Abstract: A cathode current control system comprising an electron discharge device having a tubular envelope wherein an electron gun is disposed to beam a cathode current of electrons onto a spaced anode collector electrode, the gun including an electron emitting cathode, an apertured control grid electrode spaced from the cathode, and a further spaced series of beam forming electrodes; and external circuit means connected to the cathode and other electrodes of the device for maintaining the electrodes at respective predetermined electrical potentials relative to the cathode and for maintaining electron emission from the cathode sufficient for providing a predetermined level of cathode current associated with the predetermined electrical potentials of the electrodes.