I have this old tube PA head made that I'm use as a guitar amp. Well I bought a Celestion Blue recently and using the head with it is awesome. One of the best sounding od's I've ever heard. Well anyways, yesterday I turned it on to let it warm up. Came back in 15 mins and it was off. I tried the switch a few times and finally it turned on and was fine. Later on that night I unplugged it to take to a jam and went to turn it on an nothing. I shook it a few times tried it again, it turns on. While I'm playing it shuts off and now it won't turn on at all. I opened it up and saw that the ground was not connected to the input jack. I resoldered it and still won't turn on. Does anyone have an idea what could be wrong? I'm a beginner when it comes to working on amps but I am starting to learn. I would really like to try to fix this myself but I have no troubleshooting skills. I just got a multimeter and was wondering if I could use that to see if any the parts are bad? Thanks in advance for any help!!!