Do you see anything in these traces that would explain a harsh fizzy distortion? Amp is an Ampeg Reverberocket R12-RT (schematic is here.) Working well except starting around 60% volume the distortion has a harsh fizz. The pre-amp sections are clean all the way to 100% volume, and distortion starts in the 12ax7 floating paraphase PI. The PI distortion is strange in 2 ways. 1. When the PI starts distorting, the first triode clips asymmetrically then the second triode clips the other lobe, so the output tubes are getting different signals once the PI starts distorting. 2. There is some other non-clipping distortion in the waveform. Trace 1: 100% volume. The smooth trace is the input to the first PI triode (from a 0.2V ptp 500 Hz sine at jack) and the other trace is the input to the second power tube (from the second PI triode): Trace 2: Same signal but the distorted trace is at the speaker. Scales are different, obviously. The input to the PI is 1V/div (5V ptp amplitude). PI Cathode voltage is ~2V so I expect the PI to be clipping here. PI output is 10V/div.