Magnavox Color Roundie

[DaveWM]

the way it works is the pot outside fixed resistance is in the cathode to ground circuit of the video amp, its in p with a fixed 220 ohm cathode resistor. This allows for a voltage drop across the resistor and pot when the video out tube conducts. The result is the cathode of the tube has a positive voltage with respect to the ground. This sets the bias of the tube (control grid to cathode). Proper bias is needed for the tube to work as designed.

the Cap is connected in such a way (neg to ground/pos to wiper) so when the pot is fully CCW (least contrast, least gain) the wiper is at ground so the cap does nothing. As you turn CW the cap is introduced into the cathode circuit, acting as a bypass to the ac signal generated by the tube conducting a normal video signal. With out the cap there is neg feedback (strong signal, more voltage drop, more voltage drop higher cathode voltage, higher cathode voltage greater bias off of tube. With the cap in place the bias is held constant for max gain (most contrast) of the tube.