98 



BELL SYSTEM TECHNICAL JOURNAL 



upon the design of the tube but it will usually be in the 10- to 60- 

 megacycle range. By successive modifications of the circuit arrange- 

 ment and size this decrease in power output and efficiency can be 

 minimized. With optimum circuit arrangements, however, this de- 

 crease continues until finally a frequency is reached beyond which 

 oscillations can no longer be produced. 



In Fig. 1 are shown typical data for a standard 75-watt tube, the 

 Western Electric type 242^, operated with reduced potentials over 



40 50 60 70 80 



FREQUENCY IN MEGACYCLES PER SECOND 



100 



Pig 1 — Power output and anode efficiency as a function of frequency for a 

 standard triode. These curves are typical of all tubes as they approach their upper 

 limiting frequency. 



the frequency range from the point where oscillation frequency 

 noticeably affects performance to the point where oscillations can no 

 longer be produced. The plate potential was held constant at 750 

 volts throughout the entire frequency range. The oscillation circuit 

 was modified at each point in order to obtain maximum output and 

 efficiency, keeping the anode dissipation within the maximum rating 

 of 100 watts and the anode current within the maximum rating of 

 0.150 amp. It can be seen from the curves that the output and 

 efficiency are independent of the frequency until about 20 megacycles 

 is reached, when they begin to decrease. At 100 megacycles the output 

 power is only 2.5 watts and the efficiency only 2 per cent. The tube 

 will not oscillate at 105 megacycles. 



