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BELL SYSTEM TECHNICAL JOURNAL 



The cut-off point, or the potential which the grid must have to make 

 the plate current just zero is given by 



e„ cut-off - - — ' . (13) 



M 



If we bias the grid with this negative voltage, it may be said to be 

 biased at cut-off, and if the grid voltage becomes more positive than 

 this value, plate current will flow. Thus if we apply a sinusoidal ex- 

 citing voltage, the plate current will always flow during 180°, or half 

 the cycle, and for this condition K = 0.465 from Fig. 7, so the dynamic 

 output current curve will be obtained by multiplying the ordinates of 

 the K = 0.465 curve of Fig. 9, by 0.465. However, in the upper 

 portion where the static characteristics depart appreciably from the 

 three-halves power law, K will increase gradually (Fig. 8), so that it is 

 more accurate to use slightly increasing values of K in this portion. 

 (See table of calculations for Ec = 300, Fig. 10.) 



Eb = 3000 Volts Zo = 2000 ohms 



Fig. 10 — Calculation of dynamic output characteristics — No. 251-.\ tube. 



If the grid bias is more negative than the cut-oft" point (i.e., if the 

 tube is biased "below cut-off"), the plate current will not begin to 

 flow until the grid potential has reached the cut-off point (13), and 



