Patent GB-633156-A (GB)
Title: Improvements relating to apparatus for determining characteristics of electric discharge devices
IPC: G01R31/25
Filed: 1947-06-24
Abstract: 633,156. Determining characteristics of thermionic valves. HARDIE, A. M., and METROPOLITAN-VICKERS ELECTRICAL CO., Ltd. June 24, 1947, No. 16658. [Class 37] [Also in Group XL (c)] Voltage pulses of relatively short duration are applied to the control grid of a valve to be tested and the effective value of these pulses is varied, two or more voltages or currents associated with the valve being simultaneously measured during the voltage pulse periods. By this means the instantaneous electrode dissipation may be very high whilst the heat generated is relatively small so that the valve is not damaged. As shown in Fig. 1, a pulse generator B supplies pulses to the grid of the valve VT under test against the action of the steady grid bias voltage supplied by a unit GB. This bias may be varied to vary the magnitude of the pulses. The anode and screen grid voltages are supplied from 50 - cycle mains through adjustable rectifiers HT1, HT2 and resistors R4, R5. During the grid pulse periods the anode current flows through a condenser C1 and a resistance R1, the voltage across which is a measure of the anode current. The condenser C1 is of capacity large enough to sustain the peak anode current without appreciable potential drop over the duration of the pulse, R4 being of such value that C1 returns to its full potential shortly before the succeeding pulse occurs. Similarly the screen grid current flows through a condenser C2 and resistance R2 and is measured by the voltage across R2, whilst the control grid current flows through a condenser C4 and is measured by the voltage across a resistance R3. The voltages from the resistances R1, R2, R3 are' fed through gating circuits G1, G2, G3 and an amplifier A to the Y-plates of a cathode-ray oscillograph CRT. The gating circuits G1, G2, G3 are normally biassed to anode current cut-off and allowed to operate by pulses applied from a gate pulse generator unit. The gate pulses are arranged to recur over a period overlapping the pulse on VT so that the amplifier receives the voltages from R1, R2, R3 in succession. The gate pulse generator consists of a linear time base generator K and three strobe circuits S1, S2, S3 set to commence at the appropriate instants on the time base, which extends over three of the pulses on the valve VT under test. Several characteristics of the valve may be traced. simultaneously, for instance as the anode potential is slowly raised anode current control grid current and screen grid current may be simultaneously traced, a shift potential proportional to anode volts being applied to the X-plates of the oscillograph. Brightness pulses are applied to the grid of the oscillograph so that the trace is only visible during the pulse periods or a part thereof. The pulse generator B consists of two - thyratrons V1, V2, Fig. 3, the anodes of which are supplied through a rectifier V3 and inductance L1. The thyratron V1 is tripped to initiate the pulse and V2 is tripped to terminate the pulse. The inductance L1 is large enough to prevent appreciable current change during the period of the pulse. A condenser C4 has a capacity large enough to sustain the initial potential when the largest grid current is supplied to the valve under test. A resistance R7 is large enough not to cause appreciable discharge of the condenser C4 when V1 is tripped. Extinguishing of the thyratrons V1, V2 is effected by a highly damped oscillatory circuit L2, R8. L1, C4 forms an oscillatory circuit having a periodic time less than the period between the pulses. The impulses for tripping these thyratrons are supplied from a trip generator unit comprising a phasing circuit G and' trip circuits T1, T2, T3 which respectively supply tripping pulses to the initiating and terminating thyratrons of the pulse generator B and a brightening pulse to the grid of the oscillograph CRT. The phasing circuit G enables the phase of the main grid pulse to be shifted with respect to the supply voltage thereby causing the pulse to occur at various points in the cycle of the voltage supply. By this means the characteristics may be obtained with the valve operative at any point in the filament heating cycle and the magnetron effect of the filament heating current in large valves may be investigated. A square wave input is applied from the circuit G via the line 7 to the time base circuit K, in which it is differentiated to produce positive going pips B, Fig. 11, which are half a cycle out of phase with the main pulse. These are applied as tripping pulses to a small thyratron, the constants of the circuit being chosen so that the thyratron strikes at every fourth tripping pulse, and a negative-going saw-tooth voltage E is produced which overlaps in time three of the main pulses applied to the valve under test. The saw-tooth voltage is applied to the strobing circuits S1, S2, S3, Fig. 1, which are set to trigger at instants t1, t2, t3 along the saw-tooth. Triggering pulses S1, S2, S3, Fig. 11, are thereby produced and form gating pulses g1, g2, g3 which are applied to the gating circuits G1, G2, G3, Fig. 1. A monitor circuit M comprises a sawtooth generator fed from the brightness pulse generator T3 and strobed at the required time instants to trip a flip-flop which in turn trips a time base circuit applying a saw-tooth voltage to the X-plates of a monitor cathode-ray tube. The main grid pulse is applied to the Y-plates of this tube. The monitor circuit enables the main grid pulse to be accurately adjusted so that measurement is made at instants corresponding to that at which the brightening pulse is applied to the cathode-ray tube.