A cathode heater supply for a microwave source comprising: switched mode power supply, SMPS, inverter means; and characterised by: isolation transformer means (12; 42) comprising: primary windings (122; 422) arranged to be powered by the SMPS inverter means; a monitor winding (123; 423) wound through core assemblies of the primary windings (122; 422); and a secondary winding (121; 421) arranged for connection to the cathode heater (11; 41); current monitor means (141; 441) arranged to monitor a current in the primary windings (122; 422); and signal processing means (14, 131; 44, 431) arranged to receive a first input signal from the monitor winding (123; 423) indicative of a voltage (Vh) across the cathode heater and a second input signal from the current monitor means (141; 441) indicative of a current (Ih) through the cathode heater (11; 41) to calculate a comparison signal comprising a division and/or product of the first input signal and the second input signal and to compare the comparison signal to a reference signal, the signal processing means further being arranged to output a control signal to the SMPS inverter means (13; 43) to control power supplied to the cathode heater dependent on the comparison of the comparison signal and the reference signal. switched mode power supply, SMPS, inverter means; and characterised by: isolation transformer means (12; 42) comprising: primary windings (122; 422) arranged to be powered by the SMPS inverter means; a monitor winding (123; 423) wound through core assemblies of the primary windings (122; 422); and a secondary winding (121; 421) arranged for connection to the cathode heater (11; 41); current monitor means (141; 441) arranged to monitor a current in the primary windings (122; 422); and signal processing means (14, 131; 44, 431) arranged to receive a first input signal from the monitor winding (123; 423) indicative of a voltage (Vh) across the cathode heater and a second input signal from the current monitor means (141; 441) indicative of a current (Ih) through the cathode heater (11; 41) to calculate a comparison signal comprising a division and/or product of the first input signal and the second input signal and to compare the comparison signal to a reference signal, the signal processing means further being arranged to output a control signal to the SMPS inverter means (13; 43) to control power supplied to the cathode heater dependent on the comparison of the comparison signal and the reference signal. isolation transformer means (12; 42) comprising: primary windings (122; 422) arranged to be powered by the SMPS inverter means; a monitor winding (123; 423) wound through core assemblies of the primary windings (122; 422); and a secondary winding (121; 421) arranged for connection to the cathode heater (11; 41); primary windings (122; 422) arranged to be powered by the SMPS inverter means; a monitor winding (123; 423) wound through core assemblies of the primary windings (122; 422); and a secondary winding (121; 421) arranged for connection to the cathode heater (11; 41); current monitor means (141; 441) arranged to monitor a current in the primary windings (122; 422); and signal processing means (14, 131; 44, 431) arranged to receive a first input signal from the monitor winding (123; 423) indicative of a voltage (Vh) across the cathode heater and a second input signal from the current monitor means (141; 441) indicative of a current (Ih) through the cathode heater (11; 41) to calculate a comparison signal comprising a division and/or product of the first input signal and the second input signal and to compare the comparison signal to a reference signal, the signal processing means further being arranged to output a control signal to the SMPS inverter means (13; 43) to control power supplied to the cathode heater dependent on the comparison of the comparison signal and the reference signal.