Patent GB-780531-A (GB)
Title: Improvements in or relating to electron discharge switching valve
IPC: H04N7/167
Filed: 1955-05-20
Abstract: 780,531. Cathode ray tubes. ZENITH RADIO CORPORATION. May 20, 1955 [June 29, 1954], No. 14579/55. Class 39(1). [Also in Group XL(c)] A cathode ray switching tube comprises first and second accelerator electrodes 15, 19, Fig. 5, between which is arranged a gating electrode 17 and two "controllector" electrodes 21, 22 having deflection control portions and collector portions. A gating electrode 55 is also provided, and two electrode systems of identical form are mounted on opposite sides of a common cathode 12, which generates two oppositely directed sheet beams. The complete circuit is shown in Fig. 4. Controllector electrodes 21, 31 are connected by potential divider 65, 66, 67, resistance 67 having a variable tapping to load 35 and also by resistances 61, 64, 68, 69, signal sources 59, 62 being coupled via condensers 60, 63 and resistances 61, 64 to controllectors 21, 31 respectively, while controllectors 22, 32 are interconnected. The switching electrodes 17, 25 are energized in push pull by square or rectangular wave generator 34 to cut off one beam while leaving the other beam on, and gating electrodes 55, 57 are connected to one another and to signal generator 75, and the accelerating electrodes 15, 19. 23, 27 are all connected together. The first and second accelerating electrodes may, however, be at different potentials to improve the beam focusing. If beam 54 is on while beam 53 is cut off, the beam 54 shunts resistances 66, 67 so that none of the signal from 62 can reach load 35, but the signal from 59 can reach 35. When the polarities of switching electrodes 17, 25 are reversed, only the signal from 62 can reach the load 35. The gating signals gate the beams on and off at a frequency much greater than the switching frequency e.g. a high harmonic thereof, to avoid transient switching signals appearing in the load (due to and during reversal of polarities of 17, 25) since the beams are always cut off when the polarities of 17, 25 are reversed. The current collected by the switching electrode is used by passage through resistor 72 to keep the switching electrode potential close to the accelerating electrode potential when the beam is on, the switching electrode current being mainly secondaries emitted by second accelerator 19. The two electrode systems may not have identical operating characteristics, e.g. they may have slightly different maximum beam currents in which case the external circuit may be balanced to compensate for this, e.g. by adjusting the tapping on resistor 67 to get no switching frequency signal in load 35. The device of Fig. 4 may be used in a "subscription type" television receiver system, Fig. 6 (not shown), as the audio decoding switch which will apply either one of two balanced output signals from a preceding audio circuit to a loudspeaker. The electrodes are stamped out of sheet nickel and many electrodes can be integral with one another, e.g. the gating electrodes, the accelerating electrodes and the control and collector parts of the controllectors. The electrodes can be U-shaped or separate elements on opposite sides of the beam path i.e. they need not completely surround the beam. The tube is operated in a region where the beam convergence is constant with fluctuations in switching voltage, i.e. where there is a focusing lens between the cathode and first accelerator but little convergence in unipotential electron lens 15, 17, 19. The second accelerating electrode and controllector electrodes form an electrostatic lens focusing the beam on the collector portions the controllector potential being 1/5 to 1/3 of the accelerator potential. The beam focusing is not as sharp as might be desired due to space charge defocusing of the high current beam, but nevertheless application of a signal to controllector 21 varies the beam current distribution between 21 and 22. The load may be a cathode ray tube instead of a loudspeaker and the device can be used in multiplexing systems. The apertures in the gating and switching electrodes are bigger than those in the accelerating electrodes to follow the changes in cross-section of the beam. In a modification, Fig. 1 (not shown) there is only one signal source and no gating electrodes the signal being highly attenuated or allowed to pass to the load according as to which beam is switched on, the beam acting as a shunt path to ground as in Fig. 4. The external resistances, similar to 61, 65, 68 ; 66, 69, 64, of Fig. 4, are balanced to avoid application of a signal of switching frequency to load 35. The collector parts of the controllectors need not overlap and may be straight or curved, Figs. 7, 8 (not shown) or the controllectors may be simply inclined plates, Fig. 9 (not shown).