THE OPERATION OF VACUUM TUBES 



47 



Fig. 15 were taken with the screen voltage and output impedance con- 

 stant at 400 volts and 1500 ohms respectively. It will be noted that 

 these curves show the same general characteristics at the lower por- 

 tions as the curves of the three-element tube, Fig. 13, but as predicted 

 in the theoretical consideration, they tend to saturate more rapidly 



100 200 300 400 500 600 700 

 PEAK EXCITING VOLTAGE 



Fig. 14 — Dynamic output and efficiency of a three-element tube. £& = 3000 volts, 

 Ec= - 300 volts, M = 10. 



in the upper portion. The efficiencies appear to be about the same as 

 obtainable from the three-element tube at the same outputs. 



Fig. 16 shows a comparison of the dynamic output curves at 1500 

 ohms impedance and bias at cut-off of the screen grid tube for three 

 different values of screen voltage, and of the three-element tube. In 

 addition the d-c. screen currents and grid currents are shown. The 



