THEORY OF MULTI-ELECTRODE VACUUM TUBES 73 



results in very large values of transconductance. Practically, the 

 ideal condition is not fully realized, largely because velocity components 

 other than radial are imparted to the electrons in passing through the 

 space-charge grid. Consequently, these electrons reach any given 

 cylindrical surface outside the space-charge grid with rather widely 

 varying radial components of velocity. This, as will be seen later, 

 places a rather serious limitation on the performance of such tubes. 



The arrangement and functioning of the other electrodes in Fig. 13, 

 outward from the virtual cathode, correspond with that of the screen- 

 grid tetrode. The screen grid, gz, is maintained at a fixed positive 

 potential necessary to accelerate the electrons from the region of the 

 virtual cathode. The plate must be maintained at a potential higher 

 than that of the screen for the same reason as in the screen-grid tetrode. 

 It will be shown that the characteristics of this pentode correspond 

 roughly with those of the screen-grid tube previously discussed. 



If gz were omitted, the structure outside the virtual cathode would 

 correspond to that of a triode and the characteristics in the resulting 

 tetrode would correspond roughly to those of a triode. In Fig. 14, 

 characteristics are shown for a pentode of this type, the cathode and 

 general dimensions of which are the same as those of the power 

 pentode, the characteristics of which were shown previously. The 

 plate-current and screen-current curves are seen to correspond very 

 closely with those previously shown for a screen-grid tube. The 

 characteristics exhibit the same "folds" due to secondary electrons, 

 although this portion of the characteristics is not shown. The net or 

 space-charge-grid current, /„, increases as the plate current decreases 

 with increasing negative control-grid voltage. This is to be expected 

 since, as the control grid becomes more negative and reduces the 

 current passing through it to the plate, the excess current returns to the 

 net rather than to the cathode as in the screen-grid tube. 



If values of the amplification factor, plate resistance, and trans- 

 conductance are plotted as functions of the plate voltage, families of 

 curves are obtained similar in all respects to those for a screen-grid tube 

 as shown in Figs. 3 and 5. These characteristics are not shown for this 

 tube. 



In Fig. 15, plate-current, screen-current, and net-current charac- 

 teristics are shown as functions of the control-grid voltage with different 

 values of the screen voltage as parameters. As in the case of the screen- 

 grid tube, only a slight displacement of the characteristics results from 

 variation of the plate voltage. It is of interest to note the high values 

 of net current, particularly as the plate current drops toward zero with 

 increasing negative grid bias. For example, if the operating point is 



