Thought I'd share this experience with you guys, this has happened to me. During the tweaking of an OTS, if you unbolt the main circuit board from the standoffs, be especially careful with the wire from the bias pot wiper and the other two wires from the output tube grids when you replace the board and bolt it down. They have a tendency to ride up on the shoulder of the stand-off post, which very neatly snips a divot out of the insulation when you bolt the board back down. The wire then shorts to ground, and causes a lot of headaches and damage.