WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
Hopefully bricktop will find this thread ... I sold him my old MO-200, and he redid it with a 6CL6 differential amp front-end. After tuning the global feedback, he got some great square waves and very low distortion.