Fender Twin-Amp Feedback

[Nathanmiller]

I have a fender twin-amp that is a few years old. The reverb worked great until recently. It's starting to produce this midrange to high frequency feedback/hum when I turn the reverb up. I've done some research and everything that was said to do to fix the problem was already there before. The cardboard, the vinyl bag, not screwed down tight to the wood. I've even trien another reverb tank and it does the same thing. I don't know much about amp repair and was wondering if anyone had any ideas. Thanks yall!

Nathan

[52 Bill]

Check to see if you have a microphonic preamp tube. It will be the 4th one from the right.

[Enzo]

Isolate the problem.

Pull the screws from the bag and stick the whole reverb bag/pan/whatever OUTSIDE of the amp cab. Does that make any difference?

That should tell us if it is the reverb pan somehow feeding back or if it is the amp itself.

Turn up the amp and get it to start feeding back. Now go down the row of tubes and firmly grasp each one. ise a rag or glove if they are too hot for you. Might as well start with the 4th one, but try them all. If a tube is microphonic, usually holding onto it tightly will shut it up.

Tubes and pan are most likely feeder backers, but it is possible something inside the chassis has gone sensitive.

[Nathanmiller]

I moved the whole reverb pan out of the amp and it stopped feeding back. When the pan is inside the cab or right next to the cab, it feeds back. All the tubes are fine. I just replaced them all about 6 months ago. Even with all the other knobs turned all the way down, if the reverb knob is turned up, it feedsback. With the reverb turned all the way down, the amp sounds just fine.

[Enzo]

Then set it back inside to troubleshoot. Your pan is feeding back, you tried another pan and it fed back. Don;t rule out that the possibility the other pan is sensitive IN THIS CAB too.

With the pan feeding back, push down on the top surface of the pan with your fingers. Does that stop it? SOmetimes the wide flat top surface of the pan resonates and causes feedback. Running a length of self adhesive rubber weather stripping down the centerline of hte top cures that.

One trick - flip it over on its back. ANy help?

[Nathanmiller]

The second reverb tank that I used already had a piece of weather stripping on the top of it. I tried turning it upside down and that seemed to help a little bit. The pan with the strip was better but still not 'feedback-free'. After cranking the reverb and playing for 10 minutes or so, I could still hear faint high-pitched resonances.

Any other ideas? Could it be something inside the chassis?
It's just weird to me that the pan has never fed back since I bought the amp and now it's completely out of control.

[Enzo]

You pulled the pan from the cab and it stopped feeding back. Nothing inside the chassis would know you moved the pan, so anything inside that was sensitive would still be sensitive with the pan removed, So no, I don;t think it is inside the chassis.

[cjlectronics]

I'm having the same exact problem with a Fender Twin Reverb. The feedback is very intermittent and seems to happen more often at power up (when standby switch is turned on). This is a repair that I did and it has come back for the same problem, the feedback. The first time around I replaced the main power caps and thought the reverb tube was bad. ( I replaced it the first time)

I will try the reverb pan idea and see what happens. My problem is that it is very intermittent.

CJ

[cjlectronics]

So far, I have found V2 (12AX7) to be the problem. I was able to reproduce the feedback enough times to isolate the problem to V2. After swapping it out, I cannot reproduce the feedback. I am testing it all week just to make sure.

CJ

[pdf64]

Is the cathode bypass cap on V4 good? If that develops high ESR, a positive feedback loop is created when the reverb intensity control is turned up.