Patent GB-490710-A (GB)
Title: Improvements in and relating to electric testing and measuring apparatus
IPC: G01R31/25
Filed: 1937-07-29
Abstract: 490,710. Measuring mutual conductance ; detecting faults. WALKER, H. S., and MAYO, C. G. July 29, 1937, No. 21058. [Class 37] In an apparatus for measuring the mutual conductance of a thermionic valve by applying an A.C. voltage to the grid and balancing the amplified A.C. voltage in the anode circuit by applying an adjustable A.C. voltage in phase opposition, phase-correcting means are provided for eliminating the effect of the regulation impedance of the D.C. source of anode current in preventing a null reading being obtained when a condition of equality obtains. The valve V under test has its anode supplied with direct current from a rectifier 7, the cathode being heated by current from the secondary winding 10 of a transformer 11. A transformer 1 has a secondary winding 2. connected between the grid and cathode of the valve V, and also a secondary winding 3 by means of which a voltage of opposite phase to that supplied to the grid is fed into the anode circuit. A third secondary winding 4 feeds the lamp 5 of a vibration type mirror galvanometer 6. The A.C. balancing voltage injected into the anode circuit by the secondary winding 3 is variable by means of a potentialdividing resistance 21, the slider on which is connected to the anode of the valve V through two equal resistances 22, 23 in series. The resistance 21 is provided with a scale by which the mutual conductance can be read directly. The vibrating coil 27 of the galvanometer 6 is connected through a condenser 26 and switch 25 to the anode of the valve V, the switch 25 in its left-hand position shortcircuiting the resistance 23, thereby halving the sensitivity of the apparatus. In order to compensate for the effect of the phase shift introduced by the regulation impedance of the D.C. source of anode current, an acceptor circuit comprising a choke coil 29 and -condenser 30 in series is connected as shown and tuned to the operating frequency, the effective regulation impedance of the anode current source at the operating frequency being thereby substantially reduced to zero, enabling a null galvanometer reading to be obtained. Other means of compensating for the undesired phase shift may be employed, for example further impedance elements may be arranged to form a network which presents to the valve under test a resultant impedance having a phase characteristic such that the desired condition of balance can be achieved. The anode current is read on a milliammeter 20, which may be shunted by a resistance 31 to provide two current ranges. A potential divider comprising resistances 33, 34 provides a screen grid voltage for testing valves having a screen grid. A voltmeter 39 is connected to a two-way switch 37 so as to read the .anode and screen grid voltages. Tapping switches 43, 14 and a rheostat 13 enable the heater current to be adjusted to a desired value, which is measured by means of an ammeter 45 fed by a current transformer 44, a switch 42 giving two ranges of measurement. A voltmeter 46 is connected across. the heater of the valve and is shunted by a potential divider resistance 47, the slider of which is connected to the movable contact of a switch 48, linked to two other switches 40, 41. When these switches are moved to their right-hand position, there is no connection between the voltmeter 39 and the cathode of the valve V and no reading is obtained if the cathode is properly insulated from the heater. On account of the presence of the acceptor circuit 29, 30 the upper end of the galvanometer coil 27 is at the same A.C. potential as the cathode of the valve V, the A.C. potential of the lower end being that of the anode. Consequently when a null reading is obtained, the anode of the valve is at the same A.C. potential as the cathode. A number of valve holders may be provided to take various kinds of valves, and may be connected to selector switches to enable the correct potentials to be applied. In a modified arrangement the apparatus is arranged as an A.C. bridge circuit, the indicating galvanometer being arranged to give a null reading when the bridge is balanced.