69 Twin Reverb Crackle

[madkatb]

I just recapped a 69 Twin Reverb. The owner is a happy man but after playing at quarter to half volume for half an hour the amp develops sporadic crackle. (This issue was present before the recap but I suggested to the owner that we isolate problems one at a time.) We tried switching tubes and using both inputs but it's still there. Doesn't seem to be controlled by either volume. The tube bases were cleaned and retensioned when the recap was done. I'm guessing that its an old cc plate resistor thats going (gone?) bad and I've already discussed with the owner the possibility of changing out all the plate resistors. He's OK with that but it's still pretty much a shotgun approach. What I'd like to know is what is the best method for finding the actual culprit without just blindly changing all the resistors. Or am I even on the right track? Opinions, ideas, suggestions anyone? Thanks.

[Chuck H]

More info would help. Like, is the crackle simultaneaous with hard notes and chords, does it just happen sometimes playing or not or does it crackle constantly with the volume up playing or not? What is the bias reading on this amp? Does it have a known good set of tubes in it? Have you tried any tube substitutions? Are all the voltages accurate. Did you replace the bias supply electrolytic caps when you did the cap job? How about the preamp bypass caps? When you say you cleaned and retentioned the tube bases are you talking about the tube bases or the tube sockets?

Chuck

[Sweetfinger]

You can sequentially ground signal at the grids until the crackle goes away, then you'll know where the funky one is, or you can just change all the plate resistors and be done with it....now and in the future.

[Amp Kat]

First you should sweap all of the preamp tubes 1 by 1 with a known good one. If it's not that take the signal from the first preamp tube after the coupling cap and send it to the PI grid ect... down the line. Still there ? Then you likely have a grounding issue as in cold solder joint or bad ground connection.

[Chuckl]

To figure out approx where in the circuit, I would start pulling the tubes 0ne by one, starting with the preamp tube furthest from the power tubes. When the noise goes away you'll know your in the area anyway. Also check the switching contacts on the lo gain input 1/4" jacks, sometimes they'll get cruddy & start causing noise, it's something easy to overlook.

[madkatb]

^Chuck H - Crackle is intermittent but persistent when it happens, not controlled by volume controls. Tubes are good. Tried tube substitutions. Voltages are down about 15%. Meant to say tube sockets. Wiggling tubes makes no difference.
^Sweetfinger- I'll try that. I wanted to find a way to isolate the problem, for future reference, without just replacing all the resistors (although I will be replacing all of them.)
^Amp Kat- I'll try the signal "transfer" idea. Maybe it is a cold joint. Good way to isolate it.
^Chuckl- Tried that and I think it's in the PI area. Had the lo gain input problem in a Bassman recently. Took me a while to find that (although it did improve my very rudimentary skills with an oscilloscope.)