All things LA2a related

[AnalogPackrat]

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

 

[CJ]

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

 

[API]

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

 

[AnalogPackrat]

[quote author="API"]
But who cares now, its working!!
Send me another kit, i need more!!!
[/quote]

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

 

[CJ]

You da man!
:thumb:

 

[emx]

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.

 

[CJ]

gonna need a schemo if ext pwr sup.

 

[emx]

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.

 

[CJ]

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.

 

[Bluzzi]

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

 

[yosh]

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

 

[dripelectronics]

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

 

[sintech]

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.

 

[emx]

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?

 

[emx]

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

 

[sysexguy]

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

 

[AnalogPackrat]

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.

 

[sysexguy]

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

 

[AnalogPackrat]

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

 

[sysexguy]

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