Color display apparatus including a CRT with internal switching valve

Claim 1

A color display apparatus comprising: (a) a color display tube comprising an envelope containing: (1) a screen having first and second luminescent layers and a screen electrode parallel thereto, said layers luminescing in respective colors when excited by electrons accelerated through respective higher and lower accelerating potential differences; (2) a channel plate electron multiplier having an input and an output side, said output side being parallel to and spaced apart from the screen; (3) an electron gun for producing an electron beam; and (4) means for scanning the electron beam across the input side of the channel plate electron multiplier; and (b) means for controllably producing said higher and lower accelerating potential differences between the screen electrode and the output side of the channel plate electron multiplier, said means including: (1) a thermionic valve contained within the color display tube envelope and comprising an anode electrically connected to the screen electrode, a cathode adapted for electrical connection to a reference voltage, and a grid; and (2) high voltage power supply means for selectively applying higher and lower voltages to the screen electrode in response to an applied switching signal, said power supply means including a variable voltage power supply having an output electrically connected to said screen electrode, feedback circuitry electrically connected to said screen electrode for sensing the voltage at the screen electrode, screen voltage control circuitry having an input for receiving said switching signal, having an input electrically connected to said feedback circuitry, and having an output electrically connected to said variable voltage power supply for applying a control voltage to the power supply to maintain the screen electrode voltage constant at the selected voltage despite fluctuations of electric current in the screen electrode; and (3) a pulse source electrically connected to the grid for applying a pulse thereto for momentarily increasing the valve's anode current whenever the voltage applied to the screen electrode by the high voltage power supply means changes from the higher voltage to the lower voltage.