I have a mesa walkabout head pushing a 4 ohm ampeg 4-10 cabinet and my head has gotten hot as hell and overheated twice now on stage. the head is about 2 yrs old. both times it shut off were at the end of a 3 hour set. The fan is working. help me people...i don't want to pay to have it fixed if i can do it myself. I don't think the tubes have ever been changed...could this be why it is screwing up?