70 BELL SYSTEM TECHNICAL JOURNAL 



increases progressively with load resistance. It readily is shown that 

 such dynamic characteristics, having points of inflection at which the 

 curvature changes sign, give rise to peculiarities in the harmonic output. 



If such a characteristic be expressed by a power series in terms of 

 grid-voltage variations from the operating point P, there usually is 

 found a relatively large contribution by third and higher odd-power 

 terms the coefficients of which are predominantly negative in sign. 

 Since odd-power terms yield odd harmonics, this accounts for the 

 relatively high levels of third harmonic at input voltages sufifiiciently 

 large that the flat portion of the characteristic is traversed. However, 

 positive and negative signs are about evenly divided among the 

 coefficients of the even-power terms, which yield even harmonics. At 

 some value of the input voltage, which varies with the load resistance, 

 the contributions to the second harmonic by positive and negative 

 terms are approximately equal, resulting in a very small value of this 

 harmonic. This accounts for the cusps in the second harmonic curves 

 of Fig. 12. For inputs less than the value at the cusp, the contribution 

 of positive terms (largely the second-power term) prevails over that of 

 negative terms, while at higher inputs the reverse is true. Conse- 

 quently, there is a reversal in the phase of the second harmonic at the 

 cusp. 



If the point of inflection were at the operating point P in Fig. 11 

 and, if the dynamic characteristic were symmetrical about P, then only 

 odd-power terms would appear in the equation of the curve. Conse- 

 quently, in this special case, even harmonics would vanish from the 

 output and only odd harmonics would remain. 



Plate-circuit efficiency of pentodes is higher than that usually found 

 in triodes. The underlying physical reasons for this difference are as 

 follows: In the triode, the plate simultaneously performs two functions. 

 First, it is the element in the output circuit whose fluctuating potential 

 is impressed across the load resistance. Second, assuming that the 

 grid potential is not positive at any time, the plate is the only positive 

 electrode providing the necessary driving force for the space current. 

 These two functions militate against each other to a certain extent, for, 

 as is evident from consideration of the dynamic characteristic of either 

 a triode or a pentode, the plate voltage reaches its minimum value at 

 the instant when the plate current reaches its maximum value. This 

 minimum voltage, which must be sufficiently large to draw the peak 

 current through the tube, is a very substantial fraction of the operating 

 plate voltage, particularly when the latter is comparatively low. 



In pentodes and in some other multi-electrode tubes, the positive 

 grid, maintained at a constant voltage, provides the necessary driving 



