DR clean channel humming

[shredding]

HI, I just noticed that my DR's clean channel has this low humming (almost like 60 Hz) even if the guitar's not plugged in. Volume wise it's not too loud but isn't the clean supposed to be very quiet without a guitar plugged in? When I kick in the the Red channel, the humming's mostly gone.

Also this is the 2 channel DR. When the clean channel is switched to the "more gain" in the back, the hum is mostly gone.

I just changed all the power tubes to JJ's and changed the V1 preamp tube to a tungsol. I changed V1 back to my old mesa boogie 12ax7 but the hum is still there.

Any comments?

Also it is normal to hear a mechanical transformer hum when in standby?

 

[tele_jas]

Tap on the other preamp tubes to see if any are going microphonic... use the eraser end of a pencil and see if one is louder than the rest. Or you could just try swapping around 12ax7 tubes and see if the hum moves?

 

[primerib]

Make sure the grounding in your electrical is right. Some folks pull the ground prong out of their power cord, or use adapters/extension cords to circumvent the grounding.