I fired up a newly rebuilt amp with no problem... Plates,screens and grids of the power tubes looked right. All voltages in the preamp were fine as well. I hooked up a cable between my Ditto Looper and the input of the amp, started the loop and tried to adjust for volume... With the amp cranked, I could barely hear the audio coming out of the speaker (the OT's natural resonance is louder, for reference). The speaker is a 2x 12" cab.. good for about 300 watts at 8Ω. This is strange to me and when I thought about it for a minute, I hadn't heard the big pops I usually get when probing the tubes for voltage... Something wasn't right. First suspect is the rebuild in front of me. So I grabbed the scope probe to try to trace the fault. As soon as I connected the ground of the probe to the chassis, I got full volume... Turn that down!
The other thing I notice is that the scope is indicating a perfect representation of the signal as it should be, but only the ground of the probe is connected to the chassis. The actual probe is touching nothing. 
To eliminate obvious things, I unplugged all and took the amp to one of the cabs that are used for regular jams. All connections made... Sounds just fine. Full volume, no extra noises. Amp works exactly as expected. Back to the bench, and back to the same issue. Oh well, it must be a fluke... I will try to sort this out soon, but for now on to the next amp. When all is connected and audio applied I get the same condition as before... To the known-good jam setup... No problems there. What gives?
I picked up a known-good guitar to eliminate the possibility of the looper being the issue. Guitar plugged in, no signal (actually it is just barely audible). I plug the ground of the scope probe in and all sounds normal.
I can't figure out how I don't have any reasonable volume without the probe ground connected. I checked continuity between the chassis of both amps to the receptacle that it is plugged into. That receptacle is tied to the same line supply (120 vac) as the scope, but there is a variac in line with the receptacle. Continuity was positive. The speaker cab is not connected to anything but the 1/4" jack on my bench, no sampling of the signal going anywhere. I have another jack that goes to a dummy load and I sample off of that for probing and re-amping at the volume needed. With the scope probe ground disconnected, I get close enough to zero volts, ac or dc, from the receptacle ground to the chassis. Same results when probing from the chassis to the scope's front panel ground terminal. Zero Ω between the scope and receptacle grounds. Probing the receptacle for voltage, I get 120 vac from hot to neutral as well as hot to ground. The above condition happens when the scope is powered off as well as when it is on. I have never had this problem at this bench in all of the years I have had it set up. I don't know of anything that has changed relative to the bench that would cause this.
What am I missing? Has anyone had similar issues? I'm stumped for now.
