Hi there,could be a couple of things,you might have a fault in the power supply,(bad/leakey diode or filter cap),output tubes could be going bad,or the breaker might be damaged or worn itself.It could also be the power transformer,(hope not).I'd check my voltages and do a visual on the filters to see if they're swollen or leaking.You could check your tubes also,but a lot of the time it's the breaker,they get banged around quite a bit where they're located.Pull the output tubes and fire up the amp,let it run for a couple of hrs.,if the breaker pops,it's either the power supply or the breaker.Hope this is of some help,good luck Mel.