ELECTRON TUBES FOR A COAXIAL SYSTEM 



1105 



eral, a given factor of improvement in the figure of merit can be translated 

 directly into a wider transmission band providing more communication 

 channels. 



For a two-terminal type of interstage such as that used in the L3 ampli- 

 fier, the figure of merit is 



F = GB = 



KGm 



(1) 



2t(C, + C2) 



where F is the figure of merit, G is the voltage amplification, B is the band- 

 width between the frequencies where the gain is 3 db below that at the 

 center frequency, K is a, constant whose value depends on the particular 

 interstage design, Gm is the transconductance of the tube, Ci is the input 



Q-*\ 



Tir 





^1 



^. 



b — 



2r 



T 



CONTROL 

 GRID 



1 SCREEN 

 I GRID 



-CATHODE 

 COATING 



Fig. 1 — Geometry of the 436A tube. 



capacitance, and C2 is the output capacitance. This figure of merit is di- 

 rectly applicable to a tetrode operated as a small-signal voltage amplifier 

 and is a well known relationship.^ It will be used to show how the tube 

 design factors influence the figure of merit of the W.E. 435A and W.E. 

 436A tubes. 



Using equation (1) and applying certain simplifying assumptions which 

 can be made without materially affecting the results, expressions are de- 

 rived in the appendix showing the relationship between the figure of merit 

 and the tube parameters. Equations (2) and (4) in the appendix show how 

 the figure of merit is affected by the grid-cathode spacing "a", the grid- 

 screen spacing "b", the screen-plate spacing "c", and the grid wire radius 

 "r". See Fig. 1. Equation 3 gives the required screen voltage for the as- 



^ "Characteristics of Vacuum Tubes for Radar Intermediate Frequency Amplifiers," 

 G. T. Ford, B.S.TJ., Vol. XXV, p. 389, July, 1946. 



