I have a blackface Vibro Champ (date code PI, 1966, September) that has always had a low-level 120 Hz hum for as long as I can remember. The hum is constant, and is independent of the volume setting, or even whether there's any input. It hums even at idle, with nothing plugged in. It's not a loud hum, but it's audible, and it makes the amp unsuitable for recording at low (clean) volume settings. The amp is stock (no mods), with factory wiring. I have just replaced all the tubes, so the 5Y3GT rectifier isn't a suspect. I'm more inclined to suspect the original GE "can" filter capacitor (it's a 20µF/20µF/20µF/20µF, 450VDC, with two sections wired in parallel to make 40µF). After 46 years, some leakage is likely. In fact, there are other problems with the amp that new tubes didn't solve, so I'm going to replace all the capacitors anyway. I'm seriously considering disconnecting the can cap and substituting individual individual 500V electrolytic caps for filtering. I've been advised that it might reduce the 120 Hz hum somewhat, but I've also seen posts (elsewhere) that say the hum is an intrinsic result of the power supply design, and even new filter caps won't eliminate it entirely. I'm committed to the recap job anyway, but I'm curious as to whether anyone knows if it's possible to eliminate the 120 Hz ripple entirely — I mean short of completely redesigning the power supply.