A microwave heating system uses a magnetron powered by a full wave full bridge inverter. The magnetron has its power output stabilized against variations which might be caused by fluctuations in the AC input line. The inverter powers the magnetron by way of a power transformer having a primary and a secondary. The current is sensed in the primary in order to change the switching frequency of the inverter. As the current in the primary will depend upon fluctuations in the AC input line voltage, sensing of the current in the primary may be used to sense variations in the voltage of the AC input line. Alternately, current in the secondary of the power transformer may be sensed and used to detect variations in the AC input line voltage. In either case, the sensed variations in AC input line voltage are used to change the inverter frequency. Because the secondary includes a series resonant tuned circuit, changes in the difference between the inverter switching frequency and the resonant frequency of the tuned circuit may be used to counteract AC input line variations and stabilize the magnetron output power.