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BELL SYSTEM TECHNICAL JOURNAL 



shown in Fig. 15 in which C\ is the capacity between filament and grid , 

 C 2 capacity between filament and plate, Cz capacity between grid and 

 plate, and r h r s , are leakage resistances. As the action of the tube is 

 such as to produce an equivalent voltage jue between filament and 



Fig. 15 



plate, a generator of voltage fj.e is shown in series with the internal 

 resistance r P of the tube. Calling Y g the input admittance of the tube, 

 that is, 



_I_ Z 

 Y . « 



ii + iz 



in which e g is the alternating voltage between filament and grid, the 

 solution of the above circuit gives for Y g the value, 



! (~+JC 3 p)[jC 2 pr p Z 2 + r, + Z«(|i + 1)] 



(jC*prpZt+rp+Z») + (— +JCap) r p Z 2 



(5) 



Case 1, Low Frequencies. For low frequencies the admittance of 

 the condenser Ci is negligible compared with that of r p . Dropping 

 the terms containing C 2 gives the equation, 



r,+Z,(M+l) 



F,= 



I +| - Crf+ (i+ iC| ^ 



(rp-\-Z 2 )+r p Z i Q-+jCzP^ 



which yields important interpretations. In case the load impedance 

 Z 2 is a pure resistance r 2 , the admittance of the filament-grid branch 

 of the tube may be much greater than the admittance which would 

 result from Ri and C\ alone. This is due to the influence which the 

 alternating component of the plate voltage exerts upon the input 

 circuit through the condenser C 2 . Figs. 16 and 17 show respectively 

 the effective capacity and effective conductance between filament and 

 grid as a function of the external resistance. For the particular tube 

 studied (W. E. Co. 102-A) Fig. 17 shows that, if r 2 = 40,000 ohms, the 

 effective capacity between filament and grid is not the capacity C\ 



