Based on the voltages measured for the manual 3 years ago, and simulations I have just now repeated, I will stand by the current design calling for a total DC resistance of 1200 ohms. This should produce the right voltages with a 120v power line, and variations within spec if the line varies within its spec. If you know the amp will always be used at a different power line voltage, you can adjust the voltage for that power line.
Increasing the resistor will reduce the plate voltage and output power, but will treat the tube a little more gently. Because of the resistor bias, it will remain close to "optimum" performance (based on my idea of optimum of course!). Do not exceed 2000 ohms unless you can use a resistor of greater than 10 watts rating. Even a 2000 ohm resistor will have a small effect, less than 20% dissipation and output power reduction.
Meanwhile I will try to measure the voltages again the next time I am at the headquarters - I don't have a Stereomour in my lab at the moment, and I suspect a small inconsistency in the manual voltages.
