. The Bell System technical journal . the control element. The second grid, g2, ismaintained at a fixed positive potential, £,,, and provides the maindriving field for the space current. As in the screen-grid tube, the total space current is determinedalmost wholly by the geometrical dimensions and spacings of thecathode, gi and g2, and by the voltages applied to these , in the design of this portion of the structure, the sameconsiderations apply as in the design of an ordinary triode to deliverlarge power output. By making the inner grid comparatively coarseand by desig
. The Bell System technical journal . the control element. The second grid, g2, ismaintained at a fixed positive potential, £,,, and provides the maindriving field for the space current. As in the screen-grid tube, the total space current is determinedalmost wholly by the geometrical dimensions and spacings of thecathode, gi and g2, and by the voltages applied to these , in the design of this portion of the structure, the sameconsiderations apply as in the design of an ordinary triode to deliverlarge power output. By making the inner grid comparatively coarseand by designing the second grid to operate at comparatively highpotentials, a structure ha\ing a low amplification factor is obtained THEORY OF MULTI-ELECTRODE VACUUM TUBES 63 which draws a large space current from the cathode at a control-gridbias sufficiently negative to permit relatively large swings of thecontrol-grid voltage. In pentodes designed to operate at low fre-quencies, screening between the plate and control grid is unimportant;. CATHODECONTROL GRID, g,SCREEN GRID,g2SUPPRESSOR GRID,q3PLATE Fig. 7—-Schematic diagram showing the arrangement of electrodes in a power pentode. hence, the second grid can be comparatively coarse, thus permitting aslarge a portion as possible of the space current to pass through it to theplate. To permit the largest possible swings in plate voltage, it is necessaryto remove the fold in the plate-current characteristics, caused bysecondary electrons emitted from the plate. This is accomplished bythe 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 lowerpotential than the lowest instantaneous potential reached by the plate,and is usually maintained at the cathode potential by connecting it tothe cathode inside the tube. The suppressor grid exerts a retardingforce on the primary electrons flowing toward it from the cathode, but,because of its coarse structure, all but
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