Abstract: A power supply is switchable to apply a low kilovoltage and a relatively higher kilovoltage alternately to the anode of an X-ray tube that includes a filament and a control grid. A grid bias voltage generator uses an inverter driven in the kilohertz frequency range to feed the primary winding of a first transformer whose parasitic capacitance and inductance are used to produce a peak ac output voltage from the secondary of the first transformer at resonant frequency. The secondary output voltage is rectified and the resulting negative bias voltage is applied to the control grid synchronously with the high kilovoltage being applied to the anode so the X-ray tube current is then relatively low. A less negative or zero bias voltage is applied to the grid synchronously with the lower kilovoltage being applied to the anode so the X-ray tube current is then relatively high and substantially limited by the temperature and emissivity of the filament.