Last year I purchased an early, used, Falcon Grande amp through Reverb.com. It worked very well with only minimal use in my home since then. Yesterday I decided to try some good (Sencor tested) vintage 12AX7’s that I had used in other amps to see if I could make the clean/rhythm tones less boxy sounding. After trying several tube combinations the amp started to hum, loudly. I put the original TAD 12AX7’s back in, but the hum continues. It sounds like 60hz hum, not fizzy at all, sort of like when you bring a guitar too close to the amp. The hum is not affected by volume, tone or reverb controls. The attenuator does reduce the hum to almost nothing at its lowest setting. My guitars will play through the amp, but the hum is louder than the guitar. Unplugging the guitar cord from the input makes no difference, the hum continues. I moved the amp to another room and plugged into another circuit – no change in the loud hum. My other amps continue to work fine. I tried removing one tube at a time to see how that would affect the hum: V1 – no change, V2 – no change, one power tube – lower hum, both power tubes (all tubes removed) – no hum. There is no hum with the amp in standby. With the power tubes installed and turning the standby to operate before the tubes are fully warmed up, the hum goes from nothing to full noise as the tubes warm up. One other interesting thing is that prior to the hum, the vintage USA 5881 power tubes measured 41- 42ma at 400VDC (amp is cathode biased). After the hum started, one power tube measures 56ma and the other 38ma, both at 400VDC! I have tried a dozen different 5881 and 6L6 combinations and the results are similar. The higher current socket is always higher regardless of tube rotation. I have taken the chassis out of the cabinet to look for any obvious sign of bad component(s) or loose ground connections – nothing. I have looked at the solder joints through magnifying glasses and checked for continuity on all ground connections from the mains to the end of the ground conductor in the pcb. I have tap tested all component connections to the board. The voltages going to the power tube coupling caps are identical. Filament voltage is 6.7VAC. This amp is not covered by warranty. I am an amateur guitar and amp player and repairer (my retirement hobby). I have overhauled/fixed dozens of tube amps, but am not an expert as many of you are. This one has me stumped, so I am looking for any suggestions of what else I can try before I have a professional tech look at it.