Hello. I’m working on a Fender Twin Reverb AB763 that might have a grounding issue… that’s intermittent? It happened once. Here’s the situation. I’ve done some general work replacing old electrolytic capacitors, testing it, finding a few other faulty areas. It came to me with a grounded power cable installed… and also a death capacitor and wired polarity switch. I removed both. I tested various voltages in it against the schematic, tested signals, confirmed the bias, played it at my workbench and thought it was solid. I took it for a final test to a rehearsal space with a guitar. A friend (who has played for many decades so I trust his experience) said he felt it tingling <strikethrough>without</strikethrough> touching the strings, so we shut it down. EDIT: He just clarified that he touched the strings to feel this. He said it was like a live wire at a very low voltage. (I thought he felt it like static in the air.) He said that it didn't feel like it discharged like a static shock, that it maintained its low voltage "tingle." At the same time, we were testing another amp. After stopping with the Twin Reverb, he used the same guitar, cable, and outlet/power strip with no issue on the second amp. Other points that might be factors but just could be noise: -We plugged both into the same power strip and powered them up for several minutes. -He wore the guitar, plugged the cable into it, then plugged the cable into the amp. -He tried the normal channel. Then after stopping, he wanted to retry with the vibrato channel. Same process of plugging in the cable with the same result. I know of no power issues in the space. I tested the power strip the next day and found no issues. (Expected AC between Hot and Neutral and Hot and Ground, no voltage between Neutral and Ground.) I retested at my workbench and then again in the space with the guitar, measuring from the case to the power strip ground but got no voltage. I get 4mVDC between the instrument cable tip and case ground but that seems irrelevant (yes?). Before the incident, I had no power problems. I don’t know how putting the amp back together and moving it could have caused one? After retesting at home (no issues), I brought it back to the space and couldn’t recreate the issue. In-between at home, I found: -Moderately cold solder points inside the speaker 1/4-inch cable I fixed. (Eh.) -Deteriorated heater wiring, especially on the first power tubes closest to the transformer. Small parts of the wiring had lost the colored outer shield, but I couldn’t find continuity with my meter. It seemed like there was an inner shield that was exposed but not conductive, but both leads were twisted together in a space without the outer shield. I replaced this section of wire. -Mesh shielding on the top of the wood case has a few strands poking loose and down. They run about a cm long and don’t look like they’d poke/reach anything. But I wonder if reassembling this could cause some kind of fault where one of those touched something wrong? EDIT: Old heater wiring pictured below. I’d very much like to repeat this problem and/or find a clear fault to fix. Does anyone have any tests to try to hunt it down or reconfirm the safety of this otherwise? (I’ve run many more tests than noted here, but welcome any ideas or repetition.) Any speculation about the cause? The mesh shielding strands seem like my best guess, and that still feels tenuous. Thanks much! Long time listener, first time caller.