Fender ProTube Twin-Amp reverb feedback

[Mick]

Okay, I tried swapping out all of the preamp tubes, one at a time.

No joy...

I'm about 99.999% positive that it's not a microphonic preamp tube causing the feedback.

So, I pulled out the reverb tank...



...but I didn't see any obvious problem with the tank, like a loose wire, broken spring, or whatever. So I cleaned the RCA connectors where the cable goes into the tank, then put it back in the amp.

I still have nasty feedback when the reverb is turned up.

Could the reverb transformer be causing the feedback? Is there some way to test it to see if it's within specs?

What should I try next?

[oldguy]

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.

[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...