1108 



THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



It would appear from Fig. 3 that the grid-screen spacing ''ft" should be 

 as large as possible. However, the required screen voltage increases as "6" 

 increases, and it is desirable to keep the screen voltage low. Therefore, 

 "6" is made as low as possible without causing too much penalty on figure 

 of merit. A good compromise value for "6" depends on the range of grid- 

 cathode spacing ''o" being considered, but it will usually be from 0.005 cm 

 to 0.020 cm for close spaced tubes. 



Figure 4 shows that the figure of merit increases as "c" increases, but 

 there is very little advantage in making it more than 0.040-0.050 cm. 

 Making it much larger also increases the outside dimensions of the struc- 



5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 SOxlO"^ 



GRID -TO- SCREEN SPACING ^'b' IN CENTIMETERS 



Fig. 3 — Figure-of-merit vs. grid-to-screen spacing. 



ture unnecessarily, and eventually leads to a spacing which will cause ir- 

 regularities in the plate current-plate voltage characteristic due to space 

 charge effects in the screen-plate space. 



Although it is not apparent from the curves or from what has been said 

 above, it is desirable to have 'V" as small as possible. It is obvious that 



"r" must be less than — , otherwise the grid is completely closed. Under 



the assumption that na - \, this means that 'V" must be less than 0.5a 

 if there is to be open space between the grid wires. Actually, it is desirable 

 to have not more than 30% of the projected area of the grid closed, which 



