Is it a tube recifier?, or a solid state rectifier? If it is solid state I would say it is a bad diode, If it is a tube rectifier, I would check the bottom of the tube socket, and look for carbon tracking. As a last resort you will have to check the power transformer.. You can do this by cutting off the 2 red wires that come from the power transformer, and go to the tube socket for the GZ34 ( or in the case of the solid state rectifier, the 2 red wires that go to the full wave rectifier) and then power up the head, turn if off of standby, and see if it still blows the use. If it does, then the power trans is fried. Good luck.
