I 



Mr Appleton, A method of testing Triode Vacuum Tubes 239 



A method of testing Triode Vacuum Tubes. By E. V. Appleton, 

 M.A., St John's College. 



[Read 22 November 1920.] 



The circuit to be described affords a simple and convenient 

 method of measuring the slope of the principal voltage-current 

 characteristic of a triode vacuum tube. It is now well known that 

 the effectiveness of such a tube as a relay or amplifier depends 

 primarily on the efficiency with which the magnitude of the thermi- 

 onic current can be controlled by means of the grid or intermediary 

 electrode ; that is to say, we are mainly concerned in practice with 

 the rate of variation of anode current with grid voltage, and not 

 with the absolute magnitudes of either quantity. The characteristic 

 mentioned is determined if sufficient correlated values of the anode 

 current / and the grid voltage v are known. To obtain such data 

 accurate ammeters and voltmeters are required. Moreover, even 

 if these values for the characteristic are known, the accurate deter- 

 mination of dl/dv by the ordinary graphical methods takes con- 

 siderable time. As the dimensions of the slope of the characteristic 

 are those of a conductance it seems preferable to dispense with 

 ammeters and voltmeters and determine this quantity directly in 

 terms of a standard resistance. 



A circuit designed to determine the mean value of the mutual 

 conductance (A//Av) over any range of grid voltage was suggested 

 by the writer two years ago*. With this method static currents 

 and voltages were used. In developing the circuit for use since 

 then a great increase in both accuracy and utility has been found 

 to accompany the use of small alternating voltages of acoustic 

 frequency. With the sensitive alternating current detectors now 

 available it is possible to decrease the limits of the variables in- 

 volved so that the value of AI/Av obtained by this method differs 

 inappreciably from the value of dl/dv obtained from a graphical 

 analysis of the complete characteristic. We shall assume that these 

 values are identical in the following discussion. 



The circuit used is shown in Fig. 1. An alternating voltage of 

 acoustic frequency from the alternator A is applied between the 

 grid and the filament of the tube and also across the two non- 

 reactive resistances R and r, the former of which is variable while 

 the latter is small and constant. The anode and grid batteries {B^ 

 and ^2) are suitably fixed to give the operating conditions for 



* See J. A. Fleming, The Thermionic Valve and its Development in Radio- 

 Telegraphy and Telephony, p. 142. 



