Dealing with 60-cycle hum from clients' guitars (possibly?)

[???????]

if you try that experiment, make sure there are NO gaps.

Any gap in the screen will let all noise frequencies with a wavelength of that size and smaller penetrate the cage.

[goose134]

You mentioned that it is an older house and the room has fluorescent fixtures in it. If the fixtures are older as well, then the ballasts may be a culprit. Many times the old way of cutting the lights with the switch was to cut the neutral off, not the hot via a switch leg. This still delivers current to the ballast. Newer ballasts have specs on EMI on them. You can also purchase shielded ballasts. The easiest way to tell if its the fixtures is to plug it in somewhere where the fixtures are not present.

It sounds like you've done all the things I would do to find the problem. If I missed this step in your research, I'm sorry.

A Farraday cage would be great. Unless the source is in the room with you. I've built one before at a radio station, and yes, they are as labor intensive and expensive as you imagine.

[Snarl 12/8]

You mentioned that it is an older house and the room has fluorescent fixtures in it. If the fixtures are older as well, then the ballasts may be a culprit. Many times the old way of cutting the lights with the switch was to cut the neutral off, not the hot via a switch leg. This still delivers current to the ballast. Newer ballasts have specs on EMI on them. You can also purchase shielded ballasts. The easiest way to tell if its the fixtures is to plug it in somewhere where the fixtures are not present..

Shit. I have flourescent fixtures in my music room. I also have bad EMI. I just assumed the fixtures couldn't be the source because I never use them and it's the same with or without them on. Back to the drawing board. I gotta get in there and cut the wire or something to be sure. How far back was it that they cut the neutral? I'm guessing these fixtures are 20-30 yrs old.

[vvv]

Try doing it at the fuse box?

The ballast is the silver thing sticking out the fixture; the ones I have seen can be removed to change 'em; or is that the starter?

"Power quality and radio interference
Simple inductive fluorescent lamp ballasts have a power factor of less than unity. Inductive ballasts include power factor correction capacitors. Simple electronic ballasts may also have low power factor due to their rectifier input stage.
Fluorescent lamps are a non-linear load and generate harmonic currents in the electrical power supply. The arc within the lamp may generate radio frequency noise, which can be conducted through power wiring." cite

[goose134]

Well the easiest way would be to remove the fixture and replace it with an incandenscent light. If the feed is still good, you could use it with no problem with a new fixture. The silver thing is the starter. The ballast will be the ugly black thing inside. Modern fixtures employ electronic ballasts which don't have an external starter.

I'm not saying the fixtures are the problem for sure, but it surely could be.

Incidentally, while ballasts do generate harmonic currents, the effects in a single phase system are negligible. It's in three phase systems that things get hairy. Single phase systems have voltage taps at 180 degrees out of phase. Mostly cancel everything.