Abstract: A high power tube in applications requiring reduced AC hum in the amplified signal, utilizes a direct current heated filament. In order to reduce the incidence of tube failure and maintain a stable mode of operation, the direct current is produced by a regulated source of DC voltage. The regulated source includes a transformer having a primary winding coupled to a source of alternating line voltage and also includes a secondary winding at which a transformed low voltage is generated. A rectifier is coupled to the secondary winding for rectifying the transformed voltage to produce a pulsating direct voltage which is applied to the tube filament by a filter. The primary winding of the transformer is coupled to the AC source by means of a saturable reactor. A control circuit senses the voltage across the filament and controls the reactor in a feedback manner.