if ext pwr sup.
Title: All things LA2a related
Post by: emx on August 07, 2006, 11:36:32 PM
Ok,   I'm thinking something is shorting when I close the chassis.   It only seems to pop the fuse and go out when I shut it.  It seems one of the meter lights still works though.   I'm in the process of checking all wires that move with the lid and rubber coating any bare terminals that go anywhere near anything else, the power switch for example has bare terminals that go about a third of an inch near the lip of the chassis and the filter caps.
   I just wanted to double check and see that the gray wire and the two secondary center taps all go to the same grounding lug?  It looks that way on the Cayocosta layout.
Title: All things LA2a related
Post by: CJ on August 08, 2006, 01:08:38 PM
I wish Steve would have made the chassis an inch deeper. I bet Lawrence wished he had done the same thing. If you want to use a good poly cap, man, is it a tight squeeze. Especially if you ue a non standard meter.

Put some spaghetti on the cap leads, maybe the BV+ is hitting ground.
Title: All things LA2a related
Post by: Bluzzi on August 09, 2006, 02:08:33 AM
I just wanted to say thanks again to all you guys for the help I got on my first DIY here.

LA2A is working great although still loud as hell. Very quiet! Only a slight almost imperceptible hum that I am being anal about but not audible in the least. Its in the rack (but not painted yet. Ugly but beautiful!).

I have access to an oscilloscope now and I am waiting for a friend to bring down a frequency analyzer as well. Just to see what I hear!

Now on to my second LA2A which is already started. Now that I am more confident I'll keep my wires shorter and hopefully avoid that oscillation I had with first unit.

I'm hooked big time! But first some music.

jim
Title: All things LA2a related
Post by: yosh on August 09, 2006, 06:57:11 PM
Hi, I've been lurking on this board for a while now. I've had many questions answered using the search engine. So much info! it's great.

But now I have a question to ask. I just finished building a Bloo la2a. It sounds great. I have one problem though. I went to calibrate the limit resistor (R7) using Steve's instructions on page 27 of his manual. But I'm finding that I get practically no difference between limit and compression modes. The unit seems to always act as a compressor. This is confirmed by me ears and external metering. I tried increasing the value of R7 bit by bit until up around 12k I would get limiting but the gain would spike until the limiter kicked in around a second later. So with that kind of resistor value it pumps like crazy. I assume that the attack with R7 in the circuit isn't supposed to be that slow (it really does take a full second to kick in). I checked my wiring of the limit switch and the meter switch. I pulled a bunch of it apart and did it again really neatly. No change.

Any ideas? Thanks in advance.

-Josh
Title: All things LA2a related
Post by: dripelectronics on August 10, 2006, 01:06:32 AM
could someone descibe how to use  ' grid stoppers'
and on wich tubes ,

 i searched through the thread , but came up with only fragments on the concept.

 from what i understand ,
 it is inserting a 1k  resistor between pin 6 and the rest of the circuit.
 on 12ax7 #1
      12bh7a
      12ax7#2.

  is this correct ?

 does the 6aq5a get any thing  ?


 thanks in advance.

gregory
Title: All things LA2a related
Post by: sintech on August 10, 2006, 04:25:41 AM
If using sowter trafos, the Grid Stoppers (1/2 watt 1k resistors) go on just the 12BH7A in series on pin 7 and pin 2.
Title: All things LA2a related
Post by: emx on August 19, 2006, 09:52:21 AM
Ok,  Sorted out the blowing fuses problem.   I'm thinking the power caps were to close to the back of the on/off switch so I coated them with liquid rubber paint and covered the power switch with electrical tape on top of it and it sorted that problemo out completely.  
   Now for round two,   the unit now running and not blowing fuses, is sending out alot of hum and this ticking sound over the audio conections to the mixer I'm sending it to.  The meter on the unit and the meter on the mixer are showing alot of volume.  I can also hear an actual physical hum coming from the box. Checked the voltages on a few things though and they seem a little high.

6.3 V on the heaters coming off of the Power tran

360 v DC coming off of R29

and

275 V DC coming off of R34

I'm wondering if the cinemag transformers I'm using might require grid stoppers?
Title: All things LA2a related
Post by: emx on August 21, 2006, 03:40:32 AM
Wow I must have been tired when I posted last,  those voltages are all wrong that I wrote.  I guess I wrote what they were supposed to be instead of what I wrote down as my readings.  Here is what I'm getting(corrected list of voltages)...

6.3 V AC on Heaters

R29 383 V DC

R34  335 V AC
Title: All things LA2a related
Post by: sysexguy on August 21, 2006, 06:31:27 PM
Hi everyone, got a used(?) Bloo, stripped it bare and rebuilt it yesterday.

I would have a few questions for the experts who so kindly helped all before me.

The heater wires were all tested prior to hooking up the PT and then once I had everything together, both sides were shorted to ground...I immediately suspected a problem with the PT's mounting screw touching the turret but that is not it......am I correct to order a new PT?

other quickies:

q1: I have some 1.5k 1w lying around, can i use those for grid stoppers on V2? (sowters for tranny's)

q2: Is it crucial to replace the 22K 2w (R34) with a 10K if R38 is used?

q3: Other than the labeling on R12/R14, is there a difference between Cayocosta's may '04 and may '05 layouts?

Thanks in advance, :thumb:

Andy
Title: All things LA2a related
Post by: AnalogPackrat on August 21, 2006, 07:08:25 PM
Andy,

I wouldn't order a new PT just yet.  Did you power it on?  Were any tubes in?  Did the fuse blow?  I would just carefully check all the heater wiring.  BTW, you don't have a center tapped heater winding on that thing, do you?  The US (110V) ones came with untapped, but maybe you're in 220V land with a different PT...

a1:  Should be fine as long as they aren't some sort of wire wound type (inductance in series with the grid might not be a good thing).

a2: R34 and R36 are related.  If R34 is 22k, R36 should be 1k.  If R34 is 10k, R36 should be 470R.  Technically, the R38/C13 stuff should only be used in the 10k/470R version, but greater minds than mine have said that modification is unrelated to the R34/R36 thing.

a3: No idea.
Title: All things LA2a related
Post by: sysexguy on August 21, 2006, 08:17:57 PM
It's a 6K88VG, 117v centre taps on both secondaries connected to chassis (yellow striped wires, red and green) as is the shield (thin gray wire).

No tubes yet, no power yet, and no T4b  :wink:  the fuse was blown when I got it and has been replaced.

Thanks for a1 and a2......must do a bit of shopping either way

Btw, if I can put something back into the community, I replaced the hardware with locknuts from http://rtlfasteners.com/RC/index.html Great service and lots of hard to find ......fasteners. btw, the lock washers went back on in addition as they break the powder coat so all surfaces are at ground.

Andy
Title: All things LA2a related
Post by: AnalogPackrat on August 21, 2006, 11:08:26 PM
Had to look at my Bloo manual when I got home to remind me about the filament windings.  I think your PT is OK.  The filament winding is very low resistance and the CT is grounded.  If you're using a cheap-o meter or just testing for continuity it will look like a dead short.  Probably isn't.

A P
Title: All things LA2a related
Post by: sysexguy on August 22, 2006, 12:33:12 AM
Thanks AP, so I should get the 3 remaining resistors in and see if fuses are blowing or not......lots of posts re voltages so I should be OK......hopefully my next post is about how great it sounds!!!!

Andy
Title: All things LA2a related
Post by: sysexguy on August 22, 2006, 12:55:21 AM
Hi AP, you da man!!!!! All my voltages are as per the Bloo manual

thanks :thumb:

Andy
Title: All things LA2a related
Post by: AnalogPackrat on August 22, 2006, 11:19:15 AM
Glad to help, Andy.  Hope you get the rest of it together with no trouble.

:guinness:
A P
Title: All things LA2a related
Post by: sysexguy on August 23, 2006, 10:41:23 PM
:thumb: oh yeah!!!!!!  :grin: thanks for the kind assistance  :sam:

Andy
Title: La-2a voltages
Post by: emx on August 24, 2006, 03:39:30 AM
Anyone here have a link to turret and T4 pin voltages for the LA-2a.  I did a search but could only find a dead link to Cayocostas earthlink one wich isnt' online anymore.
Title: All things LA2a related
Post by: sysexguy on August 24, 2006, 08:41:26 AM
pm sent
Title: All things LA2a related
Post by: HogFish on September 03, 2006, 02:53:12 PM
Hi.Hope I can get some help, this is driving me quite mad.Ive built a Bloo La 2a, but wired the layout according to cayocostas diagram.This includes a 10k 2w instead of the 22k 2w,470 ohm instead of the 1000 ohm, Sowter Iron on input and output. Unit passes audio and sounds fine,the T4 cell is compressing properly,lights up and responds to input properly, all the voltages are within the realm of OKness,(245V on B+, 117-120V on V4 pin 5, 108 to 110 on V4 pin 6,55-56v on the neon lamp)but I cannot for the life of me get the goddamn meter to work at all. It is wired properly according to cayocostas diagram. There is no voltage difference between the + and - side of the meter voltage wise. There is about 5.8 to 6 volts on each terminal.When I close the front panel, the + meter Terminal rubs against the case of R37, and the needle jumps up to around 0VU, but when it clears the resistor case,it goes dead again.Also, if I hold the front panel in place at the point it touches the R37 case,the switch works OK when I switch to +10 and +4 modes of operation.I have been trying to work the problem for about three weeks now and have had no success at all. If you can help, by all means dont be shy. Im about to give up on this thing........
Title: All things LA2a related
Post by: sysexguy on September 03, 2006, 06:43:30 PM
My meter "-" was touching the output XLR jack's case. I just covered the terminal with heat shrink.

Andy
Title: All things LA2a related
Post by: AnalogPackrat on September 03, 2006, 11:21:17 PM
Hogfish,

There's not a lot of circuitry involved with the meter, so you should be able to work out what's wrong.  First, identify all the stuff that affects the meter and understand what the switch is doing.  In GR mode, the meter is looking at the sidechain signal (via R27 and one of the CdS cells in the T4b) plus a DC offset provided by the neon (via R4 and R25).  In output monitoring mode it's just looking at the secondary side of the output transformer through R23 or nothing.  R24 is always in the meter circuit.  Of course, the meter switch is there.

If I were you, I would go through the Bloo manual and read all the sections about wiring the meter switch, the little board attached to the meter, and the wiring for R4.  It almost seems like you are missing the link from R24 to the minus terminal.  If you don't tie the free end of R24 to the little bits that screw the small turret board onto the meter, you have not made a connection from R24 to the negative terminal.

You should also do something to prevent your meter terminals from shorting to anything when the unit is closed.  You may need to trim a mm or two from the problem terminal (being careful not to bugger up the threads).

Keep at it,
A P
Title: All things LA2a related
Post by: HogFish on September 04, 2006, 09:05:06 AM
Well, I finally got working.This was my second time building the unit, The first build didnt work, and there were so many problems,I decided to tear it down and rebuild it. On the 2nd build, I had to replace the small turret board,because I broke one of the turrets removing parts. I bought a new turret board,but it isnt the same size.Its slightly bigger,and it dosent have those metal terminals on the end so I had to drill a hole thru the TB to make it fit over the - terminal. I did link the - side via wire from turret #5 to the terminal tho.How I got the thing working is weird. I put a small wire link between the + pole on the switch the the 3rd contact (the one that is supposed to be open) and the meter sprung to life. Why this works is beyond me, maybe someone here can explain, anyways thanks for your help AP.........
Title: All things LA2a related
Post by: rubric on September 05, 2006, 02:51:17 PM
Hello all!

I'm hoping to solicit some troubleshooting help with my Bloo LA2A. I followed the Bloo manual for the wiring, and I'm using Sowter iron.

I've got two problems. Whether they're related or not, I'm not sure:

1. The unit passes audio just fine. However, as I move the gain knob to the midpoint (50 or so on the display) the audio instantly turns into white noise. The output jumps to the extreme right on the meter as well. I've never something like that before!

2. The compression doesn't work as I would expect. It does start attenuating volume as I move the peak reduction up, but once I get past a certain point (around 20 on the display) it stops compressing entirely. It also compresses very heavily when it is working...more than I would expect anyway.

I put grid stoppers on V2, and I went back through the manual and double-checked all my wiring, and it all looks good to me. Sadly, I don't own a scope for signal tracing. I'm out of ideas as to what may have gone wrong. All the DC voltages were fine before I started running audio through it.

If anyone reading this has any suggestions on what to try, I would be very grateful for your help!!

Thanks.
Title: All things LA2a related
Post by: CJ on September 05, 2006, 02:53:27 PM
that's the amp breaking into oscillation.
get the white noise going, then move wires around carefully.
someone fixed it by moving the big output cap around.
you can adjust feedback resistor or pop in a 12AY7/6072.
Title: All things LA2a related
Post by: rubric on September 05, 2006, 11:23:05 PM
Thanks for your reply CJ!

I moved the output cap around, and I was able to get more gain out of the unit.  It was still breaking to oscillation at high gain settings, but then I put the T4B back in and I can crank the gain all the way with no problems!!  Awesome!  I'll try substituting V1 for a 12AY7 as soon as I can get my hands on one.

Now on to the compression.  I took the can off the T4B and turned off the lights.  The ELP lights up as I apply Peak Reduction, but then it shuts off as I turn past 30:00 or so.  I've also got a low-frequency oscillation happening when the ELP lights up.  I'm guessing this is what people call "motorboating".  I had this problem before, and when I put the grid stoppers in it went away.  Now it's back.  Any ideas??

Thanks so much for your help thus far, I can't tell you how much I appreciate it!

 :thumb:
Title: All things LA2a related
Post by: CJ on September 06, 2006, 12:16:26 PM
the shut down thing is a weird one.
check for an un-soldered wire.
or try the wire moving thing again, only confine this to the sidechais to stop the motorboating.
check grounds for clean low ohms surface contact, no paint.
check lid to chassis ground, don't bank on the piano hinge for a ground.
Title: All things LA2a related
Post by: rubric on September 07, 2006, 12:22:08 PM
thanks again CJ.

i spent as much time on it as i could last night.  i managed to buy a 12AY7, so I popped that in.  it works great.  the amp oscillation problem i was having was really finicky based on wherever i put the output cap.  with the 12AY7 there's no problem at all.

still no improvements on the compression side.  i made sure the front panel is grounded to the chassis (i had to scrape off some paint around the piano hinge).  i'll continue checking grounds and solder points tonight after work. i need to spend some time with the schematic as well to hopefully identify the "sidechain"... although i must admit that i don't really have a clue what i'm doing.
Title: All things LA2a related
Post by: CJ on September 07, 2006, 01:00:39 PM
check your power supply caps, the ones that feed the side chain.
you using audio taper pots?
jus curious, what kind of 12ax7 did you use?

if you really have "motorboating" in the side chain, try bridging the filter caps that feed the side chain with another one of equal or greater uf and volts rating.
Title: All things LA2a related
Post by: rubric on September 08, 2006, 11:20:38 AM
yep, definitely using audio taper pots.  the 12ax7 is an electro-harmonix.

i'm afraid i may be out of my league here.  i don't know how to check the power supply caps, or any cap for that matter (my DMM doesn't have a capacitor test).  i don't know how to locate the filter caps that feed the side chain, nor how to bridge them with another cap.

can you dumb it down a few shades?  i apologize for my ignorance.

thanks!
Title: All things LA2a related
Post by: CJ on September 08, 2006, 12:33:00 PM
ok, skip the cap test.
measure your dc volts on tubes v3 and v4.
Title: All things LA2a related
Post by: rubric on September 08, 2006, 09:11:28 PM
alright!  that i can do.

V3
1 - 92.5
2 - 0 (??)
3 - 0.8
4/5/9 - 0
6 - 92
7 - 0 (??)
8 - 0.8

Should 2 & 7 be zero??  Those are the grids, right?  

V4
Center - 116.5
1 - 0
2 - 8.7
3/4 - 0
5 - 71.5
6 - 115.7
7 - 0

See any problems??
Title: All things LA2a related
Post by: CJ on September 08, 2006, 09:18:35 PM
what is center?

correct on zero volts, v3 grids are 2 and 7
Title: All things LA2a related
Post by: rubric on September 08, 2006, 09:33:19 PM
V3 center is zero as well
Title: All things LA2a related
Post by: rubric on September 12, 2006, 12:57:14 AM
Success!!!    HAHAHAHHAHAHHAHHHAHAHH!

I was about to give up ever fixing this thing, but I decided to have one last look at the wiring diagram.

I made an embarrassing discovery.  I had the wire from the peak reduction pot going to the wrong place!  I had it going to the wrong turret...the one by the T4B that provides the juice for the ELP via V4, not the one next to the input tranny!  Now my Bloo LA2A compresses like a champ and sounds awesome!  

Thanks so much for your help, CJ!  Thanks as well to Scenaria for making it possible!  To anyone who's ever contributed to this thread who happens to be reading this, thanks to you too!  

I love this forum!!

 :thumb:  :thumb:
Title: All things LA2a related
Post by: cannikin on September 14, 2006, 06:36:30 PM
I'm having the same Problem....

I turn the INPUT past 12 o'clock and it starts sounding like I'm tuning a station on an old radio eventually ending in a low flutter/farting/motorboat  ....

funny thing is... I stick a 1K signal through it and with the t4B in it compresses pretty hard

 my voltages are ballpark.....

Replaced the 12AX7 a few times --- no luck

Tried moving the output cap around made some difference but not much..

Checking the wiring of R1 & R2 seems fine.

Things sounds great when powering up and power down... theres a gremlin  in there somewhere .... just have to find it...  :roll:

Going to Dremel some of the paint in hopes its a bad ground connection
Title: All things LA2a related
Post by: Kid Squid on October 23, 2006, 08:32:28 AM
Hiya Chaps,

Just wondering, could you have a check over my turret voltages for me?
I have checked through Scenaria's bloo manual, and i'm getting a few discrepancies (most look o.k., but i dont want to fry my t4b :oops: )

B21 - 360V ( 347V), B20 - 275V (277V), A1 - 60V (56.5)
A20 - 131V (211V ???), A14 - 216V ( 231V), A17 - 90V (91V), A19 - 105V (100V)

I'm using ECC83 - V1, 12BH7a - V2, ECC83 - V3, 6AQ5/6005 for V4

Used 10K for R34, 470 for R36.

and used Metal Flim resistors, (1%), instead of carbon comp (5%), only beacause i had them lying around.
All ground points are O.K.


Here's all my turret Volts,

A1 - 56.5                               B1 - 278
A2 - 278.5                             B2 - 216
A3 - 117                                B3 - 216
A4 - 125                                B4 - 216
A5 - 1.04                               B5 - 19
A6 - 0                                    B6 - 19
A7 - 103.9                              B7 - 95
A8 - 103.9                              B8 - 80
A9 - 0                                    B9 - 80
A10 - 126                               B10 - 80
A11 - 227                               B11 - 0
A12 - 0                                   B12 - 0
A13 - 0                                   B13 - 0
A14 - 231                               B14 - 277
A15 - 0                                   B15 - 0
A16 - 3.41                              B16 - 0
A17 - 91                                  B17 - 277
A18 - 0.85                               B18 - 0
A19 - 100                                B19 - 277
A20 - 211                                B20 - 277
A21 - 277                                B21 - 347
                                              B22- 347
                                              B23 - 347

I'm thinking, the higher 6AQ5 volts are due to the different r34,r36's.

Also, I've left out the stereo adjust pot but included the 510pf cap from pin 6 (6AQ5) and put it to earth. is this o.k.?

Best Regards,
Steve :thumb:
Title: All things LA2a related
Post by: Bluzzi on October 23, 2006, 04:49:18 PM
Quote from: cannikin
I'm having the same Problem....

I turn the INPUT past 12 o'clock and it starts sounding like I'm tuning a station on an old radio eventually ending in a low flutter/farting/motorboat  ....
quote]

This is about the same thing I had. I followed CJ's advise and fiddled around with a chopstick (those Sushi take outs are useful). I found one wire that if tucked under a cap on the turret board stopped all the motorboating!

I don't know if that will help but its worth a try if you haven't already.

jim
Title: All things LA2a related
Post by: SSLtech on October 23, 2006, 04:52:08 PM
Just wanted to say....
Title: All things LA2a related
Post by: SSLtech on October 23, 2006, 04:52:19 PM
Happy 100 pages!!!
Title: All things LA2a related
Post by: Kid Squid on October 24, 2006, 09:03:50 AM
bump.........
 :thumb:
Title: All things LA2a related
Post by: dripelectronics on October 24, 2006, 04:44:58 PM
i wanted to try a couple of different powertransformers
for the la-2a and test them out.
is the hammond that every one uses the 269jx (250-0-250 / 60mA  /6.3v @ 2.5 amp) ?

 (is using larger current better or worse , or are dropping resistors needed)

 it doesnt say center tapped , so i guess you gotta do the 100 ohm
  resistors on each leg of the heater wires to gNd.

 or is it the 240-0-240 version.

noticed that adl uses a torridial x-former any one try this ?
(if so whered ya get it  :))

 thanks .

 g.
Title: All things LA2a related
Post by: Bluzzi on October 24, 2006, 05:54:26 PM
I used the Allied p/n 227-0113.

PRI:110-120V, 50/60HZ,
500VCT @40DCMA,
6.3V @ 2.0A, LEADS
I believe  Hammond makes them for Allied as they say "made in Canada" Mfr's part#:   6K88VG

at under $20.00 U.S. funds its hard to beat! I have one LA2A made and it sounds great! No problems. All the other transformers were more expensive. Why use them if they are all same specs (maybe more current but LA2A is well fed by the Allied one).

jim
Title: All things LA2a related
Post by: dripelectronics on October 25, 2006, 12:24:07 AM
ah , the 6K88VG  is the one i normally use ,

so i thought i would do a bit of exploration

and test out other makes and models.

iv never tried a torridal type xformer and wanted to
see if there are advantages .

along with the hammond wich i just ordered and will test out
on friday.

any other recomendations out there would be appreciated ,

 ill have a power x former shoot out :)

g.
Title: All things LA2a related
Post by: jho-audio on October 25, 2006, 12:29:25 AM
What about the edcor XPWR106?  I am ordering one for my dual-drip-2a
Title: All things LA2a related
Post by: dripelectronics on October 25, 2006, 01:00:29 AM
ah , holy shiznate , those folk are here in new mexico.
i should stop by and talk to them , although they are at the bottom
of the state and me here in the north :)

 that looks like a great x former for the dual pcb project.
 
 ill have to pick one up and give it a shot.

  im looking for single unit power xformers , they dont show a 250-0-250
 ct  40 ma 6.3 v @ 2a
 
 or in the ball park , i think the one thats close is the 275-0275 @ 125 ma
 (wow that would be perfect for the la/twin...good job! you found it!)

g.
Title: All things LA2a related
Post by: Kid Squid on October 25, 2006, 07:01:42 PM
Hiya Chaps,

spent all day checking through my la2a, and still i'm getting a high reading on turret A20 ( 6aq5 pin5 - c11 - t4b) .
I'm getting close on 200VDC instead of the 155V on the drawing. Also, I'm not getting any AC after C11 ( mV, nowhere near the 100VAC I should be getting)
I've spent all day , checking thru all of these posts, comparing with my la2a, and I can't for the life of me, track down where I've gone wrong.

I copied the layout from cayacostas site.R34 IS 10K.
the only things I've got different are :-
I'm using a 6AQ5W /6005 ( got 2, and tried both) instead of a 6AQ5A,This is what i got sent .
and I've omitted the stereo adjust - I've connected pin 1 of R37 limiter response (top pin on drawing) to a 1M resistor, then to earth.
the 510pF cap, comes off pin 6 (6aq5), connected to earth,
pin 1 & 7 (6aq5) coneected together, and connected to earth.

Is this correct ?

I've checked all front panel wiring a few times, and it all seems correct. :?
I've checked all earths o.k.,

with no T4b plugged in, and a 1K sine wave (0.7V), meter set to out+4, I can alter needle position with zero adjust pot,but nothing on meter in GR or out+10,
Is This Correct?

with T4b plugged in, meter set to GR, when a light is shone on the cell, meter deflects to the left o.k.

Valve Voltages.

V1 12 AX7 #1 (ECC83)

1- 119
2- 0
3- 1.05
4- ac
5- ac
6- 122.6
7- 0
8- 1.1
9- ac

V2 12BH7

1- 92
2- 0
3- 3.8
4- ac
5- ac
6- 222.5
7- 78
8- 107.6
9- ac

V3 12AX7 #2 (ECC83)

1- 91.6
2- 0
3- 0.82
4- ac
5- ac
6- 91.5
7- 0
8- 0.82
9- ac

V4 6AQ5W /6005

1- 0
2- 3.78
3- ac
4- ac
5- 197  :shock:
6- 108.7
7- 0

6AQ5W and 12BH7 are running quite hot. hotter than the ECC83's.

I hope someone can give me a bit of guidance with this one  :oops:

I got to go to bed soon, I've been up since yesterday 12.30pm, I stayed up after my 12hr nightshift , to try and sort this out !, and its getting on for 12 midnight tonight.
my head is in the shed !!

Best Regards,
Steve :thumb:
Title: All things LA2a related
Post by: AnalogPackrat on October 25, 2006, 09:29:08 PM
Steve,

You've tied the control grid of the sidechain output amp (6aq5) to ground!  You need to tie pins 1 & 7 to the other end of the 1M fixed resistor you used to replace R3.  You should then see a healthy signal at the T4b.

:guinness:
A P
Title: All things LA2a related
Post by: Kid Squid on October 26, 2006, 09:08:45 AM
Hiya Analog,

checked and done the alteration, but i'm still getting high volts on A20 /V4 - 6AQ5- pin 5 - 210VDC,
I'll try my other 6AQ5 this afternoon, I'm thinking, I've fried them maybe ?

got my daughter for this afternoon,
I'll get back this evening, and let u know how it's going.
Steve :thumb:
Title: All things LA2a related
Post by: AnalogPackrat on October 26, 2006, 11:01:13 AM
Is C7D also connected to pin 6 of your 6AQ5?  If you pop the cover off of your T4B, dim the lights, feed a signal in, crank up the GR pot, do you see the EL panel glowing/blinking?

As for the bh7 and aq5 running hotter--they should.  No problem there.

A P

<edit>  After more coffee...I wouldn't worry too much about the high voltage on the 6AQ5 screen grid.  Others have had similar readings and everything still works.  If you do the math for screen grid current you'll see that according to the schemo voltages, it should be 600-700mA or so.  You're getting more like 300mA.  I don't think it's a problem.  Try passing audio through it and compressing now that you've got the conrol grid issue sorted.  I think you'll find that its working.
Title: All things LA2a related
Post by: dripelectronics on October 26, 2006, 01:52:36 PM
switch r34 to a 22k 2watt

and r31 to a 1 k  , that will put all the voltages right ,

every time i built an la-2a if i did not do this , all the voltages were way to
high.

one of the  original la2's  used this 22k/1k config.

 as far as the 110 ac voltage on c11 , i  never have had
my meter actually register anything AC .

 so dont worry ,

 ill double check , but im sure it doesnt actually register 110 ac.

g.
Title: All things LA2a related
Post by: Kid Squid on October 26, 2006, 07:13:11 PM
Analog,

Sorted it mate !, inbetween my daughter getting some sleep, checked it thru, with a t4b in place, put input sine to 1.2v, glowing like a good one :grin:

Does what it sez on the tin  :thumb:

I'll set the final value of R25 tomorrow, and do the R7 calibration, put some sounds thru it, and post some pics.

dripelectronics,

AC on the t4b, goes up to about 65 -70 VAC, depending on the gain reduction setting. LA2a no.2 , i will try the different R34, R31.
Thanks for the input guys :grin:

the earlier problem I had with the switch in +4 reading gain reduction,
is that , on the switch's mechanical stops, I had removed the wrong stop !
so, in position 1 ( out+10), the switch poles, were not connected to anything. Only pos2 was showing out+10, pos3 was showing GR.
removed other stop - got the switch sorted !!

Cheers Lads,

Best Regards,
Steve :guinness:  :guinness:
Title: All things LA2a related
Post by: AnalogPackrat on October 26, 2006, 11:12:02 PM
Good job, Mr. Squid!  Now go forth and squish...

Iechyd da!
A P
Title: All things LA2a related
Post by: raysolinski on October 27, 2006, 09:13:02 AM
I built Drip's unit with edcor's on the input and output....just for giggles I put a UTC A25 on the output last night...REALLY different...the UTC was way more thick and vintage sounding..I prefer the Edcors hands down ..I could see a place for the UTC but the Edcor is much cleaner..I was suprised how different the two tranny's sounded..I was expecting subtle differences...

Cheers,
Ray
Title: All things LA2a related
Post by: Kid Squid on October 27, 2006, 10:00:40 AM
SOUNDS THE F**KIN' BOLLOCKS  :thumb:  :thumb:

Pics Here
http://www.groupdiy.com/index.php?topic=18913

Nice one Fellas  :guinness:  :guinness:

Steve
Title: All things LA2a related
Post by: matta on October 30, 2006, 04:00:27 AM
Hey Guys,

Here a question for all of you LA-2A builders... Resistors...

In the Uaudio Reissue they have gone with Carbon Comp Resistors, I guess to keep in step with the 'original' design.

I've been looking into reasons, or rather trying to understand WHY a particular resistor type was used and over another and I've found through Google a few bits of info and I can rightly/wrongly sumize the following...

Resistors suffer from 3 types of noise: thermal noise, shot noise and contact noise (through there are are others). In tube amps thermal and contact are the main players.

All resistors have thermal noise and this noise is independent of type so for example a 1K Metal Film will have the same amount Thermal noise as a carbon comp Resistors of the same value with the same voltage across it.

Contact Noise (1/f Noise) is the result of fluctuations in the conductivity of the Resistors materials, particularly bad in Carbon Comps, this increases in volume as you increase the current though the resistors. So if we increase the current across it 5 times, we increased the noise 5 times.

In order of least noisy you have Wirewound, Metal Film, Carbon Film, Carbon Comp etc.

I think the discussion for using Carbon over say Metal Film is that they 'sound more musical', but that for means the 'noise' is more musical, not the sound passing through the Resistors, and in that case if you used Metal Film, you would have less noise, which seems better than musical noise?

So why use Carbon Comps in something like the LA-2A? Does it make a big difference in tone if you put in metal Film instead? I'm sure you get a much bigger difference in sound say swapping the transformers rather than the  resistors, or the tubes you use,  is the tonal quality effected by using Metal Film over Carbon Comps?

I've heard people say it sounds 'cleaner' with metal film, what does that mean? Are they referencing 'cleaner' being less noisy with 'dirty' referring to the sound they get from noisier Carbon Comps?

Has anyone had the chance to A/B Metal Film vs Carbon Comps in the LA-2A project and noticed a perceivable audible difference?

Thanks in advance for your comment. Tube/Valve stuff is all kind of new to me and this is my first HV project having wrapped  up COUNLESS transistor ones.

Cheers

Matt
Title: All things LA2a related
Post by: Bluzzi on October 30, 2006, 12:02:53 PM
Matt, I can only tell you I used Metal Film in my first one and using same in my second one. It sounds great! Very quiet (not even a hiss) except for an almost imperceptible (except at full blast) 60hz hum which I haven't bothered to do away with. It sounds that good that I just want to use it now.

I'd be surprised if anyone finds that much of a difference in using carbon type. Everything does make a difference but how much and is it worth it to start worrying about it. Use good tubes and soldering technique, and proper grounding. The rest was done for us.

If there is anything to improve in the LA2A (now that  I've buit one) I'd say its the input transformer ratio. Its too high. Maybe even the output transformer could be a bit tamer. I'm using original UTC ones and I have enough for 1 more LA2A but if I ever make more I would use less gain on the input.

I was worried that my LA2A was not going to sound good (it was my first DIY in decades let alone tube one). I can't wait to have my second one finished! Its not the most versatile compressor out there but so far on some vocals it has magic.

You are going to love making and using this project.

jim
Title: All things LA2a related
Post by: guavatone on October 30, 2006, 09:45:52 PM
mat, I did mine with CC's based on Caycosta's notes.  He has a parts list with the R #'s and they correspond to this PCB.  Dunno if it make's a diff. since I never heard the same with films, but mine is super quiet.
Title: All things LA2a related
Post by: CJ on October 30, 2006, 10:04:59 PM
I like the old resitors for the plate circuit, that's about it.
Use the 1 or 2 watters.

Lousy tempco might mean usefulness in regulating a pwr supply.

I wish I knew what the V72/76 type resitors were made out of.
I do not think they are carbon comp.
Title: All things LA2a related
Post by: matta on October 31, 2006, 04:36:39 PM
Hey Guys,

Thanks so much for all the info and encourgament. I'm slowly pieceing it all together, have all the Tubes, Sockets and the Caps are on order from Mouser.

It makes sense to me to use 1% Metal Film where one can except in the PSU and where 'lousy' Tempco can be used to ones advantage.

Quote
I like the old resitors for the plate circuit, that's about it.
Use the 1 or 2 watters.


CJ,

I assume looking at the schematic that those resistors are R34, R32, R9, R13, R15, R17, R18 assuming you are talking about JUST the resistors on the plates?

Cheers

Matt
Title: All things LA2a related
Post by: CJ on October 31, 2006, 06:44:58 PM
Thats right, on second thought, I do not think the EL panel cares about carbon comp mojo, so standard metal there.

Save the carbons for the Fender.
Title: All things LA2a related
Post by: Kid Squid on November 01, 2006, 06:28:13 AM
Hiya Matta,

I think it will sound great, whatever resistors you use  :thumb:

no, in fact , I knows it will  :grin:

Seriously, I haven't got another la2a yet, but i built mine with metal films, and it sounds F**kin' terrific :wink:
just my two-penneth on the matter, Matta  :grin:


regards
Steve :thumb:
Title: All things LA2a related
Post by: CroatianSensation on November 21, 2006, 01:11:00 AM
hi all!

I've been hanging around this forum for some time but haven't posted anything...until now.
I'm nearly done building a Bloo La2a and everything seems to be dandy save for 2 voltage readings.
A20 is a mere 10v ':cry:' and A19 is a whopping
274v ':shock:' with the tubes in place.
I have yet jam in the T4B.
I've looked over my work repeatedly. It's not the neatest I've seen but I cannot find any mistakes...any suggestions?
Title: All things LA2a related
Post by: CroatianSensation on November 25, 2006, 03:58:54 PM
Awwright.
Well, I checked C10 for orientation, the cathode ground and then-the most obvious thing-the tube! ':roll:' Lo and behold the 6AQ5a had crapped out.
Luckily a buddy of mine had one laying around so now I'm back in business.
Here are new improved readings along with what they should be:

A1: 54v/60v
B21: 352v/360v
A20: 82v/131v
B20: 278v/275v
A14: 230v/216v
A17: 82v/90V
A19: 124v/105v

Are these well within range to pop in the T4B?
The only ones that weird me out are A20 and A19.
Any suggestions? Comments?
How was the turkey?
Title: All things LA2a related
Post by: CroatianSensation on December 02, 2006, 08:37:38 PM
hello?
Title: All things LA2a related
Post by: CJ on December 02, 2006, 08:44:40 PM
The turkey was great, now plug in the T4.
Title: All things LA2a related
Post by: CroatianSensation on December 02, 2006, 10:55:51 PM
All systems are go.
It hasn't blown up yet, it's passing audio and seems to be compressing.
SWEET!
The two voltages are still off but that doesn't seem to be hurting things.
This is my second DIY project....well my second successful one.
I was just a little apprehensive when it came to jammin' in the T4.
Thanks for the vote of confidence CJ. Your posts and this forum have been a great help.
Time to squash some stuff!

-Jason
Title: All things LA2a related
Post by: CJ on December 03, 2006, 12:17:19 AM
Good going!
Remember that tube voltages can be 20-30 percent off, and everything will still work fine, unlike transistor circuits.
Title: Voltage issues
Post by: emx on December 06, 2006, 03:53:19 PM
Ok quick question   I think I may possibly have a bad power tran it seems to be giving off a bit more voltage than I'm needing and what it's rated at.  I'm getting this at these points.....

A22 +23   Main set off of Power Tran =572VAC  supposed to be 500
A24 +25   Heaters  =6.84VAC  Supposed to be 6.3 I think


Should I pull the Power Tran out and double check it off the Turret board?
I would think that It would be unnecissary,   I used the Allied power supply rated at 500VAC,   heaters are supposed to be 6.3 I believe.
Title: All things LA2a related
Post by: emx on December 06, 2006, 04:44:50 PM
Edited post
Title: All things LA2a related
Post by: emx on December 11, 2006, 12:28:12 AM
Ok  Pulled both the Red and Green wires from the turret and am getting these voltages......

582 VAC off of the Main Reds
7.3 VAC  off of the Heaterwires

I'm wondering if Allied does returns this long after the sale(5 or so months)
I'm also wondering if I can just adjust the voltages by replacing say  R34 or R29,   Probably I should go with just waiting for a different power tran though.
Title: All things LA2a related
Post by: AnalogPackrat on December 11, 2006, 12:54:15 AM
Was your first measurement (572V and 6.8V) with all the tubes in?  If so, yes, it seems like you might have a mislabeled transformer.  Is this a Bloo kit or something you're doing from scratch?

A P
Title: All things LA2a related
Post by: emx on December 11, 2006, 12:33:06 PM
The last voltage readings were with both the primary and heater wires removed from the circuit,  so they are being checked outside of the circuit.   The readings from before were with the wires still attached to the turret,  with the tubes in.    I'm doing this completely from scratch.    I'm going to contact Allied and see if they will do a replacement.
Title: All things LA2a related
Post by: emx on December 13, 2006, 05:41:25 PM
hmmmm,   just bought another Allied power Tran and am getting close to the same readings off of the new one wich makes me think that my Rat Shack tester may be off on it's readings.    It Irks me to think I have possible been holding back on running the whole unit due to a crappy radio shack multimeter.
Title: All things LA2a related
Post by: emx on December 14, 2006, 04:58:05 AM
I just picked up a new multimeter and it's getting close to the same readings as the Rat Shack one.    Is it normal for Allied power Transformers to be this much higher than the label states?
Title: All things LA2a related
Post by: dripelectronics on December 14, 2006, 05:52:32 AM
are you using the allied 6k88vg ?

you should show about 278v DC after r29. (4.7k 2 watt)


it's not the tranny , somethings not getting fed it's diet of power.

give us some tube voltages , and perhaps i can help you .

i dont speak bloo , but if you could just shout out the tube pin numbers ,
that would be cool.

g,
Title: Mono only unit?
Post by: nielsk on December 14, 2006, 09:05:04 AM
I am building a mono only LA2, anyone know what I can leave out? I am looking at R30, C6, C12.....Could R3 be replaced with a fixed value? How much is R37 actually used?

Thanks!
Title: All things LA2a related
Post by: emx on December 14, 2006, 04:28:24 PM
What if I were to be getting too much current from my AC directly off the wall socket?   I'm getting 122/123 V,   how much would this affect things?
My power Tran,  the 6k88vg,  is rated at 117 V and only supposed to put out 500v and 6.3v even though both of my trans I've bought put off closer to 600v  6.8v heater when removed from the circuit?    I'll have to get back to you on the tube voltages when I have a minute.
Title: All things LA2a related
Post by: Bluzzi on December 14, 2006, 04:35:31 PM
I may be mistaken but it seems you may be measuring both windings. Measure from the center tap and see what you get.

jim
Title: All things LA2a related
Post by: AnalogPackrat on December 14, 2006, 05:24:06 PM
The transformer is rated "at load."  The voltage will drop once it has to work a bit.  An LA-2a should draw about 15-20mA from the B+ supply.  Hook it up and turn it on (with tubes) long enough to get a measurement on both sides of R29 (which will allow us to calculate the current draw).

A P
Title: All things LA2a related
Post by: emx on December 14, 2006, 06:19:43 PM
Ok before r29 is 398  after is 395v.    Also I've just noticed that my neon is only lighting up on one side and not both the bars inside the bulb.   I might have broken one side of the neon connection when I moved the turret board.  I'm going to check this as well.
Title: All things LA2a related
Post by: AnalogPackrat on December 14, 2006, 06:38:24 PM
Don't sweat the neon yet.  Mine only partially lights up and it still works.  Neon is just providing a voltage reference for the GR mode of the meter (sort of like a zener diode works).  Anywho...you are not drawing the correct current from the PSU which is why your voltage is high.  This is with all tubes in?  If so:

1) Verify that R29 is the correct value.  With the power off and the big caps COMPLETELY discharged, measure R29 (I think you can do this in circuit as R29 has no parallel path around it).

2) Check your sidechain wiring (V3 and V4).  V4 should draw about 12-14mA.  I suspect something is wrong in that area as that is the approximate amount of "missing current" if R29 is correct.

A P
Title: All things LA2a related
Post by: emx on December 14, 2006, 07:20:01 PM
Ok R29=4.6k  

I'm not exactly sure how to measure the MA  or V3 and V4,  which pins am I measuring?
Title: All things LA2a related
Post by: AnalogPackrat on December 14, 2006, 07:28:04 PM
OK, if R29 is 4.6K (4k6 for the euros), we can use...wait for it....Ohm's Law to compute the current.  We know the voltage at both ends of R29, so the voltage ACROSS R29 is 398-395 = 3V or so (your meter may not be very accurate for HV).

V = IR, or I = V/R

I = 3V / 4600ohms = 650uA (less than a milliAmp).

Your neon lights, so we know it is passing current.  Looking at the schemo and indicated nominal voltages and using Ohm's (oh, so useful) Law again, the neon needs about 500uA to run.  My guess is that you have not completely connected your B+ to the rest of the circuit.  There should be a lot of things connected to the node between R28 and R29.  Are there?  Can you trace along wires from that point to the several plate resistors feeding the tubes (R9, R13, R17, R33, R34)?

A P
Title: All things LA2a related
Post by: AnalogPackrat on December 14, 2006, 07:30:34 PM
Another possibility is that you have not connected the tube grounds.  If the plate Rs are connected to the B+, make sure your have all the ground connections.  I think you are missing a single wire somewhere (B+ or ground).

A P
Title: All things LA2a related
Post by: emx on December 16, 2006, 02:53:20 AM
Ok,   I had to pull out the turret board on my unit as it was obstructing my view of the pins on the valves.   In doing so I noticed that one wire in particular came off of (I think it was) pin 3 on V3 very easily like it was almost not even soldered on completely,  this was connected to one of the turrets on the board that I noticed was a lot lower on voltage than it should have been unfortunately I couldn't correct this without removing the turret board.  I'm going to get back to the forum when I have completed putting it back in shortly.
Title: Less gain needed - what's best way?
Post by: Bluzzi on December 16, 2006, 08:21:40 PM
I've been using my first LA2A a lot lately and although I sub'd a 12AY7 for a softer gain its still touchy.

A.P. suggested using 100K for R9 and R13. I haven't tried that yet and its the easiest thing so I probably will do that first.

The other thing in my mind is to use a lower gain Input Xfr. I believe the A10 is 1:10 right? Wouldn't using something like a 1:3 or 1:4 reduce the gain substantially and make the Gain knob's travel useful throughout its range?

The Jensen transformer JT-11P-1 is a 1:1. Would that be a drastic change?

I love my LA2A when its tame. However with a singer that has poor mike technique the high level at input causes distortion. So far I've been lucky as its a Blues project I'm doing and it actually suits the singer and style.

I guess what I mean is how do I reduce the distortion at input. I'd still like to have it available but only when I twist the Gain knob past 2 oclock maybe.

Just trying to find that perfect blend before I undertake finishing my second LA2A.

jim
Title: All things LA2a related
Post by: dripelectronics on December 16, 2006, 11:12:36 PM
i found that using an alps blue velvet really helps ,
it has a nice slow log.

found that i could dial in with more precision .
Title: All things LA2a related
Post by: NewYorkDave on December 16, 2006, 11:21:48 PM
With a line-level input signal, there's really no good reason to use an input xfmr with such a high ratio. Use a lower ratio--even 1:1--and you'll be happier, I'm sure.
Title: All things LA2a related
Post by: Bluzzi on December 17, 2006, 11:57:02 AM
Thanks Dave. Thats what I figured since some have used the Jensen ones at 1:1 and I didn't see any complaints from them. I'll have to get one and try it out.

If you notice though most of us using the originals find the input too strong.

I'll be renting a new LA2A during the holidays to try and see any differences.

jim
Title: All things LA2a related
Post by: emx on December 18, 2006, 03:36:54 AM
Ok Repatched my turret board back into the unit and I caught several things as I did.   Initially I had opted for a more functional mounting of the turret where it was facing directly out of the unit facing forward,  this mounting proved to be not so much of a good idea and I then mounted it like everyone else here does.   In the process of moving it I guess 2 wires came loose,  I fixed this.   Also I just found out why some things weren't grounding properly,  the turret's I bought to use as grounding posts didn't conduct well  I ohmed them out at anywhere from 22 ohms to several hundred in the batch as I just now checked them.   Also some of my pots didn't quite ground out right even though I dremel tooled out the inner hole(Next time I'm not screwing around,  it's star washers everywhere.    All my voltages are now roughly in check give or take about 15 or 20 %  -------except , not quite sure about my neon.   My neon doesn't light unless I turn R4 up almost half-way    and I'm getting anywhere from 17 to 57 volts off of it depending on how R4 is set,  is this normal?
Title: All things LA2a related
Post by: dripelectronics on December 18, 2006, 09:01:28 AM
your neon is ok , it does that .

your voltages can fluctuate alot , 15-20% can be ok ,
but best idea would be to list all voltages of each tube ,
the orginal resistors were plus or minus 10%

given the carbon resistors , i imagine 20% is ok too.

 we can go over them with you.

but it does sound like your on the right track.

best.

g.
Title: All things LA2a related
Post by: emx on December 21, 2006, 03:43:14 AM
Ok here is my turret voltages,  I'll get right back with the specific tube voltages.
..............................................................................

A1-17 to 57 volts
A2-266
A3-108
A4-113
A5-1
A6-0
A7-109
A8-109
A9-.002
A10-107
A11-217
A13-0
A14-217
A15-.003
A16-7.5
A17-82
A18-0.834
A19-133
A20-116
A21-275AC
A22-279AC
A23-279
A24-3.2AC
A25-3.2AC

B1-17 to 57V(depending on setting of R4)
B2,B3,B4-266
B5,B6-18
B7-111
B8,B9,B10-70
B11-2.5
B13-0
B14-265
B16-0
B17-265
B18-0
B19,B20-265
B21,22,23-370
B24,B25-123AC

I'm doing the Cayocosta Layout with the 10K R34's  and 4.7K R29's.
Title: All things LA2a related
Post by: emx on December 21, 2006, 03:55:39 AM
Ok here is the tube voltages.....

V1
____

#1-108V
#3-1V
#6-113

V2
____

#1-109V
#2-0.02
#3(through 1kResistor to TurretA6)-0V
#6-220
#7-70
#8-111

V3
______

#1 and #6-82V
#3 and #8- 0.834v
#7-0.003


V4
_____

#2-7.3 to 7.5V
#5-116
#6-133
Title: All things LA2a related
Post by: dripelectronics on December 21, 2006, 04:39:27 AM
(edit )

the 10k R34 screws every ones voltages up ,
replace it with a 22k  along with r36 to a 1k  
and your 6aq5a voltages will all fall into place.

like i said , i dont speak bloo , so i cant help you on the turret board.
but i bet the 22k replacement will help alot ,

hell it set me back a year in my designs because the voltages were
messed up due to it ,

brand x uses the 22k / 1k combo too.
Title: All things LA2a related
Post by: dripelectronics on December 21, 2006, 04:53:38 AM
V1
____

#1-108V FINE   (reissue is 133v /la2a 96v  /schematic is 146v)
#3-1V     FINE
#6-113 FINE    (reissue is 135v / la2a 101v / schematic is 120v)

V2
____

#1-109V FINE
#2-0.02 NORMALLY READS 0 (but that seems fine)
#3(through 1kResistor to TurretA6)-0V  (SHOULD BE 4 VOLTS) trace all
   conexions to this pin , something might be loose or wrong.
#6-220 FINE
#7-70 FINE
#8-111 FINE

V3  (12AX7#2 ?)
______

#1 and #6-82V     FINE ( reissue is 105v/la2a is 90v/schematic is  100v)
#3 and #8- 0.834v  FINE
#7-0.003 FINE  (USUALLY READS ZERO)


V4
_____

#2-7.3 to 7.5V   FINE  (reissue is 10.4 / la2a is 2.8/ la2 is 5.8)
#5-116   A LITTLE HOT , TRY THE R36 1K / 22K COMBO will correct this
#6-133 FINE

hope this helps ,

g,
Title: All things LA2a related
Post by: emx on December 21, 2006, 04:55:58 AM
I noticed you stated that in a previous post to someone so I ordered some 22k resistors to try it out that came in about a week ago so I'll try that too.   R36 on mine is already 1k so I won't need to change that then.
Title: All things LA2a related
Post by: emx on December 21, 2006, 04:58:47 AM
R36?   I thought it was R31 that had to be changed to 1k,   OK maybe I need to change that one too.
Title: All things LA2a related
Post by: dripelectronics on December 21, 2006, 05:01:09 AM
(edit)

r36 and r31 should be 1k

.......................................

yeah i figured as much , because your voltage was right on with pin
2 of the 6aq5a ,

the 22k will fix the other one ,

seems like your almost ready  to roll her out of the shop.

just trace the wireing to the tube pin  thats lacking the 4 volts.

g.
Title: All things LA2a related
Post by: emx on December 21, 2006, 05:01:18 AM
As for R29, does it stay 4.7k?
Title: All things LA2a related
Post by: dripelectronics on December 21, 2006, 05:15:17 AM
that one stays 4.7k

g,
Title: All things LA2a related
Post by: Purusha on December 21, 2006, 06:08:31 AM
I have a question. With my silver T4B came a resistor and on the manual it
says that I need to use it in the R25 place if I remember it right. Do I use
it on the R25 place on the Drip boards too or is this only meant for the original LA-2A boards?
Title: All things LA2a related
Post by: dripelectronics on December 21, 2006, 03:37:46 PM
yes ,youll replace that resistor ,
ill bet money that it's a 33k , they all seem to be 33k from urei :)

nice cases by the way ,

could you email me the price and shipping to the united states of america.

best ,

g.
Title: All things LA2a related
Post by: Purusha on December 21, 2006, 05:11:23 PM
E-mail sent.
Title: All things LA2a related
Post by: Purusha on December 21, 2006, 08:22:15 PM
I am almost there with my LA-2A  :grin:

Is there anything wrong if the PCB is mounted so that the tubes
come near the back side of the case? I have my doubts as in this
way the main power wires have to run all over from left back side
to the right front side of the case.

(http://www.tat-purusha.net/diy/LA-2A-mount.jpg)

Should I turn the PCB for 180 degrees around?
Title: All things LA2a related
Post by: Neil on December 21, 2006, 10:20:18 PM
Wouldn't you want the tubes in the front, since that's where your slots for cooling are? Edit: Never mind, was looking at the pic backwards! D'oh!
Title: All things LA2a related
Post by: dripelectronics on December 22, 2006, 01:01:06 AM
I usually have it so the power section is right near the power x former.
,
seems to work out better this way ,  makes it so the wires to the
 gain and reduction pots is shorter.


and less power wires on the inside.

it's hard , because you put the metering section closer to the front , but you also put the  audio path of the gain and limit controls more
near the transformer .

try it with the power section near the transformer ,

you can use shielded wire for the meter if you want ,
it would be compairable to a point to point la-2a ,
the metering section does have some nice long wire runs in it.


g.
Title: All things LA2a related
Post by: emx on December 22, 2006, 01:02:13 AM
Hey Drip,

  I did the resistor switch and was wondering what the B+ value should be with this setup, just curious as I inititially had somewhere around 265 and now am closer to 300.  
   I think their may have been some confusion as to what my reading on V2 pin #3 as I was checking the voltage on this after the resistor which on my layout goes directly to be connected to R10 and connected with Transformer #2 directly to ground  so the 0 reading doesn't worry me much,  maybe you check yours before the resistor,directly off of the tube?   My pin #3 is covered with shrink tubing so I can't get to it without problems, it's a really tight squeeze in that area as it is.

   Anyone here have any info on the output cap placement?  I ran the unit and it ticked until I pulled it out away from the unit near V1, V2 and the neon,  it is passing audio however.
I'm assuming I should place it somewhere else away from all tubes, neon, and transformers? :grin:
Title: All things LA2a related
Post by: dripelectronics on December 22, 2006, 01:09:57 AM
this is odd ,

so to sum up ,

you replaced r34 with a 22k and you replaced r36 with a 1k.

correct ?

this shouldnt have brought the b+ power rail up more.

it's normally 275 ish ,

your running hot ,

wich probably means one of the tubes is mis wired and not doing it's share of sucking juice.

(so to speak)

lemme post some voltages.

1 sec
Title: All things LA2a related
Post by: dripelectronics on December 22, 2006, 01:32:26 AM
12ax7#1

pin#________brand x___schematic_____la2_____la2a
1___________133_________146_________111_____96
2___________---------------------------------
3____________1__________1.2_________.9______1v
6____________135v_______120_________122_____101
7____________--------------------------------
8____________1___________NA________1.4_____1v

12bh7a
pin#________brand x___schematic____la2______la2a
1___________105_________100_________95______92
2___________--------------------------------
3___________4.6__________NA_________3.4_____3.8
6___________226__________220________195_____200
7___________77___________NA__________65_____65
8___________111__________NA_________100_____98

12AX7#2
pin#________brand x___schematic____la2_____la2a

1_____________98_________NA______89_____90
2_____________---------------------------------
3_____________.8________.75_______.70____.75
6_____________98_________NA______89_____90
7_____________-------------------------------
8_____________.8_________NA_______.7____.75

6AQ5A
pin#________brand x___schematic____la2_____la2a

1_____________---------------------------------
2_____________10.4_______NA________5.8_____3.8
5_____________85_________85________125_____100
6_____________134________NA________104_____70



sheesh !

'note pad' was giving me hell
Title: All things LA2a related
Post by: Purusha on December 22, 2006, 04:13:50 AM
Quote from: "Neil"
Wouldn't you want the tubes in the front, since that's where your slots for cooling are? Edit: Never mind, was looking at the pic backwards! D'oh!


That is the thing. The cooling slots are in the back...  :? Should have known better.
Title: All things LA2a related
Post by: dripelectronics on December 22, 2006, 05:20:00 AM
i think as long as the case is vented , it will be fine ,

everyting in there can handle alot of  heat.

gosh i could  cook eggs on my avalons that i got rid of.

best ,

g,

ps thanks for the email.
Title: All things LA2a related
Post by: Purusha on December 22, 2006, 01:42:45 PM
Hi,

I need a bit of a help with wiring my Hammond power juicer and both Sowter transformers. Is there any info on this in regard to the LA-2A project? I have the PDF called LA-2A hook up notes but the colours on my Hammond are not the same as in those notes  :?

Hammond 369JX - http://www.hammondmfg.com/pdf/5c0028.pdf

Two more things.

How do you ground the T4B? Mine is in aluminium case. The only thing I can do is connect the wire to the screw on the side of the T4B case.

And regarding the comp/lim switch, when 1 and 2 connected LA-2A works in LIM or COMP mode?
Title: All things LA2a related
Post by: sintech on December 22, 2006, 02:36:40 PM
Regarding In and Out trafos:

Sowter input:

XLR Pin 1:  Chassis
XLR Pin 2: Green
XLR Pin 3:  Brown

Trafo Blue : Input signal to limiter
Trafo Grey tied with Black: Ground


Sowter Output:

Trafo Yellow : Is output signal from the limiter
Trafo Green tied with Black : is ground

XLR Pin 1:  Chassis
XLR Pin 2:  Pink
XLR Pin 3:  Grey
Title: All things LA2a related
Post by: Purusha on December 22, 2006, 06:36:10 PM
OK, a few more questions:  :grin:

How do I wire the meter select switch?

P123 top
P123 bot

Mine has only +4 and GR option. Switch is dual ON/ON toggle.

And I still have no idea how to wire Sowter iron to the board. On the board is info screened for the original iron and not for the Sowter.

Thanks Sintech for the info though, it helped a lot.
Title: All things LA2a related
Post by: dripelectronics on December 22, 2006, 09:00:52 PM
lemme pull some files togeather ,
ill post in an hour.

g.
Title: All things LA2a related
Post by: dripelectronics on December 23, 2006, 03:49:59 AM
(http://driprecords.com/pcb/sowter-hook-up.jpg)
Title: All things LA2a related
Post by: dripelectronics on December 23, 2006, 03:56:21 AM
for the meter switch

not using -10

is as such if looking at the back of switch.

    2   p   3    top
 
    2   p   3    bottom

 the 'p's go in the middle

sounds wierd ,
i know  .

g.
Title: All things LA2a related
Post by: Purusha on December 23, 2006, 04:13:57 AM
Thanks Greg,

So 3 ground points are on the board?
And we have one star ground on the chassis plus Hammond is grounded to the screw where it's mounted. Did I understood this right?

What about the Hammond wiring? Can you check this too?
Title: All things LA2a related
Post by: Purusha on December 23, 2006, 04:32:53 AM
I just found this picture :razz:

(http://users.volja.net/purusha/LA-2A-BUILD.jpg)

OK, to confirm if I understood this right.

1. All the GND tags go to the chassis as close as posible to the board?

2. I cut off the rest of the Hammond wires which are not used?

3. If fuse is used in the IEC, I just jumper the fuse section on the board?
Title: All things LA2a related
Post by: emx on December 24, 2006, 06:01:27 AM
Anyone here have a good high-res copy of the 1968 schematic?    I've looked on cayocostas site as his layout is what I'm working with, and I believe he is working with the 1968 version.   The schematic on his site is a little hard to read on V4 Pin 5 but it looks like it's 120 or 130.
Title: All things LA2a related
Post by: emx on December 24, 2006, 07:19:54 AM
Another Question,   on the schematics that have the 22k R34,     is R38 used at all?    I'm looking at a schematic in front of me that's a different version than th 1968 one that uses the 22k R34 and it shows absolutely no R38 so far as I can see, just a straight shot to the filter caps.
Title: All things LA2a related
Post by: The Kid on December 24, 2006, 01:10:11 PM
Quote
Another Question, on the schematics that have the 22k R34, is R38 used at all?


About 20 pages back (out of the 105 pages of this thread) this is covered at least twice.  :wink:
Title: All things LA2a related
Post by: Purusha on December 24, 2006, 05:14:49 PM
OK, everything at it's place  :grin:

what next?  :roll:

What is the safety procedure? No tubes and T4B and measuring the tube sockets or what?

What current should I see coming back from the Hammond? I would like to measure if this thing works OK?

I did power up the unit without the tubes and T4B. Should I see the meter light bulb burn at this stage or not?

I am new to the LA-2A build so please help me out with all caution instructions  :razz:
Title: All things LA2a related
Post by: dripelectronics on December 24, 2006, 08:04:50 PM
hey there , happy cristmas.

i usually just flip the switch and duck.

the fuse will save you if any thing is wrong.

usually leave the t4 out to start.

on the hammond ,

ive never wired up a european one ,
so im a bit unsure .

did you have a center tap ?
otherwise youll have to put some resistors to ground and to each leg of the heaters.

the neon will light up with a turn of the zero adjust/

good luck ,

g.
Title: All things LA2a related
Post by: Purusha on December 25, 2006, 04:32:46 AM
Hi Gregory,

Can you please check the PDF of my Hammond 369JX - http://www.hammondmfg.com/pdf/5c0028.pdf

I will not try to power it up with tubes until someone confirms the wiring on my Xformer  :?

I bought it here
http://www.tubetown.de/ttstore/product_info.php/info/p442_Hammond-369JX.html
and I really don't know if it's center tapped or not. Someone gave me this link so i bought it.
Title: All things LA2a related
Post by: matta on December 25, 2006, 09:32:47 AM
Quote
I really don't know if it's center tapped or not.


Purusha,

If you look at the datasheet you will clearly see that it is. Look under the the 369JX info and you will see the Filament #1 is CT (which stands for center tapped) 6.3VAC.

On the 369JX the extra wires you see are the MULTIPLE primaries that the transformer has because it runs at 100, 110, 120, 200, 220 and 240 VAC.

If I read the datasheet correctly the wiring on the secondaries is as follows.

The High Voltage will be RED and RED, just like the Allied and Greg's info with a RED/YELLOW CT which goes to the chassis.

The Heaters/Filament or the 6.3VAC wires are BLACK/YELLOW and go to the LA-2A PCB marked as GREEN, the CT is orange and again that goes to the chassis just like the RED/YELLOW one (Having seen a pic of the trano i can see the datasheets is wrong as the filament is colour coded from filament #3 not one, being GREEN and GREEN witah GREEN/YELLOW CT).

I think your operational voltage in Europe is the same as ours, being 240VAC (well ours is actually 230VAC), in which case you would need to configure it as such.

In order to do this you would have to put the primary windings in SERIES with each other.

To do so connect the BROWN and the BLACK wires together (120VAC + 0V)

Then your mains wires would be the BLUE wire and the WHITE wire.

So the wires that are not connected would be the BLUE/YELLOW, BLUE/BROWN, BLACK/RED and WHITE/BLACK, these must be insulated with some heat shrink and taped up.

I would advise you DON'T cut them as one day you may want to change the primary voltage if you find yourself overseas, and would have the flexibility to do so.

I would NEVER blindly just follow the datasheet. I would test the transformer windings with a  multimeter BEFORE just hooking it up to the PCB because you may have a problem with an incorrect winding that could damage your LA-2A.

So mount it in the box and make sure you ground the center taps and throw the switch and measure your secondaries. You should get over 500VAC across the RED HV wires and over 6.3VAC the BLACK/YELLOW wires... I say over as all unloaded AC tranos SHOULD be over the loaded voltage, I forget the % and math behidn it.

I hope that helps you out, enjoy your Christmas!

Cheers

Matt
Title: All things LA2a related
Post by: dripelectronics on December 25, 2006, 05:51:15 PM
thanks matt ,

wasnt sure how to do that.

best

g.
Title: All things LA2a related
Post by: matta on December 25, 2006, 06:57:13 PM
Hey Greg,

No worries! I think I got it all down right, but someone can over look my writings to make affirm it.

Sadly Purusha was a bit heavy handed with his wiring and didn't check the datasheet, which I have warned him against as these are not Micky Mouse voltages, and they can KILL you!!!

Below is a pic of how I take it it should be wired up as, in the CORRECT section, sadly Purusha had it wound like the INCORRECT side.

(http://www.matt-allison.com/diy/LA2/LA-2A_Power_Trafo.gif)

He has since corrected it to my wiring diagram, but now he is getting noting out of either the HV or Filament secondaries :-(

Could he have toasted the transformer? Maybe CJ or one of the more 'advanced' trano guys can answer because I don't know and as I think my wiring guide is correct, I can't account for it NOT working.

Is there another way to test to see if it is working or the fault lies elsewhere?

Cheers

Matt
Title: All things LA2a related
Post by: Purusha on December 26, 2006, 03:11:35 PM
I guess CJ hasn't seen my PM yet  :green:
Title: All things LA2a related
Post by: CJ on December 26, 2006, 07:24:59 PM
Lordy, for a while there, it looks like you had a neon pwr supply.

12 times 300 is 3600 volts!

Ohm out all the windings with the wires un soldered and report back immediately.
Title: All things LA2a related
Post by: Purusha on December 26, 2006, 07:37:07 PM
Hey CJ, I am on it  :wink:
Title: All things LA2a related
Post by: Purusha on December 26, 2006, 08:34:39 PM
OK, here we go:

(http://www.tat-purusha.net/diy/Hammond.jpg)

Let me know if I missed something.
Title: All things LA2a related
Post by: CJ on December 26, 2006, 09:20:20 PM
Your cool.
Put in a new fuse and let er rip.
Pull the tubes first.
Title: All things LA2a related
Post by: Purusha on December 27, 2006, 09:20:37 AM
Yap, the fuse was dead  :thumb:

Will try to power it again tonight  :grin:   ...this time with correct wiring  :green:
Title: All things LA2a related
Post by: CJ on December 27, 2006, 03:02:47 PM
You might want to ohm out that first cap, I think it took a hit, which took the fuse out.
Title: All things LA2a related
Post by: Purusha on December 27, 2006, 07:05:16 PM
I checked all four and most of the time I see around 20M. Only the second cap has around 300K resistance.
Title: All things LA2a related
Post by: CJ on December 27, 2006, 07:21:17 PM
good deal.
self healing, ya know.
Title: All things LA2a related
Post by: Purusha on December 28, 2006, 08:17:20 PM
The Hammond works Ok  :thumb:   Many thanks to Matta and CJ!

I connected everything and it looks fine. I see around 500VDC and 6.7V out of the Xformer. I powered the thing but with no tubes and no T4B. No smoke came out so far. :green:  Is there anything I need to measure before installing the tubes and T4B. I think I checked all things a few times by now.  :wink:  

Oh, I quickly checked on 12AX7 #1 tube socket and pin 1 and 6 have 320VDC on... is this a good thing? I really can't check the rest as it's hard to come near the pins.
Title: All things LA2a related
Post by: dripelectronics on December 29, 2006, 01:48:26 AM
hey there with no tubes or t4 ,  the  12ax7#1 voltage should be around 370 plus or minus
(im here in the states so i could blame my power company)
320 sounds good enough ,

im showing  388v before r29

and most of the tubes pin 6's read 380v

sound like the moment of truth  :twisted:

g.
Title: All things LA2a related
Post by: Purusha on December 29, 2006, 08:27:01 PM
OK, made some progress  :thumb:

After I wired the heaters I powered up the unit. Everything looked fine so I set the meter scale to 0 in GR
mode and run some audio through. Sound goes through so this is a good sign but...

... the meter doesn't respond to audio in +4 or to gain red. in GR mode. Gain knob works fine but the GR
knob lowers the volume at last 15% when turning it CW. What could be the problem and where should I check first?  

Is there any way to check my T4B if it works OK? Are there any other wires to be soldered under the PCB beside the heaters?
Title: All things LA2a related
Post by: drpat on December 29, 2006, 09:15:21 PM
....
Title: All things LA2a related
Post by: dripelectronics on December 29, 2006, 11:23:41 PM
(http://driprecords.com/pcb/meter-switch-hook-up-web.jpg)
(http://driprecords.com/pcb/meter-switch.jpg)

http://driprecords.com/dripelectronics/pcb_manual1.pdf
Title: All things LA2a related
Post by: Purusha on December 30, 2006, 04:05:57 AM
Quote from: "drpat"


This isn't a problem. The GR knob is supposed to reduce gain as you turn it clockwise, thereby lowering the volume. Check your wiring to the meter...


 know this :wink: but is it OK that I notice the GR so late at 80 on the scale of 100 and not before?
Title: All things LA2a related
Post by: Purusha on December 30, 2006, 04:14:47 AM
I checked my meter wiring and it is wired OK so something else must cause the problem  :?

Can someone tell me also what is the proper comp/lim wiring to the switch?
Title: All things LA2a related
Post by: dripelectronics on December 30, 2006, 08:07:05 AM
http://www.driprecords.com/pcb/la2_hook_up_notes.pdf

a little essay on the pots/switch

g.
Title: All things LA2a related
Post by: Purusha on December 30, 2006, 08:38:28 AM
Hi Gregory, I have all your PDF pots/switch notes  :wink:  but in the comp/lim notes there is nothing mentioned about when is either of these two engaged. For example when you short those two wires is LA-2A working as comp. or limiter?
Title: All things LA2a related
Post by: Purusha on December 30, 2006, 08:42:20 AM
Hi Gregory, I have all your PDF pots/switch notes  :wink:  but in the comp/lim notes there is nothing mentioned about when is either of these two engaged. For example when you short those two wires is LA-2A working as comp. or limiter?

I will check again my wiring to be sure it's done 100% correct but other than that is there anything else I should check regarding my problems with meter? It shows 0 when in GR mode but it doesn't show either the output signal nor GR when I crank the GR knob fully CW. I hear signal going down at the max GR but don't hear any pumping effect so I doubt the GR really works as it should.
Title: All things LA2a related
Post by: matta on December 30, 2006, 09:45:04 AM
Took me 2 seconds to work out by looking at the LA-2A schematic... maybe you should try doing the same instead of being spoonfed with every little thing, You are are taking the DIY out of DIY with all your projects  :?

Cheers

Matt
Title: All things LA2a related
Post by: Purusha on December 30, 2006, 10:11:51 AM
You are right, this one was easy  :green:
Title: All things LA2a related
Post by: dripelectronics on December 30, 2006, 11:07:14 AM
hi there ,

the meter section can be a bit of a problem ,

one of the common mistakes can be the meter switch it's self.
if it's an on -off - on with a midle position in the throw , those often don't
work.

the wireing is easy , but can be confussed .

your meter might need the diode rectifier configuration.

the most common mistake is a wire is loose,  or the shield jacket
is shorting one of the pots .

check for solder or tiny wires shorting the pots or switches.

check your HAX GND make sure its  soldered.

turn your stereo adjust pots / limiter response pots full clock wise or
full counter clock wise , one of those positions will bring on the juice.

g.
Title: All things LA2a related
Post by: Purusha on December 30, 2006, 05:18:30 PM
Quote
your meter might need the diode rectifier configuration.


Thanks for the tips. BTW, I am using Sifam AL29. Don't know what meter you usually use in yours.

Will check everything what you mentioned twice and if I still can't get it fixed then you will hear from me again  :wink:
Title: All things LA2a related
Post by: matta on December 30, 2006, 05:27:42 PM
Purusha,

Your meter doesn't need a bridge rectifier, it is built into the Sifam. Happy troubleshooting, hope you get it all up and running, you are 90% of the way there already  :thumb:

Cheers

Matt
Title: All things LA2a related
Post by: jho-audio on January 01, 2007, 06:52:06 PM
Happy new year folks!

I'm using the drip boards and I need some help figuring out edcor wsm connections..  I have some loud hum and oscillation going on, likely more than incorrect transformer hookup but I want to get the in/out correct.  

CJ style rendering of my current setup..  

(http://img102.imageshack.us/img102/3141/edcorla2afs7.jpg)

Is this okay?  

Also, I'm using the edcor power transformer which has no ct on the heater secondary..  i will go buy some 100ohm 1 watt resistors to get reference to ground as im sure that this is creating some of my hum.

Cheers, Jason


EDIT:  the xlr connections in the image are incorrect..  i have pin 1 to gnd and pin 3 is going to the transformers in place of pin 1.  I am actually using 1/4" jacks and TR to transformer S to gnd.
Title: All things LA2a related
Post by: dripelectronics on January 02, 2007, 08:02:36 AM
I havent tried the edcors yet , taking a look at the diagram now.

funny thing is edcore is here in New Mexico ,
i should support my local economy :)
maybe ill order some to test them out.

those ct ohm resistors on heaters

could help.


g,
Title: All things LA2a related
Post by: CJ on January 02, 2007, 07:13:57 PM
Isn't Pin One Ground?

BTW, I went thru the whole thread, here are some pages that might be useful:

Differences between Revs:  Page 15
Close up of 6AQ5 stage: Page 55
Voltage and Resistance Charts: Pages 68, 81, and 102.
C4 Trimmer:  Pages 78 and 85
Sowter Wiring: Page 102

cj
Title: All things LA2a related
Post by: jho-audio on January 03, 2007, 03:04:02 AM
yeah, pin 1 is gnd..  the image is incorrect.  i have pin 1 to gnd and pin 3 is going to the transformers in place of pin 1.  

I spent the time to check out this thread, entirely..  its quite a read.  I think I may have retained 1% of what I read at most.

Nice job pulling the page numbers out.  Might be helpful to keep that info on the first page or in a meta somewhere as this page is likely to get buried as well.  

I'm quite sure that I didn't get my Edcor hookups right when I look at the UTC, Jensen and Sowter connections.  Just can't seem to make sense of where the wires should go.
Title: All things LA2a related
Post by: jho-audio on January 03, 2007, 11:34:24 PM
100 ohm resistors from heater secondary to gnd fixed my hum issue entirely...  still getting boat loads of oscillation.  using the chopstick to move wires around doesn't seem to be changing the oscillation either.  I'm going to re-wire my rats nest la2a and see if I cant figure this out.

Still looking for someone to verify my edcor wiring job.. any insight is appreciated.

Cheers
Title: All things LA2a related
Post by: King_Cruiser on January 04, 2007, 01:11:27 AM
Decrease R11, 68k on the UREI diagram.
Try 62k.
Title: All things LA2a related
Post by: jho-audio on January 05, 2007, 01:02:20 AM
Thanks for the tip..  tried a 100k pot to dial in the feedback.. changing this value (ended up at 56k) helped ever so slightly.  Still have lots of oscillation.  Re-wired and tidied up the compressor, tried the chopsticks again, still didnt help.

Do either of the transformers need to be isolated from the chassis?  both are bolted straight to the chassis providing a ground connection.

Not too sure what else to try.  Anyone have input on edcor connections?  I ended up with the following scheme so far:

input (WSM10k/10k)
- pri #1 to XLR pin 2
- pri #2 to XLR pin 3
- pri CT n/c
- sec #1 to HAX pin 7 connection
- sec #2 to chassis gnd beside transformer
- sec CT n/c
- HAX GND to chassis gnd beside transformer

output (WSM10k/600)
- pri #1 to A24 pin 7
- pri #2 to A24 GND (on board)
- pri CT n/c
- sec #1 to XLR pin 2 and A24 pin 6
- sec #2 to XLR pin 3 and A24 pin 1
- sec CT n/c

also changed R5 to 11k as recommended by Jensen..

Double and triple checked my ground connections and front panel to ground..

Not sure what else to do at this point.
Title: All things LA2a related
Post by: jho-audio on January 05, 2007, 01:11:33 AM
dont know if this piece of info helps troubleshoot..  the oscillation seems to go away temporarily when a loud source is sent in.  As soon as the source lets up the oscillation comes back.  Switching from GR to +4db also changes the oscillation.  In GR the oscillation is slow and almost at one constant frequency, in +4 it oscillates with shorter wave lengths and a higher pitch.
Title: All things LA2a related
Post by: King_Cruiser on January 05, 2007, 06:09:30 AM
C4
Title: All things LA2a related
Post by: jho-audio on January 05, 2007, 09:06:17 AM
Still no go.  tried 90, 180 and 360 here..  all have the same oscillation.
Title: All things LA2a related
Post by: King_Cruiser on January 05, 2007, 10:20:26 AM
LEVEL ONE>

1) Input

A) Short Out XLR Pins 2 abd 3
B) Short Out B Side of Input Magnetics>
-1 Before  UREI Sch. R6 68k
-2 After 68k

C) Short the grid of V1 at Pin 2.

2) Amplifier

A) Short Out Pin 7 Of V1

3) Output

A) Strap 610 Ohm Resistor Across Output Magnetics B Side
B) Strap A Side Of Output Magnetics To Ground And Turn Up Volume To 11-Listen For Mechanical Osc.

C) UREI Sch. C5

-1 Outide Foil
-2 Proximity Effect

4) Grounds

A) Resistance
B) Control Panel to Main Chassis
Title: All things LA2a related
Post by: jho-audio on January 05, 2007, 10:55:56 AM
nice..  thanks!  I'll be checking through this list tonight when I leave work.
Title: All things LA2a related
Post by: jho-audio on January 06, 2007, 01:13:31 AM
Quote
1) Input

A) Short Out XLR Pins 2 abd 3


Signal cut.. still oscillates.

Quote
B) Short Out B Side of Input Magnetics>
-1 Before UREI Sch. R6 68k
-2 After 68k


Not really sure where to make these connections?  short one secondary to gnd?  short one secondary to after R6?

Quote
C) Short the grid of V1 at Pin 2.


No audio passes with this short in place.

Quote
3) Output

A) Strap 610 Ohm Resistor Across Output Magnetics B Side


Same oscillation with minor motor boating.

Quote
B) Strap A Side Of Output Magnetics To Ground And Turn Up Volume To 11-Listen For Mechanical Osc.


No audio passes.. Couldn't identify any oscillations.

Quote
C) UREI Sch. C5

-1 Outide Foil
-2 Proximity Effect


No changes by moving this cap.. cap foil seems just fine.  Cap is new Sprauge Atom.

Quote
4) Grounds

A) Resistance
B) Control Panel to Main Chassis


Going to start checking resistance at my gnd points..  ill look through this thread again to find these measurements unless someone has them readily available?

Control panel to main chassis gnd is good.

pin 1 of 12bh7 is 124v, should be 100v..  going to also look into this to see what could be causing it.

Thanks again for the help!![/quote]
Title: All things LA2a related
Post by: jho-audio on January 06, 2007, 02:41:35 AM
just added a 1k resistor in series to r29 raising the value from 4.7k to 5.7k.  This brought my 12bh7 pin 1 value down 10v from 135v to 125v and my pin 6 value down to 260v from 290v..  drastically cut down the oscillations.

I have way too much b+ (316v at r29) and this seems to be the issue.   Im going to pick up a bunch of 2w resistors tomorrow between 6.8 and 9k and see if I can get r29 down to 275v from its current 316v output.
Title: All things LA2a related
Post by: jho-audio on January 06, 2007, 02:48:41 PM
yay..  success!

A 10k 2w resistor at r29 brought my b+ to 270v and my 12bh7 pin 1 to 105v...  swapped my rca 12bh7a tube for a new ei 12bh7 and oscillation seems to now be gone.  

I feel like a tool for this.. forgot that the edcor power transformer had 550v secondary rather than the 500v expected.  

the Edcor in/out transformer hookup is real simple.. ignore the ct's, ground the input to its own ground post and you are good to go.. same connection as in my text post about hookup, ignore the picture.  They sound great and are cheap, highly recommend giving them a shot!

Thanks for all the help and to those who made this project possible.
Title: All things LA2a related
Post by: King_Cruiser on January 06, 2007, 04:01:30 PM
Fantastic
Title: All things LA2a related
Post by: dripelectronics on January 06, 2007, 08:34:24 PM
GJ :thumb:
Title: All things LA2a related
Post by: emx on January 22, 2007, 03:05:21 PM
I've got a question about R5.    I've noticed that The jensen trans have a different value for r5 when they are used and was wondering if I'm using a similar 1:1 ratio transformer(the cinemag CMLI 15/15/b   http://cinemag.biz/line_input/CMLI-15-15B.pdf)  if I should be changing the value of R5?
Title: All things LA2a related
Post by: emx on January 28, 2007, 07:32:38 AM
I guess a better question would be......

What specifically does the Jensen mod address?
Title: All things LA2a related
Post by: EmRR on February 16, 2007, 05:02:34 PM
Quote from: "drpat"
What is everbody using for the 10µF 450v output cap? Has anybody played around with different ones to see which sounds best???



has anyone tried putting the output cap between the transformer and ground rather than between the tube and transformer?  This is rarely seen, but regarded by some to sound superior in some circuits.  Just a thought.
Title: All things LA2a related
Post by: dripelectronics on February 16, 2007, 05:32:27 PM
interesting .

after grounding that cap ,

then what  ?

that wouldnt pop the fuse ?

what happens to the output x former , how does it do it's job ?

and since your sending the audio into the case , how do you get it out ?
 :grin:

g.
Title: All things LA2a related
Post by: EmRR on February 16, 2007, 06:14:20 PM
no, not audio directly into the case from tube through cap to ground; swapping the relative positions of the output cap and the output transformer with one another.   Same end result; no DC flowing through transformer, but audio seeing transformer before seeing cap.  Biggest difference, and I'm talking about the UTC or similar hi-voltage rated iron, is transformer having DC potential.  The UTC is rated to handle the potential.  I'm certain the Edcor isn't.

I'm a little amazed in 108 pages that no one's tried to get a better deal on an LS-10 or LS-50 and used them instead.  They're sometimes cheaper these days, and certainly better sounding AND more level appropriate.  I can't begin to imagine why someone would pay over $150 for an HA-100X over an LS-10.   I've won LS-50's as cheaply as $50 in the last six months.  Why would I want to pay for an A-24 in this situation?
Title: All things LA2a related
Post by: EmRR on February 16, 2007, 06:17:30 PM
Quote from: "raysolinski"
I built Drip's unit with edcor's on the input and output....just for giggles I put a UTC A25 on the output last night...REALLY different...the UTC was way more thick and vintage sounding..I prefer the Edcors hands down ..I could see a place for the UTC but the Edcor is much cleaner..I was suprised how different the two tranny's sounded..I was expecting subtle differences...

Cheers,
Ray



I'd never expect an A-25 to compete with an A-24 here.   I'd only consider it to be a purposeful sonic downgrade.
Title: All things LA2a related
Post by: dripelectronics on February 17, 2007, 12:01:42 AM
hmmm that sounds like it's worth exploring ,
im kinda bored now that im done with the v2 pcb.

havent tried the LS series , it's probably one of those secrets
people keep.

best

g,
Title: All things LA2a related
Post by: EmRR on February 17, 2007, 01:49:12 PM
LS only seems to be a secret among recording types; I have no idea why.  Too big?  The taint of the hi-fi people liking them?   They got something right.    They used to cost way more, but the recording people have pushed the A and HA stuff over the top (since it used to be cheap) while totally ignoring the LS stuff.


Somewhere in this thread, use of backwards A-24 as input with lower step-up gain is mentioned by someone and not reported on further.   I've got 1970's RCA preamps using A-24's as inputs in an API 2520 based circuit, so it's been done before.
Title: All things LA2a related
Post by: CJ on February 18, 2007, 09:22:12 AM
LS 10, Gibson Hummingbird, same deal across the pond.
No listen, just stare.
Thats why. Check evilbay.

A 10, HA 100 twin sec is art work.
4 Pies = 16 times less stray C.
New  UTC has 2 Pies = 4 times less.
Somone wanted 3 pole, ended up with Bessel vacuum cleaner, vacuums up the highs, that is!



LS much easier to wind.

Can you name a compressor that uses backwards UTC?
Title: All things LA2a related
Post by: EmRR on February 18, 2007, 02:56:49 PM
Nice riddle/poem CJ!


Compressor using backwards UTC?  Hmm...
guesses:

UA 175/176 family interstage? don't have the data in front of me

Fairchild 660/670 sidechain amp section?  don't have the data in front of me; think not right.


And a question for you:  Guess the mono limiter using (3) LS-10X's and an LS-63.

Hint:  Uses a pair of #46 bulbs as variable resistors in a bridge shunt circuit (in between two separate amp sections), along with a feedback based variable-mu front end.   Actually, that gives it away if you've ever seen it.
Title: All things LA2a related
Post by: CJ on February 20, 2007, 09:35:03 PM
Hmmm Langevin Leveline?

The 670 is correct.

UTC A 26 in reverse.
Title: LA2a Capacitors C2 C3 and the 10uf 450v Capacitor
Post by: Jazz on February 20, 2007, 09:50:35 PM
I have some sprague oil filled caps 0.1uf 400v I wanted to try these has anyone done it yet? any feedback?

Also the important cap 10uf/450v I have a sexy looking Angela cap that I am tempted to try, but it is not electrolytic it is Metalized Polypropylene. It can be seen here. Wondering if this would work?

 http://www.angela.com/

Did not mean to pop in in the middle of things. I just noticed the posting of all La2a things should be posted here.
Title: All things LA2a related
Post by: EmRR on February 21, 2007, 12:49:29 AM
Quote from: "CJ"
Hmmm Langevin Leveline?

The 670 is correct.

UTC A 26 in reverse.


Ah!  didn't realize there was an A-26 in the 670.  

Correct answer is Gates 28-CO.   Most versions instead use two HA-100 and an A-11 instead of the three LS-10.
Title: All things LA2a related
Post by: babyhead on February 21, 2007, 06:24:28 PM
Hey All.

Having just finished a drip LA-2a, I thought I had a problem with the threshold. It seemed that I had to hit it pretty hard to make it work. Being a tweaker, I thought of picking up the side chain before the 68k R6  and adding a 33.1k (R6.5?) to the GR pot. It acted like a nuke, almost worthy of a switch... The scheme didn't seem as useful as the normal configuration so I left it. I have a vintage looking panel that I dont want to muck up. I might do this on #'s 3 & 4 LA-2as, though. The next one on the bench is with the Jensen output. I will compare the two and then attempt two in one box... joy.

You all Rock.
Title: All things LA2a related
Post by: Purusha on April 07, 2007, 04:04:22 PM
I have finished my LA-2A with Sowter iron and ver. 10 PCB and just read a thread at Gearslutz
where Fletcher mentioned that todays UA version doesn't sound as the real Teletronix or Babcock
LA2A which are the best sounding LA-2As you can get in his opinion. Anyway I wonder, does
anyone know what transformers are in UA version, Sowter maybe?

I wish someone could compare the Drip version with Sowter iron to the real Teletronix or Babcock LA2A...   :cool:
Title: All things LA2a related
Post by: matta on April 07, 2007, 04:15:23 PM
Quote from: "Purusha"
I have finished my LA-2A with Sowter iron and ver. 10 PCB and just read a thread at Gearslutz
where Fletcher mentioned that todays UA version doesn't sound as the real Teletronix or Babcock
LA2A which are the best sounding LA-2As you can get in his opinion. Anyway I wonder, does
anyone know what transformers are in UA version, Sowter maybe?

I wish someone could compare the Drip version with Sowter iron to the real Teletronix or Babcock LA2A...   :cool:


UTC Reissues, ALOT more expensive than the Sowters, you can buy them from UA as spares, about $300-$400 a set I think.

And Greg HAS compared them, if you read his notes over the various threads when he deisgend the board he bought an ORGINAL Tele unit and a Reissue to compare them and make tests, he will tell you how different they sound.

Cheers

Matt
Title: All things LA2a related
Post by: Stagefright13 on April 08, 2007, 06:38:04 PM
Studio.electronics on ebay sells the reissue transformers. They are also the ones that sucked up all the last Urie t4b's. Of wich I purchased 1. Think the output was like $200 and the input $150. But they have a cheaper input solution I believe. They ARE reputable and I did get a sealed NOS Urie t4b.
Title: All things LA2a related
Post by: Purusha on April 09, 2007, 12:59:53 PM
I wonder what are my alternative solutions for the Mallory TC72 if I don't find one. Will this expensive solen (http://www.banzaieffects.com/Solen-Fast-10uF-630V-pr-17735.html) work just as good or will it effect the sound in a different way?
Title: All things LA2a related
Post by: Stagefright13 on April 10, 2007, 11:20:52 PM
Alliedelec.com has the TC-72's for 11 usd. 483 in stock right now. Plus shipping to Slovenia of course...
Title: All things LA2a related
Post by: Purusha on April 11, 2007, 03:41:53 AM
Thanks! But what would I gain or loose if I put in the Solen cap instead?
Title: All things LA2a related
Post by: matta on April 11, 2007, 03:49:13 AM
Quote
Thanks! But what would I gain or loose if I put in the Solen cap instead?


A different sound....

If you read Greg's (Drips) thread he used a Sprague and found it didn't sound like his LA-2A reissue, tonally it was different, he then added the Mallory... BINGO!

It didn't sound BAD, just not the same...

I suspect sonically the Solen would be a smoother sound, poly caps normally are, just around the time the LA-2A was built/designed there were not caps of that size that would easily fit the unit, and also would have cost alot more.

Since you already have a working LA-2A with a TC72 why not get a Solen and A/B it?

Seems easy enough to do, then you can let us know what you think.

Cheers

Matt
Title: All things LA2a related
Post by: Purusha on April 11, 2007, 04:09:11 AM
Thanks Matt. I might do that but it will be a test with my first unit as the second won't have the same PCB, T4B and neither the same tubes.

BTW, what tubes do you guys prefer in this project? I plan to go with JJ this time.

It's sooooo long thread so I don't feel inspired right now to go through all the way :oops:
Title: All things LA2a related
Post by: guavatone on April 19, 2007, 01:45:32 AM
What is A11 and A14 and where are they.  Turret points?

I've been trying my best to search this monster :twisted:
Title: All things LA2a related
Post by: mcfarlane_audio on May 02, 2007, 08:30:46 AM
ok so after paroozing all 110 pages of this thread, i have a question, my la2a is working great, save for one problem, its quiet, that is t