All things LA2a related

[analogexplosions]

Okay,

This things is semi-fully functional!

It is now squishing audio and sounding great, but I'm still having some noise issues.  My chop-stick probing tells me it is the wires and their placement.  When I probe some of them, it sounds like a crazy 70's synth witch pitch modulation going on.  I can upload an mp3 if you'd like to hear what it sounds like being probed.

So hopefully this will be my last issue/question on this build:

How do I know where these wires should be run for optimal operation?  Which ones are supposed to be run bunched together and parallel?  which ones are supposed to be crossing at 90 degree angles?  Which ones should be not be touching anything at all?  Which should be twisted? 

Also, I just read up on adding grid-stoppers to the 12bh7 tube.  How exactly does this work?

 

[analogexplosions]

Well, I read elsewhere on here that people were adding 1k resistors to Pins 2 and 7 on the 12BH7.  So, i did that.  Now, I'm barely getting signal into this thing.  What did I mess up? 

 

[analogexplosions]

I am completely at a loss with this thing.  Did grid stoppers ruin something?  The unit will not compress anymore, is super low level, and still oscillates randomly.  I don't get it.  Any input would be greatly appreciated.

 

[analogexplosions]

Now it won't even pass signal.  Just a low oscillation.  How is it that it could work perfectly, and then deteriorate into not working at all so quickly?

 

[Biasrocks]

Now it won't even pass signal.  Just a low oscillation.  How is it that it could work perfectly, and then deteriorate into not working at all so quickly?

Hard to say, retrace your steps back to a working unit.

Take the grid stoppers off.

Check all your connections, it sounds like something is shorted or maybe not making
a proper connection.

Build a signal tracer (guitar cable, film cap to block DC plugged into a small amp) and find out where the signal stops.

Regards,
Mark

 

[lassoharp]

For those who may not have seen this yet.  This is a limited time offer.


http://www.groupdiy.com/index.php?topic=48100.0

 

[tom f]

Hi,
I have a friend who is building me a Drip Electronics La2a Opto 5.
There are only two questions:
- Do you know where we can get the schematics for the Opto 5 PCB?

I have a "Purusha LA-2A (DRIP ver. 5) Case" (the black one).
- Where did you get your VU- Meter? What voltage should it have?

Thanks a lot for your help and your time
Tom

 

[Sredna]

This is probably the wrong forum for these questions, but anyway...

- Do you know where we can get the schematics for the Opto 5 PCB?

From Drip Electronics. Good luck with that.

- Where did you get your VU- Meter? What voltage should it have?

Check at Drips forum.

 

[creal]

Hi all,
i finished my LA2A P2P, layout by Cayocosta.
This unit sound killer, no noise... Very good on voice and guitars, i love this new toy.
I used a utcA24 for output and sowter for input, Drip T4B fast.
The tubes are telefunken for the 12AX7, 6AQ5 GE and EH for 12BH7
the case is reused from an old hedson graphic EQ.

I just on trouble with the VU, the gain reduction mode works ok , but not +4/+10.
I check and recheck the wiring all looks good. Maybe something with the vumeter?
if someone have any idea and advises ???

The thread is here http://www.groupdiy.com/index.php?topic=48320.0
http://www.groupdiy.com/index.php?topic=48320.0

 

[Aleguitarpro]

Hi.
I've finished my second point-to-point LA-2A, based on layout of cayocosta, but made in a normal 2U rack case.

The second clone is perfect: no noise at all, sounds good.

My first LA-2A clone has a lot of gain: the new one, has about an half of gain of my first build...
What could I check?

I've used for all the 2 clones Edcor transformers, same pots, same t4b (it's really the same, because at the moment I've test my second clone with the only t4b that I've).

I've measured all the tensions on the tubes and on the board and they're ok.
Any ideas?

 

[Bowie]

  What might cause the 6AQ5 voltage to be far too high in a P2P build?  I have audio, meter function, etc, but I have not plugged in the T4 yet because of the 6AQ5 reading;
40v on pin 2
328 on pin 5
323 on pin 6 
From my research on this site, it should be somewhere around;
7.5V on pin 2
115V on pin 5
137V on pin 3

It's not the tube as I have tried other known, good tubes.  The other odd thing I've found so far was 358v after the 4.7 resistor next to the diodes.  I've read that it should be 265 or so.  Voltages after diodes is 355, which seems normal.

The 12AX7s are running a little high but not insane like the 6AQ5. 
12AX7s;
130 & 148 on pin 1
1.2 & .87 on pin 3
149 and 153 on pin 6.

12BH7 is 271 on pin 6.

The AC out of the wall is a little high here (averaging over 122v) so my tube voltages are normally a little high.

The only things I can think may be (and these are total guesses) that I did a couple of things like the re-issue, but others like the 1968 schematic.  I used a 47uf cap for C10 (instead of 100 like the schem).  I also sent a separate pair of heater wires to the 6AQ5 (like the re-issue does) and the other tubes get their heaters in series.

The audio is gorgeous but this 6AQ5 issue is concerning so I've hit a wall.
Many thanks in advance for any help.

 

[Bowie]

OK, no takers on that one...  this should be an easier question.
My UA RI has a 1K resistor between CR1 diode and R29 that's not in any schematic or layout I've seen.  I added this and it drought me down a little to 350v @ R29 where it leads to C7A.  However, I'm still getting 313v on the other end of R29...  The resistor measures proper, and is a 4.7k, 2% metal film-through hole.  Is through-hole no good here?  Should use a stronger resistor to get down to the 275v??

The more meaningful thing to look at is that my B+ (313v) doesn't drop more than 1v at the other end of R34!  On my UA RI, it goes from 320v to 180v.  I go from 313v to 312...
I pulled it out and tested, it reads dead-on.  It's a 10k Metal Oxide, 2w, 600v.  The strange thing is that it has a directional marker on it, like a cap.  I switched the direction and nothing...
Re-checked the wiring many times and when I put this together I referenced every wire with a layout and with my UA RI.
I'm stumped.

 

[Biasrocks]

The more meaningful thing to look at is that my B+ (313v) doesn't drop more than 1v at the other end of R34!  On my UA RI, it goes from 320v to 180v.  I go from 313v to 212...
I pulled it out and tested, it reads dead-on.  It's a 10k Metal Oxide, 2w, 600v.  The strange thing is that it has a directional marker on it, like a cap.  I switched the direction and nothing...
Re-checked the wiring many times and when I put this together I referenced every wire with a layout and with my UA RI.
I'm stumped.

Sounds like something isn't pulling current through that resistor. I would trace that part of the circuit after the resistor and make sure everything is as expected.

Regards,
Mark

 

[Bowie]

The more meaningful thing to look at is that my B+ (313v) doesn't drop more than 1v at the other end of R34!  On my UA RI, it goes from 320v to 180v.  I go from 313v to 212...
I pulled it out and tested, it reads dead-on.  It's a 10k Metal Oxide, 2w, 600v.  The strange thing is that it has a directional marker on it, like a cap.  I switched the direction and nothing...
Re-checked the wiring many times and when I put this together I referenced every wire with a layout and with my UA RI.
I'm stumped.

Sounds like something isn't pulling current through that resistor. I would trace that part of the circuit after the resistor and make sure everything is as expected.

Regards,
Mark

  Thank you very much for the response, Mark!  I went back and looked it over several times today, poked and prodded, removed C13 and R38 (which are not in the RI) and also arranged the path from the R34 so that it goes to C7D before pin 6 of the 6AQ5, just like the RI.  Still no dice.  Ridiculously high voltages remain on the 6AQ5 on pins 6, 5, and 2.  Incredibly frustrating.  I went over all the other tubes and while they're reading on the high-side, they're still within around 20% of the schematic and other reference voltages that I've found.
Any other ideas would be gratefully appreciated.  I'm going to be moving in a week and it will be difficult to pick this back up later on. 
Thanks!

EDIT; I was doing some more checking and found that when I remove the 6AQ5 in my RI, I get similar voltages as my DIY; over 300v on pins 5 & 6, and no voltage drop on the back side of R34.  The only difference I noticed is that Pin 2 dies not have any voltage but I assume that's because the tube is entirely absent.  What might cause this lack of draw from 6AQ5?  The 6AQ5 tubes I'm using are all good and perform perfectly in the RI.  Checked the socket for continuity and it seems ok.

 

[Bowie]

The more meaningful thing to look at is that my B+ (313v) doesn't drop more than 1v at the other end of R34!  On my UA RI, it goes from 320v to 180v.  I go from 313v to 212...
I pulled it out and tested, it reads dead-on.  It's a 10k Metal Oxide, 2w, 600v.  The strange thing is that it has a directional marker on it, like a cap.  I switched the direction and nothing...
Re-checked the wiring many times and when I put this together I referenced every wire with a layout and with my UA RI.
I'm stumped.

Sounds like something isn't pulling current through that resistor. I would trace that part of the circuit after the resistor and make sure everything is as expected.

Regards,
Mark

  Thank you very much for the response, Mark!  I went back and looked it over several times today, poked and prodded, removed C13 and R38 (which are not in the RI) and also arranged the path from the R34 so that it goes to C7D before pin 6 of the 6AQ5, just like the RI.  Still no dice.  Ridiculously high voltages remain on the 6AQ5 on pins 6, 5, and 2.  Incredibly frustrating.  I went over all the other tubes and while they're reading on the high-side, they're still within around 20% of the schematic and other reference voltages that I've found.
Any other ideas would be gratefully appreciated.  I'm going to be moving in a week and it will be difficult to pick this back up later on. 
Thanks!

EDIT; I was doing some more checking and found that when I remove the 6AQ5 in my RI, I get similar voltages as my DIY; over 300v on pins 5 & 6, and no voltage drop on the back side of R34.  The only difference I noticed is that Pin 2 dies not have any voltage but I assume that's because the tube is entirely absent.  What might cause this lack of draw from 6AQ5?  The 6AQ5 tubes I'm using are all good and perform perfectly in the RI.  Checked the socket for continuity and it seems ok.

MIGHT HAVE FOUND IT; Mark's post got me searching and i found a forum post where someone had the exact same issue and their R36 was 470K, instead of 470.  Will replace as soon as I can stop by Radio Shack tomorrow morn and then report back in case anyone else has this issue.

EDIT;  oops, took quite a while to report back but that was indeed the issue.  Unit has been running great, sounds gorgeous and quiet.

 

[thestudio]

What's everyone doing with the "gain reduction" switch???  I asked UA and they want serial numbers of my LA2a..........Lol

 

[Cashp]

All the threads I can find are old. Are there any pcb kits available?  Drip no longer seems to have any. Silent arts does not seem to have them for sale.  Any clues?  need something with good documentation.

Thanks in advance.

 

[signalflow]

Silent arts does not seem to have them for sale

If you check his thread in the White Market he still lists them in his first post, just doesn't have a pic of them.  Unless of course you've asked him and he doesn't.  At which point I guess I'm talkin outa my ass.

-Casey

 

[pH]

Check Ben's Black Market posting, he's a good guy too.

http://www.groupdiy.com/index.php?topic=49529.0

 

[radiance]

Finished a La2a, point to point, following Cayacosta's wonderful layout >
http://www.recproaudio.com/diy_pro_audio/diy_files/la2a/la2a_layout.jpg

I used UTC in and output trannies (HA100X & A24).
First I had some oscilation going but after some re-wireing it works ok I think. Voltages are ok ish...not all exact but in the ballpark.
Only thing that's a little strange is the noise level of -60dB. Its mostly low frequency noise (see picture)
Also, the V1 12ax7a is microphonic. I tried swapping it for the 12ax7a from V3 but same story. Could it be that both 12ax7a's are microphonic or is this normal behaviour for a La2a?  When gain is turned fully CW I can clearly hear when tapping on the unit. Tapping on V1 gives the strongest response...

Anyone has any ideas?

I tried both lowering R11 and adding the grid resistor but that did not work...