394 



BEIJ. SYSTEM TECHNICA L JOURNA L 



reasonable range provided "c" is not so large as to cause the formation of a 

 virtual cathode in the screen-plate space. 



Figure 6 shows that Bq rises as "a" is reduced. Reducing "a" also has 

 the advantage that a lower screen voltage is required with a given grid bias. 

 The improvement in Bq as "a" is reduced is quite rapid, but of course the 

 mechanical difficulties involved in reducing "a" below about 0.10 mm become 

 very great. 



The curve in Fig. 7 shows that -So does not increase much if "6" is increased 

 above about 0.30 mm, and increasing "i" has the disadvantage that higher 

 screen voltage for a given grid bias is required. 



The curve in Fig. 8 shows that Bq increases very slowly if ''c" is increased 

 beyond about 1.50 mm, and increasing "c" has the disadvantage that the 

 external dimensions of the structure become greater. Increasing "c" also 

 means that the plate comes closer to any shield which is placed around the 



1,00 1.25 1.50 1.75 2.00 



•C" DISTANCE IN MILLIMETERS 



2 25 2.50 



Fig. 8 — Band merit vs. plate-suppressor spacing. 



bulb of the tube, thus increasing the output capacitance and tending to 

 cancel out any improvement obtained in the structure itself. Also, if "c" 

 is made too large a virtual cathode may be formed in the screen-plate space 

 under some conditions. This would interfere with the normal operation of 

 the tube. 



In order to take full advantage of the close grid-cathode spacing, the 

 control-grid pitch should be no greater than about 1.5 times the spacing and 

 the grid wires should be as small as possible. If the pitch is too large, the 

 jjarts of the cathode directly opposite the grid wires will be cut off while 

 space current is flowing from the sections opposite the spaces between grid 

 wires. This state of affairs shows up in the characteristics as excessive 

 variation of the triode amplification factor, iLii2 , as the grid bias is varied, 

 and results in a reduction of the transconductance. That is, when the 

 grid is made more negative the amplification factor [i\i decreases so that the 

 plate current does not decrease as much as it would if \i\i remained constant. 



