Abstract: An X-ray generator has at least a first and a second cathode filament for emitting electrons, one end of the first filament being connected to one end of the second filament, the connection point forming a terminal. Still two further terminals are located at the ends of the serial connection of the two filaments. Two of the three terminals are connected via a diode in each connection line to the first pole of a d-c source, while the third terminal is coupled to the second pole of the d-c source. Both diodes are oppositely poled. The polarity of the d-c voltage supplied by the d-c source can be changed, for example, by a remote reversing switch. The polarity of the d-c source determines which and how many filaments are energized to provide focal spots of different size. Since two supply lines are sufficient in a high-voltage cable for feeding the first and second filaments, a third line in the cable can be used for the supply of a third heating filament.