Plate Resistance Explanation

[captainate]

I'm working on this circuit, and curious why R3, R7, and R12 would be of different values. Obviously, R3 was changed to a different value  in the schematic by hand, and the unit I'm rebuilding had an entirely different value from either of those (390K). What is the purpose of this resistor, and what values are appropriate for this position?

 

[PRR]

180K 150K are practically the same.

820K in a wideband audio amplifier is dubious, even before you find the much lower 100K pot after it. (It is surely a stutter-- Rg2 is 820K and the same text got on Rp.)

In *general*, early stages are designed for high gain and low hiss, possibly low supply current to cheapen B+ filtering. Later stages are designed for power output. Look at the (signal voltage) notes: 0.5V, 3V, 3V(again); and potentially 7V at clipping. Because of EQ and gain-knob this is not such a spread of signal voltages as you might expect in three cascaded pentodes, so same/similar values work for all.

I suspect they could have used one set of values no compromise, but were afraid of running out of 150K resistors.