ELECTRON TUBES FOR A COAXIAL SYSTEM 



nil 



the desired transmission band, the gain and phase characteristics must be 

 controlled up to about 200 MC. The amount of phase shift at the frequency 

 where the gain becomes unity ("cross-over point") is of particular interest. 

 In the L3 amplifier this frequency is about 40 MC. Phase shift introduced 

 by electron transit time and by lead inductance is referred to as "excess 

 phase." Ideally, of course, the excess phase would be zero. 



The time required for an electron to travel from the cathode to the plate 

 is of the order of 10~^^ seconds. This corresponds to about 5° of excess phase 

 at 40 MC. Close spacings and high electrode potentials tend to reduce the 



260 

 240 

 220 

 200 

 180 



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r 



: 140 



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20 



5 20 25 30 35 40 45 50 55 60 65 

 GRID-TO-SCREEN SPACING "b" IN CENTIMETERS 



Fig. 7 — Screen voltage vs. grid-to-screen spacing. 



70 



transit time. However, the considerations discussed in Section 2.2 have 

 been the major factors in setting the spacings and potentials because the 

 transit time, though important, is far less so than the figure of merit. 



By using relatively heavy lead wires and mounting the tube structure in 

 such a way as to make the lead wires as short as possible, the additional 

 excess phase due to the lead wires has been minimized so that it amounts 

 to about 5° at 40 MC. 



In order to insure adequate margin against a singing condition, the 

 amplifier has been designed to have about 20°-30° less phase shift at 40 MC 



