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Well, I'm stumped! I have built cbgs with disc piezos, rod piezos and mag pups. I recently had a friend come over to play each box for marketing purposes. He brought a pedal board and we plugged it all in to one of my small 3w amps. We had issues with most of the boxes getting feedback from the amp. tried various setups and even unplugged the amp and let it run on battery, but still had feedback. I've been through the forum looking for answers but haven't seen this addressed with these parameters.

So, this morning I took several boxes and two amps down to the shop to see what I could figure out. Both amps are battery powered, and one of them is battery/AC-DC. All boxes sounded perfect when plugged in to the battery powered amps. As soon as I put AC to it I am getting feedback. Touch the cable and it knocks out the feedback, indicating a grounding issue. The boxes are all wired pretty much the same - a rod piezo built into the bridge, connected to a volume pot, connected to the output jack. Nothing fancy. The rod piezos are set into the bridge but not potted. Would potting in silicone help?

So, I am coming to the conclusion that my problem lies in the AC, not in the boxes.

Would the boxes cause feedback on battery power if they were improperly grounded? Is there a way to reduce or eliminate this feedback/hum? Is there a better way to test this to determine where the culprit is? I am thinking that the building wiring around us is the problem, not my boxes. Can I use a circuit tester or multimeter to test everything and, if so, what is the procedure and sequence?

It is frustrating as a box I lent to a friend had the same issue once they plugged it in to their Roland Micro Cube.