THEORY OF MULTI-ELECTRODE VACUUM TUBES 69 



the tube. The decrease in power output with increasing load resist- 

 ance, at the higher inputs, results from the progressive turning over of 

 the dynamic characteristics as shown by the curves of Fig. 11. 



The second harmonic decreases continuously with increasing load 

 resistance at small inputs. At large inputs, it decreases at first, then 

 increases with increasing resistance. At very low resistances, the 

 second harmonic increases continuously with input. At higher load 

 resistances, it rises to a broad maximum, then falls to a very sharp 

 minimum, after which it again rises rapidly with increasing input. 

 The explanation of these phenomena will be given later. 



The third harmonic increases continuously both with increasing 

 input and with increasing load resistance. At load resistances that are 

 small compared with the plate resistance of the tube, it rises to a higher 

 level than the second, over a certain range of input. Relatively high 

 levels of third harmonic are characteristic of pentodes, screen-grid 

 tubes, and also of some other types of multi-electrode tubes. For 

 example, from the curves of Fig. 12, with a load resistance of 6000 

 ohms and an input of 15 peak volts, the power output is 3.8 watts 

 with the volume level of the second and third harmonics 31 db and 

 26 db, respectively, below that of the fundamental. At an input of 10 

 peak volts, the volume level of the second harmonic rises to 27 db 

 and of the third harmonic falls to 36 db below that of the fundamental. 



These results are quite different from those obtained with triodes 

 where the third harmonic is, in general, 10 db or more below the second. 

 Furthermore, in triodes, both the second and third harmonics decrease 

 continuously, as a rule, with increasing load resistance. There are 

 exceptions to this, however, where the third harmonic curves show 

 minimum points or cusps, similar to those shown by the second 

 harmonic curves for pentodes. 



The reason for the relatively large harmonic content in the output of 

 pentodes is apparent from consideration of the load lines drawn through 

 the operating point P in Fig. 8. At the lower values of plate voltage, 

 the load lines cut across the rapidly descending portions of the plate- 

 current characteristics. This effect is more marked and begins at 

 more negative values of grid bias as the load resistance increases and 

 the slope of the load lines becomes correspondingly less. The effect of 

 this is to produce current variations through the external load re- 

 sistance that are not proportional to the variations in grid voltage, 

 thus resulting in distortion of the output. 



The character of the distortion is made clearer by reference to the 

 dynamic characteristics of Fig. 11, for resistance loads of 6000, 10,000, 

 and 20,000 ohms. All the curves show a flattening out at the top which 



