All things LA2a related

[college101]

Hey guys!  Im back..My son was born, he is doing great, and I actually have some time to go down to the bench and get back to workin on the la-2a

Unit is passing Audio

After doing some research...First couple things I am going to check out( Pop in here with your feedback at anytime!)

First few things I am going to re-check
R38-220K or 22K res ?????

R4-does it really matter if it is linear or not?

C10-100uF/25v

C13-510pF /450V

C14- arco 465(shoud not be part of this problem)

C15- 470 pF/450V

I printed off the Voltage Chart from DJ's book- Im going to re-check all the voltages from the tubes

with respect to ground....SO basically set my black lead to the ground, and then test each point with the Red lead...((Right?))

  In terms of having the back panel control's set correctly....they are supposed to be turned counter clockwise when looking at the inside back panel((Correct?)) y/n ??  ACTUALLY- just tell me from looking at the back of the unit,,,looking directly at the unit with the tubes facing me...which way should the controls be turned!! That would help...

Thanks Guys

 

[capnspoony]

Hey guys

just finished a drip la2a version 2.0

It has low volume
A decent amount of low end hum
and after a few minutes the little neon lamp bulb went from flickering (which I know it should just be "on") to dying.
Haven't popped in the T4 cell yet because of these issues.

Not really sure where to begin this build is so seemingly straight forward...
I've built two point to point versions and had zero issues

any help would be appreciated!

thanks

 

[Hank Dussen]

low volume = input transformer backwards?
I had this in mine.

 

[capnspoony]

thanks

fixed

it was a leg of a tube that was soldered but wasn't making contact

oh the continuity tester how thankful i am you exist

also just a note make sure you put your T4B cell ceramic socket in the right way!

they are hard to remove  :-[

sounds fabulous now after some troubleshooting

 

[dannysound]

Hello, my first LA2A build and it all seemed to be working except the current draw on the 6AQ5A seemed to be too much. I get 100v peak into the EL panel at max GR but the supply line sags with the attenuator drive signal causing a low freq ripple.
I changed R34 to 22K 3W, R36 to 1K and removed C13 and R38 connecting pin 6 to the 220K and 30uF cap as on the other schematic floating around the net. After this change the B+ increased from 265V to 284V I now get:
B+ 284V
V4
Pin 7 = 0V
Pin 6 = 134V
Pin 5 = 97V
Pin 2 = 10.5V

I know the screen grid shouldn't be more positive than the plate so I checked all the resistors and caps in the area and they're good. I'm thinking it could be the 6AQ5A is on it's way out but I don't have a replacement yet.
If anyone has any suggestions of other things to check it would be much appreciated, plus it would take my mind off the sex noises coming from next door!
Cheers Danny.

 

[sircletus]

Greetings, folks.  I've been working on a two-channel LA-2A style opto limiter in my spare time.  The sidechain and metering circuits are adapted verbatim (with the exception of a 60V zener in place of the neon), and I'm using Kenetek T4s.  Everything works and sounds pretty good at this point, except for one thing:

Whether or not there is any signal at the input of the device, when I crank the "peak reduction" knob, the VU meter's needle buries itself to the left.  During this time, I'm measuring about 28Vrms at pins 3 and 5 of the T4, so something fairly significant is happening.  I suspected there was likely some oscillation that I wasn't hearing.  Sure enough, I scoped it and there seems to be a high frequency oscillation around 46kHz.  But it's only there when I crank the peak reduction knob.  There's a TINY bit of it at the output (2mV rms), but the VU meter doesn't see it.

I suspect it's coming from the 12AX7 in the sidechain, as turning the "stereo adj" pot down to ground makes the oscillation disappear.  There is no measurable AC going into the peak reduction control with no signal present (as expected).  I've tried a few different tubes in that position, but no difference.  Anyone else ever see this?  Is it kosher to put a low pass filter between the 12AX7 and the 6AQ5?  Say, maybe a cap across the stereo adj pot?

Incidentally, what's an LA-2As stereo adj pot default position?  I would assume full CCW, yes?

Thanks,
IZ

 

[sircletus]

Regarding my last post:  Problem solved.  Power supply issues.  Local .1uF decoupling cap took care of the oscillation.

IZ

 

[JCT Audio Designs]

Hey all, I tried to post this under the cases topic, but there seems to be a problem there. I am getting ready to build a couple of LA2A clones and was wondering is anybody wanted to go in on some cases I have built. Below is a couple of links to some pictures of the last ones I had done. As you can see I have the side chain control put on the front as I have seen how this control is very useful. The first link shows the actual front panel, the Vu meter will have to change because that meter is no longer available. I will probably use TN-73 NISSEI meter as it is inexpensive and looks good. The second link shows the rest of the chassis. Please email me at [email protected] if you are interested.

http://www.facebook.com/photo.php?fbid=222593859501&set=a.222591774501.175477.502824501&type=1&theater

http://web.mac.com/JCTAUDIODESIGNS/JCT_Audio_Designs/LA2A_2.html

 

[bdubya]

Good to hear you solved it Sircletus.  I've had similar phenomenon when the wire carrying the CV  to the t4B (the one going to pin 3) is physically too close to the other t4b wires.  Or if C11 is leaky.  Or if the 6aq5 is bad, like you, or someone else, was saying. 


b

 

[bdubya]

That case that ilfungo pointed out is the ****.  Why go elsewhere?  It's $57.  For what it's worth, the color variations of it are thusly (according to a rep at Allied):

5-525DLG2 is a smooth light grey
5-525DCG1 is Beige Textured
5-525DGY2 is Grey Smooth
5-525DBK2 call Smooth Black

It's steel, you can work it with a center tap, a bunch of small diameter bits for pilot holes, and one of those stepped drillbits. 

There are some really nice looking Rustoleum colors, some with texture, that are really tough with only, like, two coats.  You want something rust resistant, I would think.

Also, the "ouncer" types of transformer are totally useable in this circuit and are, as of yet, fairly affordable.  I don't need more than +8dB nowadays.

my 2sense.

 

[Aleguitarpro]

Hi!
I'm from italy and I've just finished a point-to-point version of La2a (based on cayocosta layout).
This is my first experience with point-to-point (and I think the last one!!).

I've a problem of motorboating (I think is this the name): I've read in this thread several cases of my problem.
I've done some tests that I'm going to post, to help me and other like me that find this problem to fix it.

Before posting, I've checked all previous post about this problem and I've done this tests:

- my neon (NE2) works correctly
- la2a clone startup correctly, and all tubes works.
- I've tried to move the wires in the backs of sockets, but motorboating change his velocity, but remain.
- I've tried moving OUTPUT CAPACITOR, but nothing changed. The only thing I noticed is that moving wire that connect Pin 6 with C2 on the board, motorboating change in frequency.

I put as attachment a pic of my clone, to understand if I done a mistake in placing tubes and PSU in the rack.

TESTS

1. I've done a test with all tubes and no T4b. Moving clockwise gain pot, increase motorboating and full clockwise noise disappear, leaving this sort of white noise.

2. Test with V1 and V2 no T4b, no input: motorboating is still there, without changing nothing.

3. Test with V1 and V2 no t4b, SM57 mic input: when I startup the unit, the sound pass without motorboating, but when tubes warm up motorboating starts. As you can listen in my tests mp3 in link, sound pass and unit preamp correcly. I've put my mobile in front of SM57 only to try if it works. The lack in frequency is obiouvsly due to mobile speaker.

After few seconds, problem of motorboating change and depends directly on the source with input: no input, only white noise - no motorboating. With input, motorboating starts!

You can listen my tests here:


How can I solve this problem??
It depends on the messy wiring inside my la2a rack or in the position of tubes?
I think that I could change positions of tubes, putting them toward the back of the rack. In this way I could reduce the lenght of wires, but anyone that have found this problem on his clone, could share his experience in order to fix definitively this problem??

Thanks very much!!!!!!!
Aleguitarpro

 

[lassoharp]

First thing I would check are the positions of output related wires to input related wires.  They need maximum separation to avoid oscillation problems.

 

[Aleguitarpro]

Thank you, Lassoharp !

I've followed your suggestion and I've separated more the wire of input and output.
I've changed the position of Output transfomer and change the position of tubes inside the rack: now tubes are putted toward the back of the case.
In this way I've reduced the lenght of wires.
In the photo you can see the mods.

Now all works correcly, no motor boating and no hum.

Thanks!!!

 

[analogexplosions]

So, i'm working on my first LA-2A build, and i'm still confused about the Sowter I/O Trannies that i'm using.  (1009 and 1010)

I've read conflicting posts on various places on how exactly to wire these up!  There are far more pins (wire points) on the UTC's, and obviously, Sowter has simplified their transformers to just a few wires.  Do I still wire resistors between leads like on the UTC's, or is that irrelevant to the Sowters?

Where am I going to solder all these wires?!  Any help would be greatly appreciated.

 

[Biasrocks]

So, i'm working on my first LA-2A build, and i'm still confused about the Sowter I/O Trannies that i'm using.  (1009 and 1010)

On the 1009 compared to the UTC A-10

The primary side

Red = Pin 1
Blue = Pin 6
White (CT) = Pin 4 + 3

Pin 2 + 5 are omitted from the Sowter.

Secondary

Green = Pin 7
Grey = Pin 10

There is no secondary CT available on the 1009, don't worry about it. It's internally connected.


Sowter 1010 compared to the UTC A-24

Primary side

Red = Pin 7
Yellow = Pin 10

CT Pin 9 + 8 are internally connected

Secondary side

Grey = Pin 1
White = Pin 6

CT Pin 4+3 are internally connected and not available, not needed.

Pins 2+5 are not available.

Black can be connected to Chassis Ground.

And yes, you need the exact same resistors as used with the UTC's.

Hope this helps to get you started.

Regards,
Mark

 

[analogexplosions]

So, i'm working on my first LA-2A build, and i'm still confused about the Sowter I/O Trannies that i'm using.  (1009 and 1010)

On the 1009 compared to the UTC A-10

The primary side

Red = Pin 1
Blue = Pin 6
White (CT) = Pin 4 + 3

Pin 2 + 5 are omitted from the Sowter.

Secondary

Green = Pin 7
Grey = Pin 10

There is no secondary CT available on the 1009, don't worry about it. It's internally connected.


Sowter 1010 compared to the UTC A-24

Primary side

Red = Pin 7
Yellow = Pin 10

CT Pin 9 + 8 are internally connected

Secondary side

Grey = Pin 1
White = Pin 6

CT Pin 4+3 are internally connected and not available, not needed.

Pins 2+5 are not available.

Black can be connected to Chassis Ground.

And yes, you need the exact same resistors as used with the UTC's.

Hope this helps to get you started.

Regards,
Mark

Thanks!  That's exactly the kind of translation I needed.

Time to wire!

 

[analogexplosions]

Also, according to the schematic and layout by cayocosta, R7 says "1k-2.7k".  Does this translate to either 1k OR 2.7k, BOTH 1k AND 2.7k in parallel (wired together), or any value in between?

If i've read correctly, this resistor comes into play when the LA2A is on Limit mode, correct?  My guess is the higher the value, the more dramatic the effect is.  Drip calls for i think a 3.9k resistor for their R7.

Any thoughts/clarification?

 

[Biasrocks]

I forget what I used, but it was probably 2K7.

Search is your friend.

http://www.groupdiy.com/searchresults.php?cx=partner-pub-1749889900964910%3A1545385773&cof=FORID%3A10&ie=UTF-8&q=la2a+r7&sa=Search

Also, according to the schematic and layout by cayocosta, R7 says "1k-2.7k".  Does this translate to either 1k OR 2.7k, BOTH 1k AND 2.7k in parallel (wired together), or any value in between?

 

[analogexplosions]

I'm 99% done with my wiring now. 

One thing I can't find any information on is wiring for a 3 position dpdt mini toggle switch.  I would IDEALLY like to use center position for GR and left and right for +4, +10.  Is there any way you guys know of to accomplish this?

I'm attaching an image of my switch.

http://dl.dropbox.com/u/7140566/photo.JPG

 

[analogexplosions]

Well, I've realized that this switch isn't going to do what I want it to do anyway...

Now, on to the next problem:

I've done initial fire-up, and nothing exploded!  NE2 lights up brilliantly, and the tubes are lighting up as well.  When I hook it up to my converters to run some audio through it though, all i get is a low hum.  I can't get any level going in, out, or any gain reduction with the t4b in.  I think i've poked and prodded every wire in this thing with a chopstick and nothing changes what's happening...

So here's what I have for power.  Hammond 369JX, green twisted pair to fils and VU lamp, red twisted pair to diode pair.  Primaries for 120V US power is Black/Blue pair to Live AC in and wired to the switch, and Brown/White pair to the Neutral AC in.  Red w/ yellow stripe to ground, and all other wires electrical taped together, and out of the way. 

Have I done something wrong here?  or should I look elsewhere?