I have a Fender tube amp The Twinn. 4 6l6 2 12at7 and 4 12ax7. Mu reverb is dead and I here some strange oscillation. I suspect the 12at7 in reverb and one 12ax7 Both microphonic and very dem fil. Could this solve the problem ?

Check the connection to the reverb tank (rectangular box usually sitting on the bottom of the amp). There are a pair wires that connect it to the amp. If wires are connected, and the reverb is not working, you may need to replace the reverb tank.

Not sure of how it is feeding back. Two possibilities... the reverb tank in the cabinet is no longer shock mounted. The other is your guitar can "hear" the speaker... make sure you are away from the amp. With the reverb on, the feedback stability is less than with it off so make sure your guitar is away from the amp. Acoustic guitars are more sensitive to this than solid body ones.

Do you have the speakers connected properly?? If so, maybe speaker wiring needs checked.

Try this site:

http://elektrotanya.com/fender_reverb-63ri.pdf/download.html

You can plug the output of the gt10 into the "power amp in" jack of the Fender. This bypasses the preamps in the Fender but uses a buffer amps in the Fender to drive the power amp. Please note that the reverb is STILL in the circuit when you do this. You can turn the reverb to zero if you wish to disable it.

Could have broken a spring in the reverb tank, or a wire connecting it to the amp, or the reverb driver tube, or a few other things, but those are the most likely in this situation. Thats what I'd check first. Give it a little slap and if it vibrates like normal, your spring is probably ok, you can still open it up and double check though. Check your wires. Then pull your reverb driver tube and replace it. If that doesn't work, it is probably in the circuit somewhere, and you should not work on your amp unless you know what you are doing. They store large amounts of electicity in the caps and they can kill you.

In YOUR amp, take a pencil and tap teh sides of the tubes GENTLY, listening for any crackling... note SOME microphonics is normal in the preamp stages.

Since retubing it fixed it once, it is likely either a tube was bad OR a pin of a tube wasn't quite making so replacing the tubes cleaned it and made it work. Tubes DO go bad, especially when they are vibrated like in these amps... BUT don't just replace the lot, try to find which is at fault..

On your friends unit, LIKELY an electrolytic bypass capacitor for one of the reverb preamp stages has dried out and needs replacing.

If it's blowing fuses. It probably has a bad output tube.
If it's blowing tubes. It may have a cracked tube socket.  Also may need to be biased by a tech.

This sounds like a power supply problem. The high voltage for the tubes is not right. Either you are missing an amplification stage because this or one of the coupling caps is faulty. I'd have to see the unit itself to give you any more details.

Dan

Go to http://www.silvertoneamps.com/micKfender/Fender-Reverb-Both-Channels.html
there is info on this mod. heed the warnings of hot rod activities!!
There is a solution to your misguided nods you mentioned earlier.