NEGATIVE-GRID VACUUM TUBES 



579 



Fig. 7 — General equivalent of vacuum tubes valid for moderately high frequencies. 



This is nearly identical with the well-known equivalent which was 

 shown in Fig. 2, and the modification consists only of the addition of 

 series resistors in the internal cathode-grid and cathode-plate paths. 

 A phase angle in the amplification factor is avoided by the resistances 

 in series with the capacitances.^ The impedance in series with the 

 /z-generator is the well-known internal plate resistance as given by the 

 slope of the static characteristic of the tube. The capacitances are 

 likewise those we have used all along at lower frequencies, but the 

 mathematics now enables their dielectric constants to be computed. 

 Fig. 7 will be found to be valid at any of the frequencies for which 

 negative-grid tubes are now contemplated for commercial application, 

 including those where the transit angle is about half a radian. 



Much has been said and written of late years about the active grid 

 loss.^' ^' *• ^ In Fig. 7 this would be determined by placing a large 



2 It can be shown that measurements published in a paper "Phase Angle of 

 Vacuum Tube Transconductance," F. B. Llewellyn, Proc. I. R. E., Vol. 22, August 

 1934, may be interpreted as well in terms of the phase angle of the grid-plate imped- 

 ance. If the phase angle of the latter is a, and the angle measured for the paper is d, 

 then 



, C, sin cos <i> tan 6 

 sm-a = l--^-t- ^^^ 



where C' is the grid-plate capacitance of the cold tube, C of the hot tube, and is 

 the phase angle of the inductive branch of the tuned circuit used in the experiments. 



^ Loc. cit. 



^ J. G. Chaffee, "The Determination of Dielectric Properties at Very High Fre- 

 quencies," Proc. I. R. E., Vol. 22, August 1934. 



^ W. R. Ferris, "Input Resistance of Vacuum Tubes as Ultra-High Frequency 

 Amplifiers," Proc. I. R. E., Vol. 24, January 1936. 



' D. O. North, "Analvsis of the Effects of Space Charge on Grid Impedance," 

 Proc. I. R. E., Vol. 24, January 1936. 



