It's only happened twice now, but I'll see bright orange sparks fly out from the outlet when I unplug my Peavey Classic 30, and a there's slight burning smell as well. It's only happened at a theater I play at weekly, never at home or anywhere else. Last time it happened was a couple days ago.

Went to play it earlier at home, first time I've played it since it sparked, and I noticed it wouldn't turn on. I saw the first power tube had a crack in it near the base where the prongs are and it was white at the bottom. I put an older, but still useable set of power tubes in, still wouldn't turn on.

Then I unplugged it from the wall and the power cable came right out of the plug. It's worth noting that the plug is one of those replacement plugs, the ground prong broke off the old one. I looked at the plug and it was all burnt and charred black inside, which I'm guessing is why the plug came right out.

Needless to say, I'm thoroughly confused. I'm fairly sure the broken power tube had something to do with the sparking at least.

Anyone have any idea of what's going on?
You need to replace the plug and do it properly.
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Never pull on the cable to unplug anything, grab the plug itself.

Get a new power cable and check the fuse. If it still doesn't power on (or the fuse blows right away) check for burnt spots inside the amp.
"Practice doesn't make perfect. Perfect practice makes perfect." -some dude
Never pull on the cable to unplug anything, grab the plug itself.

Yep, never ever pull on the cable.

If the plug wasn't bad to begin with, the wiring in that venue might be flaky, let hem know it may have a problem. Replace the power cable completely, and never ever break off a ground pin. It's there for a reason. If you absolutely have to use a two prong outlet, very rare these days, get a ground adapter that does not have one.

If it got far enough to physically damage a tube, there's a good chance something caused a direct short, possibly the tube itself. If the amp is an older Classic 30, it's old enough it could have dried out electrolytic capacitors, and of the wrong one fails while in use, that can cause a short that can in turn fry a transformer.

Have the amp checked out by a competent tech, sometimes but not always, if a transformer fails it will show something that looks like wax dripped down the outside, or will have a burnt smell. If you replace the power cable and it still won't turn on, take it to a tech.

Also be very careful if you open it up, tube amps have filter capacitors that can hold a high voltage charge for several months after being unplugged. If you don't know how to properly drain those, it is literally deadly in there. Yes I'm serious, it can literally kill you. The Classic 30 is also not an easy amp to work on, the circuit board is folded inside the chassis, it's built in two boards tied together by flexible wire flat cables. When I did some modifications on one it took me around 20 minutes just to get the thing out of the chassis. And you have to be careful not to bend those flat cables too much, you don't want to break a wire...putting it back together took another half hour. By comparison it takes me less than 5 minutes to get my Super Reverb out of the cabinet, nothing has to be removed from the chassis at all, it's all wide open and easily accessible. Really bad design...
Hmmm...I wonder what this button does...