ES369405A1 - Picture display device - Google Patents

Abstract

A beam concentrating electrode 5 (e.g. a convergence or focusing electrode) is connected to a potential divider including a valve 11 and cathode resistor 13 connected across the EHT supply source, the grid of the valve being biased by the potential drop across the resistor. With this arrangement the ratio between the voltage at the electrode 5 and that at the EHT terminal substantially equal to 1/Á+1 (where Á is the amplification factor of the valve) which is independent of the value R k of resistor 13 so that variation in the voltage at electrode 5 due to temperature effects in the resistor are eliminated Additionally, the resistance between the anode and the cathode of the valve is substantially-equal to R t /Á+1 (where R i is the internal resistance of the valve), which is very small in comparison with the resistor value required, i.e. ÁR k , in the absence of the valve, so that the voltage at electrode 5 is substantially independent of variations in the current drawn by this electrode. When electrode 5 is a focusing electrode point 15 may be connected directly to the EHT zero potential line when electrode 5 is a convergence electrode (e.g. in a tri-gun colour display tube) point 15 is returned to the zero line via a second valve 17 to the grid of which a convergence correction waveform is supplied, the peaks of the waveform being at substantially cathode potential (i.e. EHT zero) due to the charging of capacitor 23 by grid current flow. With this arrangement a portion Á/Á+1 of the input convergence correction waveform. (Á= amplification factor of valve 17) and which is independent of the value R k is effective at electrode 5. When a focusing electrode is present in addition to the convergence electrode 5 (e.g. in a three-beam colour television display tube) it may be connected to a tapping on a potential divider connected between the cathode of valve 11 and the grid of valve 17, the tapping being adjusted to a position at which the A.C. (convergence correction waveform) voltage is substantially zero and the magnitude of the D.C. voltage of which may be adjusted by adjustment of resistor 21. In order to accommodate different display tubes the ratio of the voltage at point 15 (substantially zero) to that at the EHT electrode is made adjustable by provided a bias at the anode of valve 17, e.g. by clamping the A.C. voltage at the grid at different levels or by adjustment of anode resistor 19. The voltage at electrode 5 may be adjusted by connecting the control grid of valve 11 to a variable tapping on register 13 or by adjustment of the anode voltage 17, e.g. by adjustment of resistors 19 or 21. The triode 11 may be replaced by a multi-grid valve and if desired, all valves may be replaced by transistors.