OPERATION OF rur.RMIOXIC VACITM TUBE ClRCllTS^ 445 



justed that the impedance of the combination to the first order fre- 

 quencies is practically that of the condenser alone and may be 

 neglected, and to the desired second order, or detected, frequency it 

 is practically that of the resistance alone. When this is the case, 

 and when the impedance in the plate circuit is a pure resistance, Rp, 

 we may write (36) as follows : 



2a2m1'g'Rp\ 



1 ^^R,[ 



r.ri^ 



''' 2 {rp^Rp)ri{r,-^R,) ''"" 



and, considering n constant, in order to obtain a physical view of 

 the result, we get 



1 T?Vr.> J^P f^>^'^p\l. 



'"'- {rp+Rp)ri{r,+R,) ^^^^ 



This equation shows a condition that is present in many grid-leak 

 detectors and which, it is thought, has not been generally appreciated. 

 The condition referred to is the presence of the term involving the 

 curvature of the plate characteristic in the grid detection component. 

 This efifect is in addition to the plate detection effect, given by (35). 

 The plate detection component and the grid detection component 

 are opposite in phase. Hence, it would seem that for best operation 

 as a grid-leak detector, the curvature of the plate characteristic 

 should be zero. This means a rather large value of Eb, the plate 

 battery potential. In practice, however, it is usual to operate with 

 fairly low values of Eb. The second term of the numerator of (37) 

 accounts for this. It will be seen that detection resulting from this 

 term and from the first term are in phase, since v is intrinsically 

 negative. Hence, it is entirely possible in certain cases for the optimum 

 operating point to be such that the effect of the plate curvature is 

 appreciable. 



We now combine once more the three components, (33 j, (35) and 

 (36), under the simplifying assumptions that m and v are constant 

 and that v is large enough so that terms containing v in the denomin- 

 ator may be neglected. The result is 



I T. 1 n" / 1 \ "T • • • 



+ 



(rg-\-qh) {rp+Zh) {r,j-{-qk) {rp + Zk) 



r - y^rpYp' 



' i.ri,-\-qk){rg-\-qk)L{rp+Zh){rp-{-Zk){rp+Zh 



:i)] 1 <3«) 



{rp-\-z,-k)Ur,+qh){r,+qk){r,+qk-k) A f '^'~'' ^ ' ' ' J 



