Grounding issue with multiple guitars and multiple amps in multiple rooms

Question

Three amps (Mustang GTX-100, Mustang GTX-50, and an Acoustic). All brand new.

Three electric guitars. Fender Deluxe Nashville Tele, noiseless pickups, 5-way switch, brand new. Fender Mustang Special "Pawn Shop," HH, used. Squire Tele, single coils, brand new.

Four outlets in four rooms on two floors.

In every case, in every combination, there's a grounding issue. There's a hum when not touching the strings. If I touch the strings or the bridge or the tuning peg or whatever, the hum stops.

So either all three of my amps are bad, all three of the guitars are bad, or all four of the outlets are bad.

Am I missing a fourth explanation?

Answer 1

Touch a water pipe (assuming you have metal pipes in the house in the rooms you are interested in) while wearing the guitar and really really not touching the guitar's metallic parts (like the strings). Pipework tends to be grounded fairly thoroughly. Does the hum go down? If so, your body is what is picking up electrical noise from the room (normal wiring to a room is a single kind of hot and neutral, meaning that it doesn't average out to being neutral overall) and the guitar is not shielded sufficiently from you to not be bothered by it. If the hum goes up when touching the radiator, your house wiring and the pipework may not agree about what constitutes ground.

Which would be a real cause for worry.

That you yourself are a source for hum is not all that remarkable: I don't think I have ever seen a guitar amp where touching just the jack tip with a finger would not have resulted in awful humming. You'd probably need to play in an open field to not see that effect. It's why the strings are actually wired to signal ground behind the guitar's front plate: not for personal safety (keeping them isolated would make more sense) but because it allows you to quiet the hum. The amp is then grounded to make it actually reasonably safe for you to touch signal ground.

If your socket ground is actually bad, it usually is derived to some degree from pipework where it enters your house, and some clamp there establishing the potential might be defective.

Answer 2

It's a process of illumination - throwing light on the problem one bit at a time.

First is to eliminate the parts one at a time. Starting with the guitar lead, which I suspect is the same for all situations. That could be the common factor, thus is there in each of the combinations.

Next is to work through. There are 3 variables - mains, amps, guitars.

Guitar and amp (and good lead) to another mains out - maybe another room, maybe another house.

Purloin another guitar and amp, try in your socket. If that's o.k., substitute guitar, then amp,to establish where the fault lies.