[Alan Douglas]

There must be a pattern to this. Read the voltage on pins 1 and 4 of the 4-pin socket, while turning the filament selector through its range. They'll be somewhat high, with no load, but they should match the knob settings pretty well. Do they start at 1 or 2 volts and increase in step with the knob position? 6V at the 5V position, 7V at the 6V position, 13V at the 12V position, and so on? Or do they really jump around?