THE OPERATION OF VACUUM TUBES 



43 



500 volts and by the same procedure we find K = 0.441 instead of 

 K = 0.45. These errors are quite small and do not alter the result 

 appreciably in these cases. 



The effect will be less pronounced, the higher the /x of the tube, and 

 more pronounced the further the bias voltage is moved from the cut- 



2.75 



2.50 

 in 



iU 



cc 



S: 2.25 



5 



< 



Z 2.00 



I- 



z 



300 400 500 



PEAK EXCITING VOLTAGE 



Fig. 12 — Calculated and experimental dynamic characteristics of Western Electric 

 No. 251-^ TubeEi = 3000 volts, Zo = 2000 ohms, ix = 10. 



off point. For extreme cases in which more accuracy is desired, K 

 will be determined quite accurately by the second approximation 

 method of Fig. 11. For the case of bias above cut-off where K is 

 always greater than 0.465 the error in determining K will be consider- 

 ably smaller as K does not vary greatly with the period of flow in this 

 region; see Fig. 7. In the upper portion of the characteristic where 

 departure from the three halves power law becomes appreciable, the 

 increase in K due to this saturation eff"ect which must be estimated. 



