Apparently I am not making the circumstances clear.
This amp is generating 60/120hz hum when NO interconnect cables are attached. The only things touching this amp are the mains power cord coming through a ground-lifted bucking transformer providing approximately 117vac, and speaker cables to a pair of unpowered Altec Lansing A5s.
No CD, no receiver, no preamp, no inputs in fact they are unplugged and unpowered. Every electrical device in the room has been unplugged from the walls including lamps, clocks, overhead lights, etc. Nothing powered by house current is within 30 feet of this amp. When the amp is turned on, it is silent for 15-20 seconds at which point the hum is very audible. If at that point shorted RCA plugs are inserted into the inputs, the amp is silent. The hum is louder if I reintroduce ground through the power plug.
The hum also disappears if the amp is powered on with no power tubes. The hum is unchanged with the changing of the 4 power tube positions. The amp functions very well and sounds great otherwise and in fact brief frequency response tests show excellent flatness throughout the audible range.
Besides looking for some advice as to where I might begin to troubleshoot without arbitrarily replacing tubes, I post this in the interest of advising that the conventional wisdom that if hum disappears with shorted inputs, it MUST be coming from outside the amp; this is simply not 100% true. THERE IS NO 100/120vac 60hz electrical device within 30 feet of this amp and it hums. The hum is not worse when my audio devices are connected and music is played, unless I remove the ground lift at which point it is much worse (ground loop).
Hope I have made the circumstances clearer.
Thanks for any input.