THEORY OF MULTI-ELECTRODE VACUUM TUBES 



75 



very similar to that exhibited by triodes of lov\^ cathode emission. In 

 fact, this phenomenon is caused by the partial exhaustion of the space 

 charge in the region of the virtual cathode. This is a fundamental 

 characteristic of space-charge-grid tubes, caused largely by the 



35 



10 



?; 30 



25 



10 



•^ 3 



Z I 



o 



•4 

 _3__ 



c <f) 



•-; y 30 



2 20 



-10 



-7 -6 -5 -4 -3 



CONTROL GRID VOLTS, Eq 



Fig. 15 — Characteristics of a space-charge-grid pentode. Plate current, screen 

 current, and net current as functions of the control-grid voltage. Eg, with the indicated 

 values of screen voltage as parameters. Plate voltage, Ep = 180 volts. Net 

 voltage, En — 15 volts. 



imperfect character of the virtual cathode, which results from electrons 

 entering the space-charge region with widely varying normal com- 

 ponents of velocity. It constitutes a serious limitation on the 

 practicable range of operation of such tubes, since it prevents the plate 

 potential from swinging over a sufficiently large range to obtain a large 

 power output without resulting in prohibitive distortion. 



