1969 Fender Twin Reverb Amp

[cyclone]

That's what I was saying in post #9

I guess I didn't realize you were backing it. I'm pretty sure if the OP say's the problem is only on the vibrato channel, the problem is only on the vibrato channel. The guy is playing outdoor gigs. I'm sure he's used the normal channel plenty in a pinch or to get by in tough situations. I think we need to focus on circuitry exclusive to the vibrato channel.

Reverb transformer failing will do that. Saw it on a Plush which is a top quality TR with tuck and roll badassery.

[cyclone]

Just to reiterate, the reverb xformer failing or cathode bypass caps in the vibrato channel becoming very high capacitance when they swell and split will cause the oscillation problems the OP is experiencing. If the xformer is bad (high primary resistance) you might want to replace the bypass caps anyway as they are very likely bad.

[g1]

Cyclone in the case of the reverb transformer being bad would it not make the reverb non-functional?

[Leo_Gnardo]

Reverb transformer failing will do that.

Good call cyclone. On rare occasions I have had Fenders (and rarer Earth & Plush copies of them) with dodgy reverb drive transformers. I take it to be arcing within the windings but who knows. Causes annoying spikes when a lot of signal's going thru that channel. Replace transformer, racket stops. And in the meantime the reverb does work which makes it more difficult to single out the transformer as the problem. Seems I have to try & prove it's anything but the transformer then there's only one answer left.

[Richard]

I had a bad solder joint on a cathode resistor that produced a similar sound. Might it be time for a chopstick test?

[cyclone]

Cyclone in the case of the reverb transformer being bad would it not make the reverb non-functional?

If it opened completely, yes. In the amp I'm referring to, the primary went high resistance (25k i think) and caused the gain to be very high. Turning up the reverb past 1 or so results in a full volume squeal. If its not as bad I'd expect it would cause other oscillation related problems. The gain of the reverb circuit was already too high because the old 25 uf bypass caps were up to 100uf or higher.