fender twin pro tube

[donegone]

Hello,

The reverb stage is feeding back (squealing) when turned up. It also produces a lot of noise (sounds like the ocean) when turned up. Replaced the tubes thinking that one of the reverb stages may be microphonic. Still occurs even with new tubes. I replaced a shorted bypass cap I found on the recovery stage and it still produces this feedback. On the reverb driver stage it does not have a bypass cap. I dont have a schematic for this so its more difficult to troubleshoot. I do know that even with a new tank it still produces feedback so I am assuming its in between the reverb driver and recovery stage. Ive tried different cables. I do know that when I short the rca cables the feedback occurs. Maybe a resistor drifting? Suggestions?

[Enzo]

I don;t know why the drive side would be involved.

When you short the cable, the feedback occurs? Not sure what that means.

Pull the return cable out of the jack on the chassis, and turn up the reverb. What happens?

How high must it be set to feed back? Turn it just below that level and tap on the chassis. Does it ring, as if it were almost ready to feed back? Tubes are not the only thing that can be microphonic, you know.

Classic reverb feedback is acoustic. the energy of the speaker gets into the springs, which is then reamplified to the speaker. The drive end has nothing to do with it.

Does it still feedback with the reverb pan sitting outside the cab?

[booj]

make sure the connectors all around are clean. I've seen that cause some nasty noises

[donegone]

Its a 100k pot. About 50k is where it almost begins to feed back. At this point, I tapped on the chasis and it does ring. Sorry, what I meant about the cable is that I omit the tank and connect the rca's together (out to in) and it immediately feedsback ruleing out the tank (?) id assume. Tried new cables. And yes it still feeds back with the pan outside the cab as well as with different reverb pans.

[Enzo]

COnnect the send cable to the return cable??????? Well that certainly wouldn;t work right. That doesn't rule out much, it just sends a super hot signal back up the return.

If it rings with no cable plugged in, then set the chassis on your bench, connect a speaker, adn tap on everything in sight until you find what is sensitive. Obviously we can probably limit our search to the reverb recovery stage.

[donegone]

I did the test. It is definitely in the recovery stage. Where do I go from here? I checked grid, plate and cathode resistors. All are with in tollerance.

[donegone]

So here is where I am at. I tried a new tank with the same specs as the old one. It didnt squeal. I tried the original tank again and it squealed. I walked away for a few minutes and the new tank was squealing when I came back. I moved it around and it stopped and only started back up in a certain position. I tried new cables and new tanks but something still isnt right. The problem is definitely intermittent. I found a schematic for it and have attached it. Something I also noticed. When I scope the plate on the recovery stage, with the reverb level all the way down, I can tap the tank and see it operating properly on the DC load line. When I turn the volume all the way up and repeat the steps, I see the wave form but my load line bounces all over the place. Is this normal behavior when observed on the scope? When I see bouncing DC I think bad cap but I am new to reverb circuits and all coupling caps test fine.