Theory of Multi-Electrode Vacuum Tubes * 



By H. A. PIDGEON 



Physical principles underlying the characteristics and performance of 

 multi-electrode vacuum tubes are presented in simple form in this paper. 

 Presentation of the subject is based, as far as possible, on the well-known 

 theory of the three-electrode tube. It is shown that the definitions of elec- 

 trical tube parameters applicable to triodes are, with certain modifications 

 in their interpretation, also applicable to tubes having more than three 

 electrodes. 



The characteristics of the screen-grid tetrode are discussed in detail since 

 they are typical of those found in a number of multi-electrpde structures. 

 Except for the effects produced by the emission of secondary electrons from 

 the plate and screen, it is shown that the characteristics of the screen-grid 

 tetrode are in general accord with those to be expected from the application 

 of simple theory. The presence of the electrostatic screen in such structures 

 inherently results in high values of the plate resistance and amplification fac- 

 tor, but the transconductance remains normal and has about the same value 

 as in comparable triodes. 



One of the necessary modifications in the screen-grid tetrode to produce 

 a satisfactory output power tube is some means of removing the fold in the 

 plate current-plate voltage characteristics, which limits the permissible 

 plate voltage swings. This is accomplished in the power pentode by the 

 addition of a suppressor grid between the plate and screen grid. The 

 efficiency of power pentodes, and of some other tubes having positive grids, 

 is higher than that usually found in triodes. The reason for this is discussed 

 and also certain peculiarities in the harmonic output of pentodes. 



The arrangement of electrodes in the space-charge-grid pentode corre- 

 sponds to that of the screen-grid tetrode with an additional grid inserted 

 between the cathode and control grid. This space-charge grid, which is main- 

 tained at a positive potential of 10 to 20 volts with respect to the cathode, 

 reduces the effects of space charge near the cathode surface. This results in 

 extraordinarily high values of transconductance and, consequently, in high 

 amplification. Practically, such tubes are limited to u.se as voltage ampli- 

 fiers, since operation over the wide range necessary for large output power re- 

 sults in prohibitive distortion. 



In the co-planar-grid tetrode the lateral wires of the positive grid are ar- 

 ranged in the same planes as those of the control grid. This results in com- 

 paratively low plate resistance while retaining the advantages of a positive 

 grid. The plate efficiency is comparable with that in the power pentode but 

 the available amplification is lower. 



TNTRODUCTION of the three-electrode vacuum tulie into the field 

 -^ of communications and in other applications represented such a 

 tremendous advance over the possibilities of any other known device 

 that, despite some of its rather obvious limitations, it proved entirely 

 adequate for the service required until comparatively recent years. 

 However, with increasing demands made by service requirements for 

 larger power output at higher efficiency, reduced distortion, higher 



* Published in November 1934 issue of Electrical Engineering. Scheduled for 

 presentation at Winter Convention of A. I. E. E., New York City, January 22-25. 

 1935. 



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