This is my first tube amp, it pops every once in a while... why? :/

Check for loose connections. That's most likely the problem.

Find out if it is popping because of something you are doing. Also are you running pedals? sometimes pedals make popping noises, specially if you are using ac adaptors and it has a bad connection.

No pedals, straight guitar to amp connection, I'll try a few different cords and report back if it fixes it.