no 



BELL SYSTEM TECHNICAL JOURNAL 



The inter-electrode capacitances of this tube are: 



Plate to grid 1.8 iifxi 



Grid to filament 10 tifxi 



Plate to filament 0.75 nni 



The output and efficiency curves, shown in Fig. 11, were obtained 

 with two tubes in a push-pull circuit. The efficiency of the tube is 



u 



200 250 300 350 400 450 500 550 600 650 700 760 

 FREQUENCY IN MEGACYCLES PER SECOND 



Fig, 11 — Output and efficiency curves for 2 tubes of the type shown in Fig. 10. 



about 28 per cent at 300 megacycles, contrasting with only 18 per 

 cent for the previously discussed tube, while the output is only 8 watts 

 as compared to 13 watts. The higher efficiency suggests that above 

 300 megacycles two of the smaller tubes are preferable to one of the 

 large tubes. The fact that the limiting frequency varies with the 

 applied anode potential indicates that the transit time effect is largely 

 responsible for the decreased efficiency. This suggests modulation 

 difficulties if the tube is used near the upper limit of the frequency 

 range. The outputs and efficiencies in the 400- to 600-megacycle 

 range, although low, represent a substantial increase over the usual 

 values obtained at these frequencies. 



The relatively low efficiency at 200 megacycles is due to insufficient 

 filament emission. In order to maintain space charge conditions near 

 the filament, at high frequencies, the electron emission must be large 

 enough to supply not only the actual electron current to the plate but 

 also the charging currents to the grid-filament and plate-filament 

 capacitances. In a tube of this small size an increase in the filament 

 emission is possible only by an impractical and unwarranted increase 

 in the filament heating current. 



