Patent GB-480762-A (GB)
Title: Improvements in or relating to beer chilling or like plant
IPC: F25D31/00
Filed: 1936-08-31
Abstract: 480,762. Resistance measurements; electric tests. WILKINS, S. R., and AUTOMATIC COIL WINDER & ELECTRICAL EQUIPMENT CO., Ltd. Aug. 26, 1936, No. 23464. [Class 37] The mutual conductance of a thermionic valve is determined by applying an alternating voltage equal to 1À4 times the rated direct voltage to the anode, applying the requisite volts to the cathode or heater therefor and measuring the change in anode current after equal and opposite potentials, differing by an amount equal to twice the difference of the normally accepted direct voltages, have been successively applied to the grid. Thus the grid could be supplied successively with -1 volt and then + 1 volt through a rectifier fed with A.C. Preferably, the grid is supplied with an alternating potential of one volt in phase opposition to the anode voltage, the anode current is then balanced out of the meter in circuit therewith by a current obtained from rectified but unsmoothed A.C., after which the grid potential is changed to one volt in phase with the anode voltage, the meter then directly indicating the mutual conductance as a proportion of the rated figure. The apparatus, which may also be used for detecting interelectrode shorts, and testing filament continuity and cathode to heater insulation, preferably comprises a panel (not shown) on which sockets corresponding to various valves are mounted and connected by a multicore cable to the output terminals Nx, D2, D1, A2, L, Gd, LT-, LT+, Dx, of the measuring panel carrying rotary switches H, SC, AN, AS. The fixed contacts of the switches H, SC, and AN are connected to various tappings on the secondaries of transformers M or L fed from 50 cycle mains whereby the correct heater, screen, and anode volts, respectively, may be applied through terminals LT + and LT- for the heater, Nx for the screen, and one of terminals D2, D1, A2, L, for the anode of the valve under test. The wiper of the anode voltage switch AN is connected in series with a D.C. milliammeter ME, and the wiper of switch AS by which the appropriate anode of a valve having more than one electrode assembly is energized. MA/V is a variable shunt resistance for altering the range of the meter. In testing a valve for mutual conductance, with the appropriate voltages applied to the heater and anode a two-way switch TWM, having contact sets A, A1, B, C, D is operated to the central position shown, when the valve grid is fed over terminal Gd, and contacts A, A1, with one volt in phase opposition to the anode voltage from one side of a centre tapped secondary Ls3 of transformer L. A " set zero " control V, comprising a potentiometer resistance V1 fed from a secondary Ls4 in combination with a variable resistance V2 actuated by a common rotary element V3, is then adjusted until the current passing therefrom through rectifier R " backs off " or balances out the meter current. The MA/V control is then adjusted to the range which should subsequently give a full scale deflection for the valve under test; if " good, " after which the switch TWM is thrown to the position MC, when one volt in phase with the anode voltage is applied to the grid terminal Gd over contacts A1 from the other half of the transformer secondary Ls3. The consequent change in anode current indicated by the meter is therefore the mutual conductance which is indicated in milliamps/volts and also as a proportion of the full scale deflection which corresponds to the rated mutual conductance figure. In addition a scale divided into three coloured zones marked " good, " "indifferent." and " bad " or " replace " is provided. The resistances V1, V2 and part of the device MA/V are wound on wedge shaped formers such that the resistances do not increase uniformly with the angular rotation of the operating arms V3, Mx, and a more uniform operation of the backing off and meter range controls is obtained. The voltage contacts of switch AN are connected to tappings on the transformers equal to 1À4 times the markings on the switch in accordance with the method of testing described. Two anode contacts thereon D12, REC, are used for emission tests with diodes and mains rectifiers, respectively, the meter measuring the rectified current in each case passing through load resistances. To measure the cathode to heater insulation resistance of a valve the switch TWM is pressed in the direction INS whereby the grid potential is changed to one volt in phase with that of the anode which is connected over a condenser K to the 75 volt tapping on transformer M, the connection between the cathode terminal DX and heater LT- is broken, the backing off and meter shunt MA/V control disconnected,' and the meter is connected through a 100,000 ohm resistance DO between anode and heater of the valve, the reading thereon being therefore inversely proportional to the insulation resistance. A neon lamp N in series with a resistance is normally lit over a switch NA from the supply to indicate that the apparatus is energized. Plugs may be inserted in sockets S to open the switch NA when test prods connected to the sockets may be used with the lamp N to test filament continuity or interelectrode shorts, the lamp glowing according to the leakage current between the electrodes under test.