here’s a 300 watt Ampeg SVT-CL that i had on the bench today. a much different kind of amp from the 70s models that i’ve been more accustomed to working on in the past. this one had no sound or output and the fault light would stay on constantly. i assumed right away that it must be circuit related. turns out that it was actually just drift in an old set of tubes that got so bad it caused the amp to fault. i switched the tubes around so that they could get a different set of voltages and then adjusted the bias until it stopped faulting. also replaced the 12AX7 phase inverter since it was showing signs of age. i figured it would help to at least have one known good in one of the most important spots. after that, i gave it a couple hours of play time with some solid signal just to make sure it didn’t drift again. a new set of 6550s should square out everything for another year.