Beam current limiting circuit

Claim 1

In an electron beam device having a high voltage supply, a bias voltage supply connected between the grid and the high voltage supply, a self-biasing resistor connected in the cathode circuit, and an operating characteristic whereby a decrease in bias voltage results in an increase in the electron beam current, a circuit for providing bias when transients cause loss of the bias voltage supply comprising a transistor in the cathode circuit having a collector connected to the cathode, a zener diode connected between the high voltage supply and the base of the transistor, an impedance connected between the high voltage supply and the emitter of the transistor, the value of said impedance being selected to provide a voltage drop across the impedance within a range of normal operating currents which is much less than the conduction voltage of the zener diode, and a first resistor connected between the base and the collector of the transistor.