In my home studio I have several amplifiers:
- Fender Blues Junior III (guitar combo, all tube)
- Gallien Krueger MB150E (bass combo, solid state)
- SWR Studio 220 (bass head, tube preamp)
- Alesis RA 300 (monitor amplifier, solid state)
All of them hum audibly when simply switched on, with the volume and gain all the way down. That hum can be heard faintly up to 4 meters away (as far as I can get without going around a corner).
With at least the Alesis and SWR, the hum persists when the speakers and all inputs are disconnected.
The Fender, being a full-tube amp, is not meant to be switched on without a speaker load attached.
In contrast to the others, the GK does not hum when the speakers are disconnected; it does however continue to hum (with a bit of hiss) even with the volume turned all the way down and no inputs attached.
The Fender is new, and as such so are the tubes. The tube in the SWR is more than a decade old (but it's a pre-amp tube, so it should have a long lifetime).
All of the equipment is connected to the same power strip (which I believe should rule out ground loops?).
I assume the hum is coming from the transformers? Is this something that a line conditioner would have a shot at fixing?
I've searched a lot trying to figure out what the potential problem could be, but most of what I've found focuses on one of the amplifiers being faulty (which seems unlikely with 4 different amplifiers), or the hum being introduced in the input signal path, which doesn't apply here.