THEORY OF MULTI-ELECTRODE VACUUM TUBES 



71 



force for the space current, thus relieving the plate from performing this 

 function. Consequently, the plate is free to swing to lower voltages 

 than otherwise would be possible, which results in a corresponding 

 increase in efficiency. 



In Table I, similar data are shown for typical pentodes and triodes. 



TABLE I 



Comparative Data for Typical Triodes and Pentodes 



Plate voltage, Ep 



Screen voltage, Eg 



Grid voltage. Eg 



Plate current, Ip, ma 



Screen current, /», ma 



Transconductance, Sm, mi- 



cromhos 



Amplification factor, tx 



Plate resistance, Rp, ohms . 

 Load resistance, R, ohms . . 



Input, peak volts 



Power output, watts 



Second harmonic, % 



Third harmonic, % 



Total harmonic, effective % 



Plate efficiency, % 



Plate-grid capacitance, ix^i . 

 Plate-ground capacitance, 



wf 



Grid-ground capacitance, 



MM^ 



250 

 250 



-15 

 42 



5 



3,000 . 



156 

 > 2,000 

 6,000 

 15 

 3.8 

 2.8 

 5.0 

 5.7 

 36 

 0.2 



15 



9 



While the triodes and pentodes are not directly comparable with each 

 other, the data are indicative of the dififerences between the two types 

 of tubes. The chief differences between pentodes and triodes may be 

 summarized, as follows: (1) Pentodes yield higher gain and require 

 correspondingly lower input voltages to drive them. (2) Pentodes 

 have much higher amplification factors and plate resistances than 

 triodes. The latter constitutes a handicap in coupling the tube to its 

 circuit, particularly if transformer coupling is employed. (3) Pentodes 

 yield high power output, generally at higher plate efficiency than 

 triodes. (4) The harmonic content in the output of pentodes is high,- 

 the third harmonic being particularly high compared with its level in 

 triodes. This requires that pentodes work into load impedances that 

 are very low compared with the plate resistance. Ratios of R to Rp 

 of 1/3 to 1/10 are common. (5) The plate-grid capacitance is much 

 lower and the plate-ground capacitance is somewhat higher in pentodes 

 than in comparable triodes. 



