So this happened at band practice... It's one of the big 70uF filter caps in series on an AB165 Bassman (blackface). I'm thinking this was caused by vibrations, but I'm still pretty surprised about it. The replacement part is on its way... in the meantime, I'm curious if removing that cap from the circuit while the amp was on would have damaged any other part of the circuit, or tubes for that matter. Would anybody care to speculate on this? Anyone run into this kind of problem before? I'm thinking about putting a strip of hot glue at the base of these caps to hold them in place and dampen vibrations. Thanks for the input!