System for the power control of high voltage electron beam generators having a cathode and a control electrode associated with the cathode. The system is provided with a high voltage isolation transformer with primary and secondary winding for supplying the control electrode with a control voltage. A system precedes the isolation transformer on the primary side and is adapted to generate square wave pulses susceptible of amplitude modulation. The pulse generator is designed such that the pulses are square wave impulses symmetrical with the null line. The isolation transformer is provided with a tertiary winding on the primary side and the primary winding receives input from an amplifier whose input is connected with the output voltage of the tertiary winding. A rectifier bridge is connected to the secondary winding.