454 



BELL SYSTEM TECHNICAL JOURNAL 



tion vanishes when 





(9) 



which is a valuable property of amplifier tubes when the equality 

 can be secured. 



A more general relation for which the second order vanishes occurs 

 when we set Eq. (7) equal to zero. As a matter of experience these 

 conditions are not satisfied with the usual type of tube; they are 

 found to hold in tubes of rather special construction. The null 

 points are, of course, independent of the character of the applied grid 

 potential, provided that the restrictions on the original development 

 for the tube characteristic are not exceeded and that contributions of 

 higher to lower order terms are negligible. 



Ml 



4E^ 



100 



* 



.003 



-18 -Ife -14 -12 -10 -8 -6 -4 -I 

 V0LT5 GRID POTENTIAL 



1(0 .08 .008 



WO .07 .007 



IZO .06 .006 



100 j05 .009 



80 .0+ .00+ 



«) .03 .003 



40 .02 .002 



ZO .01 Opl 



,0 



Fig. 5 — Variation of tube parameters with grid potential. EL tube No. 109,150. 



Ep = 120 



The expression for the third order coefficient contains six terms in 

 the numerator, three of which are of opposite sign. If we consider 

 the contribution of each of these terms as a function of the external 

 plate resistance, we find that at very low resistances the single term 

 boz is predominant, while for resistances comparable to that of the 

 internal plate resistance of the tube itself, no one of the six terms, of 

 which three are negative and three are positive, may be neglected. 



