Fender ProTube Twin-Amp reverb feedback

[Gutmann]

Feedback without attached guitar probably is caused by a microphonic tube.
Try to hit the tubes slightly with a pen while the amp is running. If one is making a strange noise, it might be a microfonic one. It could be a preamp or a poweramp tube.

Also it could be the reverb tank causing the problem - but I don't know how to check this.

Good luck - this is an excellent amp!:

[tot_Ou_tard]

Feedback without attached guitar probably is caused by a microphonic tube.
Try to hit the tubes slightly with a pen while the amp is running. If one is making a strange noise, it might be a microfonic one. It could be a preamp or a poweramp tube.

Yep, I had the same increasingly loud squeal with no input on my Gries 5.

tap, tap with a pencil eraser & lo & behold the 12AX7 was obviously microphonic. I changed that & problem solved.

[oldguy]

Welcome to TheFret, Michael.

I agree you should check the preamp tubes to start with........even new ones can be microphonic occasionally.

Your photography is stunning, btw, esp. the wildlife.

[Mick]

Thanks for the replies guys! Extra thanks to oldguy for checkin' out my pics. I'm glad you like 'em.

So, I just tap on the tubes and listen for strange sounds, eh? I guess that sounds easy enough. Even for me.

In fact, I'll go give that a try right now.

[tot_Ou_tard]

Thanks for the replies guys! Extra thanks to oldguy for checkin' out my pics. I'm glad you like 'em.

So, I just tap on the tubes and listen for strange sounds, eh? I guess that sounds easy enough. Even for me.

In fact, I'll go give that a try right now.

A microphonic tube will be obvious, kind'a the dental hygenist hitting the exposed nerve.

[Mick]

A microphonic tube will be obvious, kind'a the dental hygenist hitting the exposed nerve.

Well, I removed all of the metal shields (vibration dampers?) from the preamp tubes and let the amp warm up. Then I tapped each tube several times, including the power tubes. All I heard was a tiny bit of noise that sounded like small vibrations coming from the springs in the reverb tank. And they all sounded the same. I did the test several times, with the reverb dial set higher each time. None of the tubes had any obvious difference. Oddly enough, this time I was able to turn the reverb dial up to 8 without getting the feedback. But if I turn it any higher the feedback starts right up.

Is it possible that I have the bias set too hot? Can that cause feedback? The bias setting instructions on Eurotubes.com says "Super's and Twin's have a higher plate voltage so with the JJ 6L6GC's we see them come out of crossover distortion at about 34mA but we typically like to bias them between 38 to 44mA and most players like them best at 38 to 40mA." I set the bias on this amp to 39mA.

What should I try next?

[Moshe]

What should I try next?[/QUOTE]


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