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Hello all

I have a blackface/early silver face princeton rvb with GZ34. All the voltages look very close to published specs on the schematic. The power tubes have 407VDC on the plates and 46-49ma of current respectively. They are both showing pink & a touch of red down the center of the plates.

The power supply looks to be normal in terms of voltage as well, there is a little bump in the PI where volts are maybe 215 on the PI tube plate as opposed to the published 200 but that doesn't jump out as being way off.

What I am wondering is what the bias system here is. Both power tubes cathodes are tied directly to ground with no resistor/cap network as per cathode bias. Grid 1 however is tied to the intensity pot wiper through a network of 220K resistors. Where they meet, there is -34volts as specified on the schematic.

Is this a grid bias system? How to adjust to make the power tubes stop red plating? Or is this the proper approach?