Couple of suggestions. It is entirely possible that you do not have an earth ground.
BTW, usually, grounds from the supply are connected to water pipes.
Even if you are not electrically inclined there are some things you can verify just by
looking. Is this a very old house?
First, if you know where the electrical supply box is, go look at it.
1. Look at how many wires are coming from the pole to the pipe sticking out of the roof, in turn that is connected to the supply box below... Should be 3 wires. I have seen old houses with a single phase(2 wires). With 3 wires, you have 2 hot wires and a neutral. The 2 hot wires are out of phase with each other. If you measured from each hot to the neutral, you have approx 120 volts. If you measured across the 2 hot wires you would have approx. 240 volts. Make sense? If there are only 2, you do not have 240 in the house, and may not have a ground either.
2. Open the cover and see if you have the old round type fuses or circuit breakers?
3. Look on the outside of the box. Do you see a bare wire running to a clamp on a water pipe nearby? Is it corroded at the clamp? I've seen them even disconnected or broke too.
4. Look at the outlets in the room. Are all of them the 3 prong type(2 straight and 1 gound hole per plug? Or just 2 straight holes per plug? Or a combination. I've seen a combinations of outlets in the same room many times also.
5. Look at the amps. Do they have polarity switchs?
First scenario:
Ok, this is the reason I asked you to look at the wires on the pole. IF, and I've seen it done many times, a homeowner or someone, without permits, did some electrical work on the house, they may have hooked up 2 or more outlets in one room to one phase of the supply.(120v) Then they hooked another outlet in the SAME room to the other phase(120v also). However, IF you measured across from the hot pin on an outlet connected to Phase "A", and a hot pin on an outlet connected to Phase "B", you have a potential of 240 freaking volts. NO GOOD....ie...can kill you if you have old outlets with only 2 holes and no ground. I almost got killed in a kitchen with this scenario. Knocked me on my ass good. Modern kitchens are wired like this. Lord knows why. I even asked the NEC about this potential and they scratched their head. Go figure. Probably it would require total failure of the grounding system.
Second scenario:
Do your amps have a 3 prong plug on the power cord. Many of the older and even some new amps only have a 2 prong plug. IE NO GROUND. What they do have is a POLARITY SWITCH. Some have a power switch you can throw from center, to either direction to switch polarity. Some, like old fender amps have a polarity swich on the back of the amp. One of the two amps you are using, has a reversed polarity, OR, they are on another phase, STILL with reversed polarity.
Third scenario:
You have the old type room outlets(2 hole, no ground) but one or more of your amps have 3 prong power plugs, and you have inserted a 3 pin, to 2 pin adaptors. And one of the adaptors is turned around. Causes reversed polarity. In other words, your amp chassi, becomes the hot side in relationship to the guitar plugs. IE.....no ground.
Fourth scenario:
Someone has changed old 2 prong outlets, to the newer 3 prong type, but had no ground wire to hook up, so the ground prong is dead. NO GROUND.
Fifth scenario
A ground wire has inadvertantly got cut, disconnected, or plain not hooked up. I've seen old houses where the ground wire was seperate from the romex, and was run from out let to outlet, daisy chain, across the studs before they sheetrocked the room.
As they nailed the sheetrock, believe it or not, they severed the ground with a sheetrock nail. Took me 4 days to find THAT one. ARRGGGGGGGRRRR!!
Any of the above scenarios, and there may be more, are severe. My suggestion is to make the observations above, report back and let us know what you find. OR, simply do as spankenstein said. Use a multiple outlet strip, connected to ONE outlet in the room. That will at least garantee the audio gear is on the same phase. It does NOTHING to garantee a ground exists. This is serious stuff and shouldn't be fooled around with by renters. If you are not familiar with electricity, have someone qualified, or maybe the owner can call some one, to check for ground in the house.
But at least make the observations, as some of those will eliminate some scenarios.
Then maybe we can Really help. Some people may dispute my theorys. Here is my disclaimer. I AM NOT AN ELECTRICIAN. Only my experiences with electricity are reflected in my suggestions. Let THEM figure it out in that case.
fitZ
Besides, with electrical stuff its best that ALL the info be absolutely clear in my book. Otherwise, someone might get hurt, start a fire, get pisssd, come back and verbally kick my ass.!
