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I've modded a Carvin TS100 so-as I patch in directly at the 500k volume pot (x2 L/R), thus a gain stage. It's seeing a lower impedance source via a stereo reverb pedal of maybe 10k. I notice once rotating towards 50%, hum becomes audible but vanishes at max. I'll add a biasing resistor of 1Meg at the grid and see if that cures it. It is just something I wasn't expecting.