Abstract: In the illustrated embodiments the primary voltage of the high voltage transformer is supplied via a voltage divider so that the secondary voltage does not exceed a desired value during heating of the X-ray tube filament. As the secondary load increases with a consequent tendency to reduce secondary voltage, an element or elements of the voltage divider are switched so as to increase the voltage supplied to the primary winding of the high voltage transformer. In a monopulse circuit, diode means are associated with the voltage divider which are poled such that the voltage divider controls the positive polarity secondary half wave voltage. In one embodiment, a single switch may simultaneously bypass the series resistance of the voltage divider and remove a parallel resistance with respect to one polarity of primary current.