I've got a David Allen-built Bassman combo amp that I've had for about a year. Up until now I've had no problems. The other night, after connecting a guitar into it, I plug the amp in and hit the power switch to let it warm up for a few minutes. When I hit the standby switch I immediately got a loud high-pitched buzz (which turned into a squeal). I turned the standby off right away and it stopped. Checking the power and rectifier tubes, I found them to be glowing normally, though I didn't take the shields off of the preamp tubes to check them. I also checked to make sure that all the tubes were seated properly. Tried it one more time and got the same loud buzz/squeal. I emailed David for some advice. He said it could be a bad phase inverter tube (last tube before the power tubes). Anybody here have something similar happen to a tube amp?