62 BELL SYSTEM TECHNICAL JOURNAL 



from consideration of equation 16 which may be written in the form, 



A, = S,„ ^. (18) 



1 +- 



Since in screen-grid tubes and pentodes, RjRp is small, the voltage 

 amplification is given approximately' by 



A, - S„rR. (19) 



Now, in a comparable triode the transconductance will have a value 

 comparable with 5,„, and it may be assumed that a load resistance could 

 be used comparable with R. However, the ratio RIR,, for the triode is 

 not small and may be greater than unity. Hence, from equation 18, 

 the amplification obtained with the triode is correspondingly smaller. 

 It has been shown that the presence of the electrostatic screen in the 

 screen-grid tetrode results in a high plate resistance and a high amplifi- 

 cation factor, but that the ratio of /x to R,, (transconductance) remains 

 normal. Such tubes yield high amplification and, with suitable 

 associated circuits, are relatively free from feed-back; but they are 

 limited in their range of operation because of the fold in the plate 

 current-plate voltage characteristics resulting from secondary emission 

 from the plate. 



Power Pentodes 



In order to deliver a large power output, a vacuum tube must be 

 capable of large variations in plate current and plate voltage from their 

 normal operating values. Both of these conditions are fulfilled by the 

 power pentode. The arrangement of the electrodes, shown sche- 

 matically in Fig. 7, corresponds to that in a screen-grid tube except that 

 an additional grid, gs, is inserted between the plate and screen grid, go- 

 As in the screen-grid tube, the first grid, gi, has a negative \oltage 

 applied to it and acts as the control element. The second grid, g2, is 

 maintained at a fixed positive potential, £,,, and provides the main 

 driving field for the space current. 



As in the screen-grid tube, the total space current is determined 

 almost wholly by the geometrical dimensions and spacings of the 

 cathode, gi and g2, and by the voltages applied to these electrodes. 

 Consequently, in the design of this portion of the structure, the same 

 considerations apply as in the design of an ordinary triode to deliver 

 large power output. By making the inner grid comparatively coarse 

 and by designing the second grid to operate at comparatively high 

 potentials, a structure ha\'ing a low amplification factor is obtained 



