Magnatone 480A Bias + Plate issue

[FPALB]

I have this 480A that just came in that the owner says is red plating after playing for a half hour or so.

I put in some 1ohm resistors to check the cathode currents and put in a bias pot replacing the 47K resistor in the bias circuit.

The funny thing im seeing is that the plate voltage is changing when I adjust the bias - haven't seen this before.

Any ideas here?

100k grid to bias resistors are spot on
Changed the coupling caps

 

[CJ]

Plate voltage will come down as the rubes are biased to draw more current,

What brand tubes? They might run hot when the amp is in the cabinet,

Or there might be something else going on, like a conductive circuit board,

If that is the case, you can stiffen up the bias supply by lowering resistor values and you could also install cathode resistors line 50 ohms that will help regulate current runaway. And you can also lower screen volrage.