ELECTRON TUBES FOR A COAXIAL SYSTEM 



1109 



means that 'V" should be less than 0.15a. In addition to this consideration, 

 it is desirable to have "r" considerably less than 0.15a so that the required 

 screen voltage will be as low as possible. This comes about because the 

 amplification factor /x increases as 'V" is increased, other quantities held 

 constant, and equation (3) shows that £^2 increases as n increases. The 

 diagram shown in Fig. 5 illustrates the trend in grid-cathode spacings and 

 grid wire sizes. The W.E. 416A tube (formerly BTL 1553) represents the 



800 



700 

 600 

 500 



U 

 < 



CD 



?8 



400 

 300 

 200 

 100 



20 40 60 80 100 120 140 160 180 200 220 240 260 xlO'^ 

 SCREEN-TO-PLATE SPACING ''c" IN CENTIMETERS 



Fig. 4 — Figure-of-merit vs. screen-to-plate spacing. 



GRID 



wires' 



1 0.001" Dl A. 



CATHODE 

 COATING 



0.0003" DIA. 

 0.0025" 



yO-OOOa^DIA. 



386A, 6AK5 404A,435A, 436A, 437A 



Fig. 5 — Trend in spacing and grid wire size. 



416A 



greatest extension of this trend reported as far as grid-cathode spacing is 

 concerned.^ 



The figure of merit increases as the absolute value of the bias Ed is 

 reduced, since Iq increases. However, a minimum bias value of about —1.5 

 volts is usually necessary in order to avoid undesirable effects due to the 



' "Design Factors of the Bell Telephone Laboratories 1553 Triode," J. A. Morton and 

 R. M. Ryder, B.S.T.J., Oct. 1950. 



