454 BELL SYSTEM TECHNICAL JOURNAL 



Fig. 6 illustrates the agreement between this relation and an actual 

 circuit where the above conditions were closely approximated. 



Plate Curvature Detection 



In discussing this phase of the problem we refer to equation (35). 

 In addition to the remarks made in connection with that equation it 

 is necessary only to add a few words on the evaluation of rp and rp. 

 In general, these quantities are susceptible to the same method of 

 treatment that was suggested in dealing with rg and rg. Two funda- 

 mental circuits for plate curvature detectors are in use. In the first 

 the plate battery is placed in series with the load impedance. In the 

 case when the load impedance contains appreciable resistance the 

 normal or effective value of Ep must be obtained in the manner 

 described for finding Eg. In the second circuit the plate battery 

 potential is introduced through a low resistance, high impedance, 

 choke, and the normal value of Ep is then equal to Eb. Especially 

 in dealing with resistance coupled units these points should be borne 

 in mind. 



Amplification 



Equation (33) gives the general amplification relation. The remarks 

 made under the heading of the "Grid-Leak Detector" concerning the 

 evaluation of the 2's and g's are applicable here, as in all other vacuum 

 tube relations. The special points to be brought out are the methods 

 of applying the equations to so-called improper amplifiers of Class III. 

 In this type of amplifier the grid swings negative further than the 

 plate current cut-off point each cycle. Experience has shown that 

 even in this event, to find the tube resistances, the approximation of 

 using the secant line joining two points on the characteristic corre- 

 sponding to the extreme values of the input voltage, is often justifiable. 

 If greater accuracy is desired, the corrections given by the curve, 

 Fig. 7, should be applied. These corrections are based on the as- 

 sumption of a sine wave input and a characteristic that follows the 

 square law, and to that extent are themselves in error. For modu- 

 lated waves the dotted curves give values found by interpolation 

 between the two points shown. 



Modulation 



The detection equations apply equally well to modulation effects. 

 The only case in which a question may arise is that in which one of 

 the input frequencies is introduced into the plate circuit of the tube 



