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6GC5 the big tube on the audio board.
The 3.3meg resistor has one side to the 135v source (provided by the 6GC5 acting as a voltage divider in a stacked B+ supply).
The other side of the 3.3 meg connects to the 330k resistor to ground.
the brightness control gets it voltage from the junction of these two resistors (a 10/1 voltage divider). If the 3.3meg drifts high the junction of the two (330k/3.3meg) will have a lower voltage for the brightness control to max out at, resulting in a dimmer pic. If the 330k drifts high the reverse is true the junction will have a higher voltage, brighter pic. if the 330k goes low, the junction voltage will be lower, if the 3.3 goes low the junction voltage goes higher. Look up voltage divider for how this works if the above is not clear.
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