Ampeg SVT static/crackling on power up

[corliss1]

Hey hey! Schematic attached.

The amp works, sounds, and operates properly. It has a fresh recap and service, everything is cleaned, all that jazz. All voltages are correct. However, during the first 60-90 seconds, there is a lot of crackling noises, which would be more than enough to scare off a buyer testing it out in a store. After a few minutes, the amp settles down and operates fine.

I've isolated the issue to pin 6 of V2. Pulling v2 completely stops the noise, v3 has no effect on the crackle. Every tube in v2 causes the noise, so it's not a tube issue. Removing just pin 6 of v2 from the board also stops the noise. Running a clip lead from that pin back to the rest of the amp brings the noise back.

I have swapped the v2 socket.

Muting the AC of each pin on the socket with a cap to ground doesn't stop the issue, even on pin 6. The same goes with any of the power supply points, like R52 or D11 and whatnot.

Is there maybe a board issue around that socket that is causing arcing or warming up or something? I'm not aware of any more isolation I can do besides what I've noted.

Thanks for any ideas!

[g1]

Good to see you here RJ.
Assume you mean V2 in preamp not power amp? And how did you narrow it down to that half of V2?

[corliss1]

I'm so sorry I didn't clarify - v2 of the power amp page on the schematic.

[g1]

Ok. I'll still ask my other question, since AC grounding (via cap) did not help for any pin on that tube, how did you isolate the issue to pin 6 ?

[corliss1]

I replaced that tube socket since it was REAL loose anyway, and just lifted leg 6 so it wasn't making a connection. I also broke a trace on the board itself. That's the only pin that seems to exhibit the behavior, and the only thing that stops it is that.

[corliss1]

Actually, maybe I should phrase it this way - I suppose I technically *have not* isolated it to pin 6, but lifting that pin does resolve the issue, so it's at least that half of the triode.

I keep a couple 12AX7s (yeah, I know, an AU7 would be closer for testing...) with each triode's pins cut out for weird testing, which also confirms the issue to that half.

[g1]

pin 6 is one of the last things before the 'top half' power tube grids, so it could still be prior to that.
Signal routes through the cathode, but with the plate disconnected the cathode is pretty much dead. Also, the cathode is setting the bias, so with that tube not running, I would think it goes full cold bias and kills all sound from top half.
I guess with V2 and V3 in, but V1 removed, the noise is still there?

[corliss1]

Correct - V1 and V3 don't change anything at all. I was thinking if it was "before" that, muting pin 7 would have done something, but nope.

Any/all tube but v2 in and we're good.