Hi! I have a Marshall 800 50W head that produces hum/buzz when you turn up the REVERB control. It is less at minimum, then increases, then reduces a little at mid level, then is much louder at full rotation. Other volume and tone controls do not affect the buzz. The reverb sounds fine except for the buzz. The buzz remains if I pull the reverb connections off the pan input and output. Tried grounding the reverb pan to the chassis--no change. I just replaced the power supply filters with JJ 50uf/50 uf 500V caps. Replaced all pre-amp tubes. Power stage has 6550s which I rebiased for 21 Watts. All voltages read fine and everything seems to be grounded per the schematic. If I pull V3 12AX7A out the buzz is gone (but so is the signal of course). Anybody have a problem like this and fix it?