VACUUM TUBES AS HIGH-FREQUENCY OSCILLATORS 101 



the fixed ratio of the grid-filament to plate-filament inter-electrode 

 capacitance. Most tubes made especially for ultra-high-frequency 

 use are constructed so as to minimize these circuit limitations by a 

 reduction in the inter-electrode capacitance and lead inductance and 

 by adjusting the capacitance ratio. 



Effect of Transit Time on Performance 

 The second fundamental effect has to do with the time required for 

 the electrons to travel from the cathode to the anode within the tube 

 structure. This time, the so-called transit time, is very small in 

 present-day commercial types of power tubes, usually much less than 

 one micro-second. Obviously at low frequencies it can be neglected 

 and, in fact, for many tubes it still plays a minor role either in de- 

 termining the output and efficiency in the high-frequency range or in 

 establishing the limiting frequency for oscillations. When the fre- 

 quency range of oscillation of a tube is extended by an adequate de- 

 crease in energy losses and by improvements in electrical design, 

 transit time becomes a dominating factor in the reduction of output 

 power and efficiency and in establishing the limiting frequency of 

 oscillation. 



This comes about in two ways. In the first place, the relative phase 

 of the alternating grid and plate potentials for best operation must be 

 altered to compensate for the time required for the electrons to travel 

 from the region in which the grid has its greatest effect upon their 

 motion to the region in which their motion has the greatest effect upon 

 the plate current. The available control over these phases is usually 

 insufficient to permit a realization of the optimum adjustment. In 

 terms of the measured characteristics of the tube, the transconductance 

 has become complex. But even with the optimum phase adjustment 

 the efficiency is reduced by losses which occur because of the variations 

 in grid and plate potentials during the transit time. Electrons 

 arriving at the plate will in general have velocities greater than the 

 velocity corresponding to the potential of the anode at the instant of 

 their arrival. The excess energy corresponding to the greater velocity 

 is obtained from the oscillating circuit and is dissipated at the plate in 

 the form of heat. Again in terms of the measured characteristics, 

 the input conductance has been increased above its low-frequency 

 value. 



The mechanism which enables electrons to take energy from the 

 oscillatory circuit in their passage across the tube is evident from a 

 consideration of a somewhat simplified case as shown in Fig. 4. 

 Assume that the anode is held at a constant positive potential of 100 



