1106 THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



sumed current density and geometry. Since these expressions are rather 

 involved, the manner in which the various factors influence the figure of 

 merit can be brought out best by a series of curves. Figures 2,3, and 4 show 

 how F is affected by changes in "a", "6", and "c". Figures 6 and 7 show 

 how the screen voltage required to get the assumed current density with a 

 given bias Ed varies with "a" and "6" (equation 3). The screen voltage is 

 essentially independent of "c". 



These relationships are also applicable to the W.E. 437A tube with 

 minor modifications. 



2.21 Design Considerations 



How the various factors in equations (2), (3), and (4) affect the figure 

 of merit will be discussed in detail. They are listed in Table I. 



The factor M is the ratio of the plate current to the cathode current. 

 The figure of merit is directly proportional to this factor. M can be increased 



Table I 



by using smaller wire in the screen grid, the minimum practical wire size 

 being determined by the mechanical rigidity and heat dissipation capa- 

 bility required. M can also be increased by reducing the number of turns on 

 the screen, but this is limited by the necessity for sufficient shielding effect 

 to meet the requirement that the plate-grid capacitance be less than a speci- 

 fied value. 



Since the figure of merit is directly proportional to the cube root of the 

 cathode current density /o , the improvement with increasing /o is not 

 very rapid. The problems of obtaining uniform initial performance and 

 long life are aggravated by increasing the current density, for several rea- 

 sons. There is no direct evidence to show that high current density per se 

 causes accelerated loss of available emission. In fact there is some evi- 

 dence to the contrary.^ However, there is ample evidence that phenomena 



' "Influence of Density of Emission on the Life of Oxide Cathodes," S. Wagener, 

 Nature, p. 357, Aug. 27, 1949. 



