Fender ProTube Twin-Amp reverb feedback

[Mick]

I think Jim P gave a fair evaluation of his prognosis after perusing the schematic of the amph. I would suggest taking it to an amph tech to confirm (or disprove) his theory as to the cause of the feedback.

Pay someone that knows what they're doing to fix it, eh? That's quite a concept you got there!

Nawt yet... Methinks I'll try to verify that all the existing components are within specs before I throw in the towel.

I checked the tubes.
! checked the tank.

What's next? Maybe pull the chassis and have a look inside? How do I do that without electrocution? How do you drain capacitors safely? Lick the ends?

[Mick]

C'mon people! Doesn't anyone have any helpful suggestions?

[Gutmann]

Some prednisolone for your tongue
No other idea than the tubes, the tank and the tech - sorry.

[Mick]

Some prednisolone for your tongue

I found a website that tells you how to drain capacitors without using your tongue.

How to drain the filter caps

What I'd like to see is detailed instructions on safely removing the chassis on a Fender Twin. Ever done that?

[Mick]

I wonder if my problem might be similar to this...

Cold solder joints causing amp to "feedback" whether a guitar is plugged in or not.

Hmmm...

[Mick]

In case anyone is following this...

Someone from another website suggested replacing the RCA cable. Apparently, they had had a situation where a cheap unshielded cable caused reverb feedback. So, I removed the tank and connected it with a new RCA cable. It worked! With the reverb tank sitting on a table next to the amp and the reverb dial at max, no feedback!!!

Unfortunately, putting the reverb tank back in the amp case causes it to feedback as before. Even with the new RCA cable installed.

Somehow, the magnetic field of the two 12" Eminence speakers is interacting with the reverb tank to cause the feedback. It's like the tank is microphonic, but only when it's near the speakers. I tried placing various materials between the tank and speakers (rubber mat, wood, etc.), but nothing helped. Although a 1/2" thick piece of oak did change the frequency of the feedback.

The search for a cure continues...

[Mick]

I installed a new reverb tank and that fixed the problem. No more feedback, even with the reverb set to maximum. I'm happy.

With the new tubes and reverb tank this amp is awesome! I have to be careful with the volume though because If I turn it up too much it'll blow out every eardrum in the place, and the windows too!