Fender ProTube Twin-Amp reverb feedback

[markb]

Why does the amp only feedback when the reverb is turned up?

If it's a microphonic preamp tube causing the feedback, then shouldn't it feedback even when the reverb is turned off?

Why does the reverb setting have any effect on the feedback?

If there's no feedback when the reverb is turned off, doesn't that indicate that the problem must be somewhere in the reverb circuit?

That was my thought, Mick so it should be the reverb driver tube, assuming that that model Twin has a tube driven reverb. Another thing you could look at is the way the tank is positioned and mounted. The older Fender amps had their reverb tanks in a vinyl bag (which damped them a bit) screwed to the bottom of the cabinet with rubber isolating washers. You could still turn round and kick the amp for the whipcracks in Rawhide, say but they were quite well damped.

I'm not familiar with the more recent models. The most recent Fender tube amp I've owned was a 1982 Concert which was built the old way.

Oops, that's not true! I had a Pro Junior (no reverb) and a Blues Junior (naked tank, no isolating washers) and the BJ was touchy if you turned up the 'verb. Your fix could be the price of some rubber washers and slightly longer fixing screws.

[Mick]

That was my thought, Mick so it should be the reverb driver tube, assuming that that model Twin has a tube driven reverb. Another thing you could look at is the way the tank is positioned and mounted. The older Fender amps had their reverb tanks in a vinyl bag (which damped them a bit) screwed to the bottom of the cabinet with rubber isolating washers. You could still turn round and kick the amp for the whipcracks in Rawhide, say but they were quite well damped.

I'm not familiar with the more recent models. The most recent Fender tube amp I've owned was a 1982 Concert which was built the old way.

Oops, that's not true! I had a Pro Junior (no reverb) and a Blues Junior (naked tank, no isolating washers) and the BJ was touchy if you turned up the 'verb. Your fix could be the price of some rubber washers and slightly longer fixing screws.

This amp does have a tube-driven reverb. I think the 12AT7 tube in the V7 position is for the reverb. The tank is mounted as you described, in a vinyl bag attached to the bottom of the cabinet with screws. I can't tell if there are any rubber isolating washers. I haven't done anything with it yet. If swapping out the tubes doesn't fix the problem, then I'll probably pull the tank out and have a closer look.