Have a 65 Twin Reverb about 3 weeks old. Turned it on at a gig and had low volume and thin sound. Loud pop when turned it off and the two left power tubes were glowing red. Found one of the 12AX7 preamp tubes was bad but not sure if it was cause or symptom. Either way replacing it didn't help. Tried a new set of power tubes but the left bank from rear of amp are glowing red within a few seconds of turning the standby swith on. I assume something in the bias circuit has gone stoopid. I haven't pulled the chassis to take any voltages at the test points. I've located the schematic but not a chassis layout. Hoping someone had seen this before so I knew where I was heading. Any help is much appreciated.