a rotating x - ray tube comprises a cathode assembly including a cathode and a cathode current supply , and an anode assembly having an anode current controlled via the cathode current supply . a filament for emitting electrons is separated from the cathode . a cathode cup supports the filament and provides electron field shaping assistance . the x - ray tube further comprises a distributed capacitance for affecting the cathode cup electron field , and an insulator leakage resistance . the cathode cup is allowed to float on the insulator leakage resistance , whereby voltage associated with the distributed capacitance and the leakage resistance is allowed to remain relatively constant .