Mesa Boogie Mark IV one 6L6 red plating and loud hum.

[jvm]

Haven't been into a lot of Boogies. I have a Mark IV simul class with loud hum and one 6L6 redplating. Any suggestions as to where to start? All resisitors in output section are in tolerance. I have heard that this could be caused by a leaky coupling cap but I would think that it would affect more than one tube. Any suggestions appreciated.

[km6xz]

Swap the tube positions and see if the high current tube is the same or if the socket is involved.
If the same position is red plating, with a different tube in it, pull the power tubes and measure the pins to see is negative bias is present on that socket and whether the socket connections are formed so that they make good contact with the tube pins. That is the first brief tests to start with.

[jvm]

Thanks, km6xz. I've tried three different tubes in the same socket. Definitely a problem with the amp. I am at work at present and will follow your suggestions when I get home. Thanks for the help.

[jvm]

Red plating was being caused by leaky coupling cap. I was getting about -220 volts at pin 5! Replaced cap and 220k resistor. Works like a new one!