Hi, I have a Vintage Bandmaster that is really strange. It is an AB763. Interestingly, it has a high pitched squeal when turned on. This is not the result of the out of phase Brn/Blu lead on output tube socket being on wrong socket (tried both ways, still squeal). This sounded like a pre amp squeal. I did find the following, and according to the owner he took it in for service about five years ago. I imagine a tube went south and took out the CC resistors. there is still some carbon scarring And some heater wire insulation was burnt and Peeled back and separated (no insulation for about And inch. Output tube socket resistors replaced with wire wounds and metal oxides (grid). Mains filters replaced--however they were mis-wired. I found them in with in series with a common ground, and amplifier wiring leads to the caps were: red to pos and black to the other positive. (on the second cap) Because of the age of the amp, and the mis-wire all the electrolytics have been replaced: mains, bypass, and bias. Result amp still squeals. TUBEs, have been installed with a known good test set. They make no difference the amp squeals. Stand by voltage = 450V. Operating voltage = 413V. Bias voltage to the tubes = -48.2 The other voltages are all close to spec. The amp is stable and no squeal with out a PI tube. With a PI tube installed it squeals. With a PI tube and the .001 cap removed from the circuit There is not squeal. However, when my Fluke is measures the voltages On the PI resistors .1M, 470, 1M. On any of those it squeals again. Every CC Resistor in the amp has drifted out of Spec. That is it in a nut shell. Any Ideas? I have the tools, and will post results and Voltages accordingly.