VACUUM TUBES AS HIGH-FREQUENCY OSCILLATORS 121 



the grid current and equation (2). Some variation in the required' 

 negative plate potential is observed. A maximum efficiency will be 

 noted at a frequency of approximately 530 megacycles. The output, 

 however, continues to increase with increasing frequency, the limit in 

 output as well as in frequency being set by the safe grid dissipation. 

 The outputs over the 500- to 600-megacycle range vary from 4.5 to 8 

 watts, comparing with outputs from 6 to 3 watts for the negative grid 

 tube. The low efficiencies of 5 to 6 per cent are to be compared with 

 the somewhat higher efficiencies of 19 to 11 per cent for the negative 

 grid type tube. 



The influence of the grid voltage on the frequency and on the output 

 and efficiency is shown by the curves in Fig. 22. These data are for a 



= in 



>-cc 535 



z°- 

 ^^ 



So 525 



5 ^ 



300 320 340 360 380 400 420 440 460 480 500 



GRID VOLTAGE 



Fig. 22 — Result of variation of grid voltage, tuning fixed, on tube of Fig. 19. 



fixed circuit tuning, the grid voltage being adjusted to the values 

 indicated. This corresponds to the condition that might obtain if a 

 grid voltage modulation scheme were to be used. The lack of linearity 

 of the output curve and the large shift in frequency indicate that 

 amplitude modulation by this method would be unsatisfactory. The 



