84 BELL SYSTEM TECHNICAL JOURNAL 



circuit efficiency of the triode and co-planar-grid tube at varying 

 harmonic levels expressed in dh below the fundamental. Curves are 

 shown for the triode with the load resistance, R, equal to the plate 

 resistance, R,,, and also with R = 2Rp. In both tubes the plate 

 voltage is maintained at 130 volts, and the plate current at 35 milli- 

 amperes. 



With the harmonics 26 db below the fundamental (5 per cent) and 

 with R — 2Rp, the output from the triode is 0.65 watt. At the same 

 harmonic level the output from the co-planar-grid tube is 1.65 watts 

 with a resistance load matching its plate resistance. The efficiencies 

 are 14 per cent and 35 per cent respectively. 



The most significant differences between the co-planar-grid tube and 

 the power pentode are as follows: (1) The former has a much lower 

 plate resistance which simplifies coupling the tube to its output circuit 

 and is particularly advantageous in applications where it is desirable 

 to operate the tube into an impedance matching its plate resistance. 

 (2) The pentode yields higher amplification and, consequently, requires 

 a correspondingly smaller input voltage to drive it. (3) The plate- 

 circuit efficiency is not greatly different except at low plate voltages 

 where the co-planar-grid tube is somewhat more efficient. 



It should be borne in mind in considering the efficiency of output 

 power tubes used at audio frequencies, that the usual criterion of 

 available power is the harmonic content. As has been shown, funda- 

 mental physical factors tend to make the harmonics, particularly the 

 odd harmonics, relatively high in pentodes and co-planar-grid tubes. 

 On the other hand, the recent trend in the design of triodes has been 

 toward tubes of very low plate resistance, which operate into com- 

 paratively large load resistances. This tends to decrease the harmonics 

 on the one hand and increase the efficiency on the other. The net 

 result has been that triodes of more recent design, particularly those 

 operating at 250 volts or higher, are not far below the pentode or 

 co-planar-grid tube in efficiency. 



