Silverface Twin Motorboating....

[Buschman]

severely enough to rip the cone out of my junk test speaker. Only does it on the reverb channel. It's not a tube and re soldering a bunch of joints has not fixed it. It seems related to the bass pot and volume. Only does it when cranked. Chopsticking has not revealed anything. I thought it might have been part of the bias balance circuit but it came back after a while. Any ideas?

[Structo]

I would look at the coupling caps in the reverb channel.

[tubeswell]

The usual source of motorboating is failing filter caps. If the amp is a ~70's era silverface, it may never have had a cap job and I would suspect the electrolytics have haddit. Have you tried tacking on new caps to check?