THEORY OF MULTI-ELECTRODE VACUUM TUBES 



65 



teristlcs of Fig. 9. These curves were obtained from a tube of the 

 same type as that for which the characteristics are shown in Fig. 8. 

 One set of curves was obtained with the suppressor grid operating in 

 the normal manner. The other curves were obtained with the 

 suppressor grid tied to the screen grid and maintained at a positive 



300 

 PLATE VOLTS, E( 



Fig. 9 — Curves showing the effect of the suppressor grid in a power pentode. 



potential of 250 volts. In this latter case, the number of secondary 

 electrons escaping from the plate is practically the same as if the 

 suppressor grid were removed from the tube. The presence of the 

 suppressor grid not only permits the plate to swing to very much lower 

 potentials than otherwise w^ould be possible, but also permits the plate 

 and screen to operate at the same potential, which is 250 volts in this 

 case. 



The characteristics of Fig. 8 closely approach the form that would be 

 expected from simple theory. As is usual with power pentodes, the 

 plate-current curves are not quite as fiat as those for screen-grid tubes. 

 This is because of the more open character of the grids which permits 

 the plate to have a slightly greater effect on the magnitude of the space 

 current. This is evidenced also by the tendency of the curves to turn 

 up at the higher plate voltages. 



The normal operating point for this tube is at point P in Fig. 8, at 

 which the plate and screen potentials are both 250 volts and the control 



