Patent GB-817212-A (GB)
Title: Improvements in or relating to methods of and apparatus for measuring the transconductance of an electron discharge device
IPC: G01R31/25
Filed: 1955-10-17
Abstract: 817,212. Electric discharge tube testing. SANGAMO WESTON Ltd. Oct. 17, 1955 [Oct. 18, 1954], No. 29595/55. Class 37. The transconductance between two electrodes of an electron tube having a cathode and at least two electrodes is determined by applying normal working voltages to the tube electrodes, applying a first known alternating energization voltage to the circuit of one electrode, and applying a second known alternating energization voltage to the circuit of the other electrode after the first alternating voltage has been disconnected; the ratio between the resultant alternating voltages appearing between a selected one of the two electrodes and the cathode as the successive alternating energization voltages are applied being a measure of the trans-conductance. In Fig. 1, an A.C. energized transformer 10 has identical secondaries 14, 15 respectively connectable by ganged switches 16, 17 to (1) superimpose a first alternating voltage on a standing direct voltage 21 for application over resistance R to the anode-cathode circuit of triode 20 while connecting the grid and cathode thereof together, or (2) connect direct voltage 21 to the anode directly over resistance R while connecting a second alternating voltage equal to the first between grid and cathode. The alternating voltages e 1  and e 2  appearing between the anode and the cathode in each case are measured by vacuum tube or rectifier voltmeter 22 in series with variable resistance Rm, and it is shown that the transconductance is given by where K is a constant. R m  may be adjusted so that e 1  gives maximum deflection of meter 22, which may then be calibrated to read e 2  as a direct indication of transconductance. In Fig. 4, the known alternating voltages from secondaries 14, 15 are respectively applied to the anode and grid of triode 20 in the two positions of ganged switch 25; the standing grid and anode voltages being derived from potentiometers 27, 29 energized from batteries 26, 28 and resistance 30 being the anode load, while the resultant alternating voltage developed between the anode and cathode is capacitance coupled to an amplifier 33 and rectifier voltmeter 34, through potentiometer 32 adjustable for full-scale deflection when the known alternating voltage energizes the anode, so that the transconductance is directly indicated on the calibrated meter scale when the known voltage is applied to the grid. Alternatively the known alternating voltage may first be applied to the grid and the potentiometer may be adjusted for full-scale deflection, after which on application of the known voltage to the anode the meter will indicate values of transconductance on an inversely calibrated scale rising to infinity at zero deflection (Fig. 5, not shown). In modifications (Figs. 6, 7, not shown), the range over which transconductance is measured may be extended by switching alternative anode load resistors into the triode circuit, or by varying the values of alternating voltages applied to the anode or grid by means of switchable tappings of the supply transformer secondaries or of adjustable resistive voltage dividers. The device is applicable to measurement of the transconductance between any two electrodes of a multi-electrode tube, and may be associated in known manner with switchable multiple sockets and adjustable power supplies adapted to accommodate a wide variety of different types of electron tube.