All things LA2a related

[JMan]

The can of the transformer making contact with the case is not a problem.

Regarding the 6aq5 sitting a bit askew, I wouldn’t worry. It shouldn’t affect its performance as long as it is securely in its socket. Both of my units have similar “imperfect” seating angles, and I don’t think it’s uncommon for these skinny tube types with small pin diameters (I’m talking a few degrees; obviously if it’s significant enough that the electrons are legally required to use their turn signal, that’s a different story ). Same thing with the 6x4s on my Pultecs, but they work just fine.

Hopefully the new tube(s) will prove to be the solution. The good thing is that you said you are getting the expecting gain reduction behavior, and that narrows down where the gremlin could be a bit. So the issue is this distortion in the audio path, as I understand it. We’ll hunt it down eventually.

 

[Toobme]

@JMan Thanks, just reading your words has brightened the evening and given some hope as well!
Yes, the noisy distortion is only present if I speak in regular conversational levels. If I whisper, it sounds pretty clean.
But of course, that’s not going to work for anyone, so your narrowing my hunt is appreciated!!!

 

[beatnik]

If V1 pin 6 is 246V, there is a problem there. We don't know the voltage on cathode pin 8 which is missing from your chart. Check the cathode connection of V1 and that resistor value is correct.

You could try swapping the 12AX7 in the audio path with the one from the sidechain and see if the voltages on V1 get closer to the nominal values.

Check the value of R34 because the voltage on the 6AQ5 plate is also high.

 

[Toobme]

Well, thanks everbody, this is officially buttoned up!!!!!!!
I went through again and found I was missing the ground wire from V2’s added center pin.
Added that up and my voltages are on spec +/- 5-10% and the thing sounds . . . A-MA-ZING!
I just can’t wait to do some hotrodding and compare some of these esoteric caps like the Mundorf or something.
Just so I know what if any diff there could be.

Anyway, you have no doubt all been in the weeds on SOME build at SOME point, but your continued support and expertise was what kept me going and you saw me right through to the end. I’m so grateful to all you folks and look forward to the day when I can help out someone in the boat you just helped me out of.

Cheers to all and, by the way, I just finished an M-49 clone which sounds absolutely disgusting through this newly minted LA-2A clone.
Happy camper here!
If I may indulge a photo. Case by Tat Perushka of DIY-Racked.com (He cut out a larger meter hole for me, cause I liked the larger meter).

 

[beatnik]

Great news. Glad to hear it's now fully working

One last piece of advice

If you have a variable capacitor in the C4 position, its purpose is to tune for flat frequency response. Or perhaps a gentle treble boost if you are into that.

The other variable capacitor in the sidechain affects compression at the high frequencies.

The sidechain filter control R37 is very useful to have that on the front panel for extra flexibility on how the compressor reacts to low end frequencies.

Perhaps it could go in place of the meter adjustment, which doesn't need touching so often.

 

[Toobme]

I was thinking of that right from the start, but now hearing you suggest it, am gonna do it!
Chears, @beatnik

 

[craptical]

Alright, long time no see! ))

Anyway, I'm trying to help a friend with his LA2A build and it now passes audio and gain control is under control BUT I can't get the meter up to zero in GR mode (regardless of T4B plugged in or not). R25 is 33k, Sifam meter, zener regulator. Zero adj in middle pos gives 62V at the zener, maxed out (or min) zener drops severely (normal?) but meter only reaches around -10dB maxed out. I then have around 15V in the meter "loop", differing .2V across the meter. Meter looks OK when playing music in both +4 and +10 pos. I haven't made any actual signal tests here, but maybe I should, the background being that the "meter board" array of resistors were originally soldered in reverse order, R25 being 3.9k and the meter resistor R24 @33k, R26 @10k (to ground) and R27 @330k. None of them looked burnt and they measured ok when I put them back in, but I can't say anything about the meter as I never turned the compressor on before I noted and corrected the resistor board errors. No shorts detected and I also tried elevating the main board to be on the safe side. Any hints greatly appreciated.

/Dave

 

[craptical]

Alright, long time no see! ))

Anyway, I'm trying to help a friend with his LA2A build and it now passes audio and gain control is under control BUT I can't get the meter up to zero in GR mode (regardless of T4B plugged in or not). R25 is 33k, Sifam meter, zener regulator. Zero adj in middle pos gives 62V at the zener, maxed out (or min) zener drops severely (normal?) but meter only reaches around -10dB maxed out. I then have around 15V in the meter "loop", differing .2V across the meter. Meter looks OK when playing music in both +4 and +10 pos. I haven't made any actual signal tests here, but maybe I should, the background being that the "meter board" array of resistors were originally soldered in reverse order, R25 being 3.9k and the meter resistor R24 @33k, R26 @10k (to ground) and R27 @330k. None of them looked burnt and they measured ok when I put them back in, but I can't say anything about the meter as I never turned the compressor on before I noted and corrected the resistor board errors. No shorts detected and I also tried elevating the main board to be on the safe side. Any hints greatly appreciated.

/Dave

Ok, rant continues.
I replaced all resistors on the meter board. No difference. I then tried to adapt some values, raising the 33k to 68k and lowering the meter resistor from 3.9k to 3.3k. I got closer but still not fully up to zero, which suggests either:

1. Meter problem (but works fine with music, so probably not?)

2. T4 problem. But the circuit should be able to calibrate without T4, as it's very highly resistive when not active/compressing, right?

3. The zero trim pot. It measures fine out of circuit, though.

4. Supply voltage. Here I get 62V when in middle position, but dropping as I trim the meter up. Trim fully CCW I get around 30V in the meter circuit, and on the T4. Voltage drop across meter is about .4V.

Anyone?

And one more thing: upon start the meter first went up a tiny bit past zero, but then dropped to the ca -2dB. This was only one time when it had been off a good while, but my own (functional) LA2A build never does this. Jumps straight to zero and stays rock steady.

 

[bxershrts]

Good evening everyone,

I have been building la2as for a decade now and recall someone creating a pcb to be able to bypass the La2a compression and a/b the source signal and compressed signal. Has anyone integrated a bypass switch successfully on a LA2A? I would love to hear how it was done!

Thanks!

 

[pinebox]

I just found this thread, I will be looking through it tonight, but might as well post what I am trying to track down in the mean time. I have a vintage LA2A that is oscillating at 15k when no signal is applied. Seems to come down when white noise is injected. Very clear 15k waveform at the top of the neon lamp. Im measuring 110vdc at the top of the lamp where it is supposed to be 65. Voltage before is fine, ~285. Without input signal, sometimes I will probe the wiper of Zero Adj and the lamp will shut off, and away goes the 15k. Is this indicative of a dying neon?

 

[beatnik]

The neon lamp has a limited lifespan so very possible it needs changing, just replace it with a 65v zener and you won't worry about it again. In a vintage unit I would also check the value of all carbon comp resistors, it's not unusual to find dodgy ones particularly in the HT lines. If the oscillation is still there I would perhaps try without the T4B plugged in and eventually testing a different one.

 

[pinebox]

Thanks, I read a bit about everyone swapping the bulb for a zener after reading the thread more, but I always see them mentioned at 65v, and I cannot find any in this value, I'm guessing 68v will be fine but confused with why the 65v value referenced when I cannot find any manufactured at that value?

 

[thelivingroom]

Thanks, I read a bit about everyone swapping the bulb for a zener after reading the thread more, but I always see them mentioned at 65v, and I cannot find any in this value, I'm guessing 68v will be fine but confused with why the 65v value referenced when I cannot find any manufactured at that value?

I used 1N5372 (62v) and didn’t have any issues.

 

[CJ]

Check your front panel grounding, try moving the front panel open and closed while looking at 15k hz on scope

This is a problem due to the hinges being in the ground path,
Also check pots on front for tightness. Corrosion on star washers etc

 

[Toobme]

Anyone see a problem with using 1w resistors instead of .5w in an la2a build?
This is my second one and I thought about overbuilding it.

 

[fallout]

No issues, you're good to go.

 

[Toobme]

Thanks, @fallout - was hoping that was the case!