Fender Twin Reverb - Random Feedback - Troubleshoot and Repair

Here we have a twin reverb on the bench, and this is a strange one. The sound, which as described sounds like a feedback condition, did not happen when presented to me for inspection on-site. and while I think I know why, I still have to observe all safety procedures.

Just in case the speaker explodes ill be using an auxillary speaker for testing. and having found an extra set of tank cables ill be going through that too to ensure that the connections are correct as well as clean, just to rule anything there out.

Firing up the amp I notice that the bias has drifted on the final since last inspection. That is to say, they have gone further out of balance since last time, with a delta of 6mA. These are all original tubes for an amp that sits on most of the day, probably time for a swap.

I also check some of the higher wattage resistors under the finals as well. Using a plastic tweaker I touch through components to listen for noise on the output, I identify a very microphonic 12ax7 in the preamp stage. I hit all the sockets with De-oxit and clean them. Immediately after I found all of the pots to also be crunchy and had to clean all of them too. The combination of the pots and the microphonic tube on a hanging cable in a small room could have easily caused the feedback condition if it caught in oscillation.

The owner decided that the original tubes would go on a bookshelf and all tubes would be replaced. All of the new tubes were tested on the Hickok and were shown to be good. The matched quad set was a good march and the amp was biased to 31mA.