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BELL SYSTEM TECHNICAL JOURNAL 



volts, and that the grid is held at 50 volts positive just long enough to 

 allow an electron to come from the cathode to the grid plane (very 

 near one of the wires), where its velocity will correspond to a fall of 50 

 volts. The potential of the grid is then suddenly changed to 50 volts 

 negative. The electron will then fall through an additional potential 

 difference of 150 volts, arriving at the anode with a velocity corre- 



(A) 



(B) 



Fig. 4- 



-lUustrating the mechanism which enables electrons to take energy from the 

 oscillating circuit. 



sponding to 200 volts, producing just twice as much heat as it would 

 have done had the grid potential not been changed during the transit 

 time. This added energy must come from the source which produced 

 the change in the grid potential. In the actual case the change in 

 grid potential is not abrupt but a similar loss occurs. This limits the 

 useful frequency range of a tube to values for which the oscillation 

 period is long compared to the electron transit time. 



Special Designs Required for Different Ranges of High Frequencies 

 Most standard power tubes reach their upper frequency limit of 

 oscillation somewhere in the 10- to 100-megacycle frequency range. 

 For frequencies above this, specially designed tubes are required. 

 The frequency range in which a given design is near the optimum is 

 limited. Therefore, there is a succession of tubes, each rated for a 

 band of frequencies. Characteristics such as a high mutual con- 

 ductance and a sharp cut-off which make a tube a good oscillator at 

 low frequencies, while still of importance at ultra-high frequencies, 

 are apt to be secondary to the special frequency requirements 



