Fender ProTube Twin-Amp reverb feedback

[Mick]

I would swap out the tubes one at a time until the feedback goes away.

Okay, I'll try that...

But what if the problem is actually a combination of two or three bad tubes? This amp has a total of 12 tubes (seven 12AX7's, one 12AT7, and four 6L6GC's) and any of them might be microphonic, right? I could be swapping tubes in different combinations until hell freezes over!

When I was a kid they used to have vacuum tube testing machines at the store. I remember my dad taking tubes from our TV in for testing. Why don't they have something like that nowadays?

[markb]

Okay, I'll try that...

But what if the problem is actually a combination of two or three bad tubes? This amp has a total of 12 tubes (seven 12AX7's, one 12AT7, and four 6L6GC's) and any of them might be microphonic, right? I could be swapping tubes in different combinations until hell freezes over!

When I was a kid they used to have vacuum tube testing machines at the store. I remember my dad taking tubes from our TV in for testing. Why don't they have something like that nowadays?

So that people can charge you extra for doing the testing for you. But really, without the cynicism, a tube tester like that won't pick up a microphonic tube. For that you need an audio amplifier like, say, a guitar amp. Get two new 12AX7s and a new 12AT7 and swap each one with the new ones. If you swap one out and there's no difference you can say that's a good one. The problem won't be in the power section so you can save yourself the cost of the 6L6s and a bias adjustment straight away. And when you've found the bad one and thrown it away, you'll still have a spare or two for when the next one goes bad :

[Mick]

So that people can charge you extra for doing the testing for you. But really, without the cynicism, a tube tester like that won't pick up a microphonic tube. For that you need an audio amplifier like, say, a guitar amp. Get two new 12AX7s and a new 12AT7 and swap each one with the new ones. If you swap one out and there's no difference you can say that's a good one. The problem won't be in the power section so you can save yourself the cost of the 6L6s and a bias adjustment straight away. And when you've found the bad one and thrown it away, you'll still have a spare or two for when the next one goes bad :

I have two extra 12AX7's. I ordered two Ruby 12AX7A's from Guitar Center before I decided to replace all of the tubes with JJ's (Note: The Ruby tubes I got from GC are just JJ tubes with a Ruby label added). I don't have an extra new 12AT7, but I do have the original Groove Tube 12AT7 that came with the amp. I'll swap those in and see what happens. If that doesn't work, then I'll buy a new 12AT7.

[Mick]

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?

[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.