I recently shuffled two rooms around so now all my amps are in the same room. They are at the far end of the room while my desk is at the other next to a tall lamp.
Well, the other night my solid state Fender Bassman 10 combo was humming a little bit and when I moved towards the desk, the hum noticeably got worse. It stopped as soon as I put my fingers on the strings and dwindled when I went towards the amp.
Thinking the cable was wonky I changed it out for another. Same thing. Tried it with another bass and the same thing happened. Then I switched the cable back to the original. Same thing.
All the basses I tried had round wounds on it so I tried one of my Bachs with flats. It was noticeably quieter but it was still there.
Getting really curious about it, I turned the amp to face the wall but the sound got worse as I went towards the desk. Then, I thought maybe the wall outlet was weird so I ran an extension cord from the kitchen. No dice, same thing.
So, I thought, "Hmm, maybe something in the solid stateness was weird. So, I pulled out the Sunn 200S, and all tube head, sitting on the 4x12 cabinet. SAME THING!
Perplexed, I turned the lamp off, turned on the ceiling light, and sat down at the desk. The Sunn was dead quiet.
I switched lamps and OILA! Nothing!
Turns out the first lamp has a slider which allows you to dim or brighten the lamp as desired. The slider must operate like a transformer and was creating interference with the bass as soon as the bass came near it.
Odd and interesting but I figured it out.