I'm fairly new to working on amps, so this is foreign territory for me. Is it common for tubes to pop and burn up like this when they die? I just acquired a Musicmaster Bass Amp. One of the very early ones, probably all original tubes and caps. I added an inline fuse, made sure plug was properly grounded, took out death cap, removed and jumpered input cap (typical mod for guitar use). I have not changed caps or tubes. I just wanted to make sure everything was working before I did any other work on it. It worked gloriously for about 2 minutes, then started cutting out, distorting in a nasty way, and eventually died. Fuse didn't blow. Tubes and light were still glowing. I changed the output tubes around and no change. I then swapped the preamp tube for a known good one. No sound for a minute or so. Then a pop and smoke. When I opened it up at first I couldn't see any noticeable physical damage. So I went about sniffing around the chassis to see if I could locate the area that burned with my nose. I took out the tubes - one of the output tube smelled, and has a clear burned area. I checked the socket, and sure enough Pin 5 on the socket is black. That's the plate for 6A5Q tubes. The other tube and socket have no burn. There was no noticeable redplating before this. But the plates are actually hard to see in these tubes. I will go about getting some numbers on components, and will change caps and tubes before I do anything else. But right off the bat, what are some causes of tubes blowing up like this? Just getting old? Too much current? Poor bias? Could it be a power issue, or one of the transformers failing? Is the plate the typical point of failure? Can bad filter caps be culprits? Any insight will be helpful. Thanks!