My 74 champ has a problem just a slite hum no sound I did switch out the power tube,12ax7 and 5Y3 with no luck. I took the cassie out to look for burnt leaky caps all looks good to my untrained eye. Now I'm know amp tech but I do know how to use a volt meter/drain caps , anybody have any advice as to how to proceed trouble shooting? I can post pics if needed...