Abstract: A source of variable DC gain control voltage for a gain controllable video display system including an image display kinescope comprises a manually adjustable potentiometer, and a variable conduction ambient light sensing device located in proximity to the kinescope display screen. A gain control voltage provided at an output terminal of the source normally varies both manually as a function of the potentiometer setting, and automatically as a function of varying conduction of the light sensing device in response to varying ambient light levels. Automatic gain control provided by the light sensing device is defeated when the potentiometer is set to an extreme position corresponding to maximum gain, so that maximum signal gain (i.e., maximum image contrast) can be produced in low ambient light conditions.