What I’ve done so far: Added shorting plugs to all unused inputs. Seems to have brought the hum level down. Added red silicone tube dampers to all tubes except the rectifier. Typically run the Phono with the (L) and (R) gain knobs turned down 5 clicks. I’ve tried it with and without the variac. No difference noticed. I mostly listen to jazz on vinyl and not very loud. Typically I can only hear the hum in between tracks and in quiet passages but if I turn the volume up so its loud it is present in the music.
I like to use NOS tubes from the 1950’s and 60s. I find the new production tubes do not sound toobie to me. However, Roy suggested I try some JJ 12AX7 and add on the low microphonics/noise option. Same hum. Some tubes seem to cause more hum than others. The best combo so far is a GE 7025 in V1, a GE 12AX7A short plate in V2 and a GE JG5157 in V3. The GE 7025 has pins that are a tad too big and its difficult to install and I can only get in in half way. I have an NEC 12AD7 coming for V1.
Will tube hum decrease with use i.e. ‘burn-in’ time ?
Is it tube hum or an internal or external source being picked up via EMI, RF, ground loop ?
What else should I try ?