THE OPERATION OF VACUUM TUBES 51 



Stages from the modulating stage through the final amplifier stage. In 

 certain cases distortion in one stage might be fairly well compensated 

 for by suitably shaping the characteristic of the following stage through 

 choosing proper values of bias voltage and output impedance. 



It may be concluded from both theory and experiment that the 

 screen grid tube functions in general similarly to the three-element 

 tube and is capable of giving about the same output efficiency. It 

 possesses the property of the high ^t tube in giving greater output for 

 a given exciting voltage in the portion of the dynamic characteristic 

 where saturation has not become noticeable, but the dynamic charac- 

 teristic shows saturation much sooner than that of the three-element 

 tube. The screen grid tube also has the property that the output 

 impedance has very little effect on the input circuit which will allow 

 some shaping of the dynamic characteristic without affecting the driv- 

 ing power. 



The fact should be mentioned that the so-called internal impedance, 

 or plate impedance of the tube, such as might be measured on a bridge 

 for small amplitudes of plate current swing, bears no very close rela- 

 tion to the impedance of the output circuit into which it should work 

 in Class B or Class C operation. Reference to this plate impedance 

 is quite misleading unless the part of the characteristic from which it 

 is taken is given, since it varies greatly over the characteristic. For 

 the screen grid tube mentioned the internal plate impedance is of the 

 order of 100,000 ohms, w-hereas it w^as working with an output im- 

 pedance of the order of 1500 ohms. 



Table of Symbols 

 Eh = d-c. plate voltage 

 Ec = d-c. grid voltage 

 Eg = screen grid voltage (held constant) 

 ep = instantaneous plate potential 

 eg = instantaneous grid potential 

 Cpm = minimum plate potential 



e\ = instantaneous value of fundamental frequency component of 



alternating plate voltage 

 Cn = instantaneous value of n^'' harmonic component of alternating 



plate voltage 

 h = d-c. plate current 

 Ip = peak value of plate current 

 ip = instantaneous value of plate current 



