Tube Amp Archive

I finally got around to trying out a couple of the suggestions, and here's what i've found: There's definitely crosstalk in the first tube, with the input going into channel 2, i can see a large output on the plate of V1B and a small signal on V1A. Unsoldering the connection from the loudness wiper ...

The pots are old. I wonder if 0 and 1 are the same. IOW, 0 used to really be off, but isn't any more. Maybe you can unsolder the pot and see what it meters when on the 0 position. Maybe the pot needs replacement? There are a variety of possibilities. If you want to solve the problem, I suggest you ...

What is the amplitude of your test signal? If connected to the input jack you should set the amplitude to about 200mV peak to peak. Do you have bleed-through when playing a guitar through the amp? It starts being audible at around 20-30mV? With a guitar plugged in and guitar pots set to 10 it's at ...

Phil_S wrote: ↑Tue Dec 06, 2022 6:48 pm How did you check there is no problem with the loudness pot?

I think i forgot to respond to this part - i verified it by going so far as to unsolder that wire from the board, which didn't fix the problem. Grounding it didn't help either, and i didn't see anything there on my oscilloscope.

I notice that there’s no coupling caps before the 330k and 100k mixing resistors originally. Are you 100% certain that the signal bleed you talk about is not a normal condition of the amp? Well, interesting you mention that. I tried to find out and the response is mixed: Someone on reddit said that...

Thanks for the suggestions. I'll take the amp apart and update.

Thanks for taking a look at the schematics! I have actually tried a shielded wire to pin 8, didn't help. I had it grounded to a grounding point on the board, IIRC it made it more staticky. I'll try your suggestion of jumping from wiper to the grid, we'll see what happens. I did try removing a bunch ...

I checked the ground on the pots with a multimeter, it looked like everything was grounded properly. I'll triple-check it though.

I have a 1964-ish Gibson GA30 RVT amp (9 tube version). I bought it a couple months ago with a known audio popping problem, which turned out to be a failing output transformer. The transformer died a couple weeks ago, taking out a couple diodes with it. I replaced that with a model suggested by Hamm...