Abstract: A filament winding forms a part of the step-up transformer in a flyback-type high-frequency, high-voltage power supply for supplying filament heating energy, in addition to the anode potential supplied by a secondary winding of the transformer, to a magnetron microwave power generator, which is a self-rectifying load. The voltage across the transformer primary winding, and the current flowing through a controllable current path in a switching device connecting the series with that primary winding, are both monitored and compared to reference voltage and current values for establishing the conduction time interval of the switching device to provide active control of the power input to the load.