guitar amplifier with hum and overheating transformer?

It would have been a whole lot more helpful if you could have said what make / model of amp you have. It makes a big difference in the diagnosis if it's tube or solid state.

But to answer your question simply, you need to take it to a qualified electronics tech to figure it out. If it's a new amp, take it back where you bought it. If it's under warranty, contact the manufacturer.

There's a lot of things that could be wrong with the amp. Unless you've got a fair amount of electronics experience, test equipment, and know which end of a soldering iron to pick up, you'll likely do more harm than good working on this yourself.

Good luck.

Greetings from Austin, TX

Ken