| Yep. Ground loops have plagued audio setups since time immemorial. Okay, the three prong adapter, that's a clue. There are two possibilities: either the house wiring is bad (which is very likely, the vast majority of the clubs I've played at have bad wiring), or, one of your amps isn't grounded, or is grounded incorrectly. The former is way more likely. You can tell whether an outlet is wired correctly by using one of those little tester gadgets you can get at any Home Depot for about 10 bucks, just plug it into the outlet and it has three little lights that tell you what the wiring's like. But one thing this won't find for you is ground wiring that's not to code and is likely to cause ground loops. For instance, you can have ground wiring tying back to a common point in the building, like a pipe strap or a ground rod, and if it so happens that you plug two pieces of equipment into different sides of the ground, and then connect them together with an audio cable, chances are high you'll get a ground loop and it will result in audible hum. One way to fix this is to plug the amp into the same circuit as the board, so that "ground" means the same thing in both places (or as close as it's gonna get for practical purposes). Another way is to lift the ground on your amp by using a three prong to two prong adapter, but this is slightly dangerous because it means the amp's ground depends on the audio cabling for its connection to the house ground, so this method should only be used temporarily and as a last resort. I'd check to make sure your Redhead is properly grounded, you'll need an ohm meter or continuity checker to test that. If you happen to read about "star grounding" in any of the guitar or amp rags, the same principle applies when you're putting together large audio systems. Try to get the grounds as close together as possible, and keep all the cables as short as possible.
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"When all other possibilities are eliminated, whatever remains, however improbable, must be the truth."
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