THE OPERATION OF VACUUM TUBES 45 



where m is as defined above, and p is the ampHfication factor of a three- 

 element tube considering the screen as the grid. However, if p is large 

 compared to /x and /x is greater than 1, the cut-off point is given quite 

 closely by (14). 



In operation the screen is by-passed to ground by a large capacity 

 so that the screen potential will remain practically constant at E^ to 

 insure the effectiveness of the screening action. As the minimum plate 

 potential approaches the screen potential, the screen will begin to draw 

 appreciable current and thus cause a saturation effect in the charac- 

 teristic similar to that of the three-element tube when the minimum 

 plate potential approaches the maximum grid potential. Also, as the 

 maximum grid potential approaches the screen potential, the grid will 

 begin to draw current which will subtract from the plate and screen 

 currents, thus tending also to cause a saturation effect in the charac- 

 teristic. It would seem desirable, therefore, that the screen voltage 

 be chosen somewhere between the maximum grid potential and the 

 minimum plate potential, the actual point depending on whether grid 

 current or screen current is the least desirable. In operation the 

 screen grid tube should give the same type of characteristics as indi- 

 cated in Fig. 12 and have some advantage from the fact that as a 

 high fj. tube it requires less driving voltage. However, the dynamic 

 characteristic may tend to saturate sooner than it would for the equiv- 

 alent three-element tube because of the tendency of both the screen 

 and the grid to absorb current under the conditions mentioned above. 



Experimental Results 



In Fig. 12 along with the curves calculated from the theoretical 

 development are shown the actual experimental curves obtained at 

 3000 kc for the same conditions. 



The following dynamic output current and efficiency curves of the 

 three-element tube. Western Electric 251-A, and the screen grid tube. 

 Western Electric 278-A, measured at 3000 kc are submitted to show 

 the effects of various factors on the dynamic characteristic. They 

 allow a direct comparison of the three-element tube and the screen 

 grid tube. 



The curves of Fig. 13 show the dynamic output currents and efifi- 

 ciencies obtained from the three element tube for the conditions indi- 

 cated. The upper portions of these curves show some saturation effect 

 and they are practically identical except that they are displaced along 

 the abscissa by approximately the differences in grid bias. Fig. 14 

 shows the effect of varying the output impedance for the case of bias 



