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I'm working on a Kenwood 1100U tubed receiver. I fixed a ground loop that I caused a few years back, now when I have the unit plugged in, I get a slight hum (5-7mvp-p). If I reverse the AC plug, the hum almost goes away, but when I turn it off with the AC plug reversed I get a hum for about 3 seconds after I turn it off, about as loud as the hum with the AC plug in normal.

To be safe, you could replace the power supply caps. With a receiver, you could install a three pronged plug. A three pronged plug in my Dynaco fm-3 generated incredible hum, but it was probably due to the amp.