THEORY OF MULTI-ELECTRODE VACUUM TUBES 



63 



which draws a large space current from the cathode at a control-grid 

 bias sufficiently negative to permit relatively large swings of the 

 control-grid voltage. In pentodes designed to operate at low fre- 

 quencies, screening between the plate and control grid is unimportant; 



CATHODE 

 CONTROL GRID, g, 

 SCREEN GRID,g2 

 SUPPRESSOR GRID,q3 

 PLATE 



Fig. 7 — -Schematic diagram showing the arrangement of electrodes in a power pentode. 



hence, the second grid can be comparatively coarse, thus permitting as 

 large a portion as possible of the space current to pass through it to the 

 plate. 



To permit the largest possible swings in plate voltage, it is necessary 

 to remove the "fold" in the plate-current characteristics, caused by 

 secondary electrons emitted from the plate. This is accomplished by 

 the insertion of a third grid, gs, between the plate and second grid, g2- 

 This grid, known as a suppressor grid, must be maintained at a lower 

 potential than the lowest instantaneous potential reached by the plate, 

 and is usually maintained at the cathode potential by connecting it to 

 the cathode inside the tube. The suppressor grid exerts a retarding 

 force on the primary electrons flowing toward it from the cathode, but, 

 because of its coarse structure, all but a small fraction succeed in 

 passing through it and are accelerated again, finally reaching the plate 

 with the same velocity they would have if the suppressor grid were 

 absent. On the other hand, secondary electrons emitted either b>' the 

 plate or screen grid find themselves in a retarding field, which they are 

 unable to traverse because of their low velocity, and are constrained to 

 return to the electrode from which they came. 



