MODULATION IN VACUUM TUBES 



457 



level is 27.5 T.U. down on the fundamental — the external resistance is 

 twice the internal, the grid potential is — 10, and the improvement in 

 the relative reduction of Ji over customary conditions (for which 

 R = Rq and Ec = — 9) is about 3.5 T.U. A similar survey made 



Z^^KKi? 



o25 



SI II O 



"O 5,000 10,000 15,000 20.000 Z5,0OO 30.000 



0HM5 EXTERNAL RESISTANCE 



Fig. 9 — Ratio of second harmonic to fundamental as a function of load resistance. 

 EL tube No. 109,150. Ep = 120, e = — Ec (variable). Power output constant 

 = .056 watt, 



with regard to the maximum fundamental power obtainable with 

 constant plate potential, and with plate resistances matched, shows 

 it to be had at — 13.5 volts grid potential (Fig. 10), and to represent 

 a gain of about 35 per cent in output power over that obtainable at 

 the customary operating point. Other considerations such as stabil- 

 ity with battery voltage variations operate in repeater practice to fix 

 the grid potential at the customary values. 



There has been considerable discussion recently as to the maximum 

 undistorted power obtainable from a tube with fixed plate potential, 

 and variable load resistance and grid potentials. The above analysis 

 shows that in the strict sense of the word, distortion (Co, Cz) always 

 exists, while with some arbitrary criterion of distortion, results must 

 depend upon the specific criterion adopted. Now as to the maximum 

 fundamental power obtainable, it may be shown that with a parabolic 

 tube characteristic the maximum is obtained for 



R = ^ Ro, 



Ec ^ - 0.58Eb/i. 



