48 



BELL SYSTEM TECHNICAL JOURNAL 



high IX effect of the screen grid tube is responsible for the steeper dy- 

 namic curve. The reason for the greater saturation effect at the upper 

 portion of the curves for the screen grid tube is apparent upon com- 



Z 2.25 



200 300 400 500 



PEAK EXCITING VOLTAGE 



60 z 



Fig. 15 — Dynamic output and efficiency of a screen grid tube. Eb = 3000 volts, 

 £s = 400 volts, Zo = 1500 ohms. 



paring the d-c. screen and grid currents with the grid current for the 

 three-element tube, since the plate is being robbed of the current taken 

 by grid and screen in one case, and only of the current taken by the 

 grid in the other. In Fig. 17, the effect of the output impedance on 



