VACUUM TUBES AS HIGH-FREQUENCY OSCILLATORS 119 



tained constant, the maximum available power output (assuming the 

 same efficiency) will vary as the square of the desired wave-length. 



, Circuit of a Positive Grid Oscillator 



In Fig. 20 is shown a diagram of a positive grid tube of the straight- 

 wire-grid type and its associated circuit. Tuned circuits, in this case 

 in the form of so-called Lecher systems, are connected between the 

 grid and plate leads, extending approximately a half wave-length 

 (30 cm) beyond the lead seals. Because of the existence of preferred 



Fig. 20 — A typical positive grid oscillator circuit. 



frequencies of operation fixed by the potentials applied to the tube 

 electrodes, distributed-constant circuits, if used, may be operated at 

 frequencies corresponding to harmonic modes of oscillation. In this 

 case the length of the leads within the tube envelope has been ad- 

 justed so that the glass seals come at or near potential nodal points 

 for the Lecher systems of which the leads form a part. This minimizes 

 dielectric losses in the glass. The effective paralleling of the two sets 

 of leads greatly reduces the resistance losses, while the balanced 

 arrangement decreases radiation losses. Strict attention to these 

 details is required because of the already low efficiency of the mecha- 

 nism of generation. 



Characteristics of Positive Grid Oscillators 

 The dependence of output and anode efficiency on frequency is 

 shown in Fig. 2L These data were taken by adjusting the circuit 

 tuning, filament current, and the grid and plate potentials to their 

 optimum values for each frequency. The curve showing the grid 

 voltage will be observed to follow equation (1) above, at least roughly, 

 and a similar correspondence will be observed between the curve for 



