72 



BELL SYSTEM TECHNICAL JOURNAL 



Space-Ciiarge-Grid Pentodes 



One limitation in the three-electrode tube and in the multi-electrode 

 tubes considered thus far in this paper, is the resistance offered by space 

 charge to the flow of space current. Tubes having so-called space- 

 charge grids overcome this limitation to some extent by having a 

 positive grid close to the cathode, which partially neutralizes the 

 negative field very near the cathode surface due to space charge. A 

 comparatively large current is drawn from the cathode by the space- 

 charge grid. A portion of this current (usually about half of it) is 

 collected by this grid, while the remaining portion passes through it 

 and is acted on by the remaining elements of the tube. 



The arrangement of the electrodes in a space-charge-grid pentode is 

 shown in Fig. 13. The space-charge grid, gi, is maintained at a 



CATHODE 



SPACE CHARGE GRID.gi 



VIRTUAL CATHODE 



CONTROL GRID,g2 



SCREEN GRlD,g3 



PLATE 



F"ig. 13 — Schematic diagram showing t he arrangement of electrodes in a space-cliarge- 



grid pentode. 



relatively low positive potential with respect to the cathode, usually in 

 the range from 10 to 20 volts. The second grid, g2, is the control grid 

 and is maintained at a negative potential with respect to the cathode. 

 Ideally, at some cylindrical surface between these two grids (as- 

 suming the structure to be cylindrical) the electrons are retarded to 

 nearly zero velocity forming a second space-charge region which may be 

 regarded as a virtual cathode. Since this space-charge sheath is larger 

 in area than the original cathode and is very close to the control grid, it 



