I've got a Marshall AVT20 Budget Amp that was accidentally left on for about a month. Could this have damaged the amp in any way?
I dont know about damage but your electrical bill is gonna be insane. a month??? how do you not notice that?!?
its only 20 watt... the bill wont be too much.

you might want to check the tube in the preamp (does it have one?) otherwise it should be fine.

Quote by KileManA7X
I remember my first erection. I went to my dad and was like "Do I have Aids???". I seriously thought there was something wrong with me.

i dont think it would damage the amp itself, but the tubes. im not certain though.
It only has one tube (12AX7) in the preamp. If it still works and sounds the same don't worry about it.