THE OPERATION OF VACUUM TUBES 



37 



ditions it is desirable to know what the shape of the curve of output 

 current versus exciting voltage will be. The following method deter- 

 mines points on the output curve by the use of the static characteristic 

 of the tube. 



In Fig. 6 is shown a family of plate current curves obtained by apply- 

 ing a sine wave grid voltage to a three-halves power characteristic 

 with a plate voltage consisting of a steady voltage plus a sine wave 

 180° out of phase with the grid voltage. The different curves show 

 the relative shapes obtained by variation of the grid bias so that the 

 portion of the cycle during which plate current flows is varied. It can 



0.8 

 0.7 



180 160 140 120 100 80 60 40 20 20 40 60 80 100 120 140 160 180 



DEGREES 



Fig. 6 — Curves of ip obtained from a three halves power characteristic with sinusoidal 

 exciting voltage for varying periods of plate current flow. 



be shown that for any tube, no matter what the actual values of volt- 

 ages and currents are, as long as the portion of the characteristic under 

 consideration obeys the three-halves power law, the plate current waves 

 will correspond to those of Fig. 6 in shape for the same respective 

 periods of plate current flow. By means of harmonic analyses, the 

 value of K corresponding to each of these shapes was calculated and 

 Fig. 7 shows the variation of K with the number of degrees during 

 which plate current flows. Fig. 8 shows a comparison of three possible 

 shapes for 180° flow with the corresponding values of K. A is the 

 unsaturated curve of Fig. 6. B shows a curve for which the charac- 

 teristic departs from the three-halves power law at its upper end, 

 presumably due to the effects of grid current, etc. This curve if un- 

 saturated would have a peak value about 17 per cent higher. C shows 



