912 



THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1953 



single tubes without feedback check the measured values of amplifier 

 modulation to within a couple of db if the third order coefficient of the 

 tube is corrected to take account of the fact that some third order modu- 

 lation is generated by the interaction of the fundamentals and the fed- 

 back second order products. In general, the effective third order coeffi- 

 cient of the tubes is approximately equal to the voltage sum of the 

 tube's uncorrected third order coefficient and a coefficient 6 db worse 

 than the square of the tube's second order coefficient. If this interaction 

 correction is not taken into account, the correlation of tube modulation, 

 feedback and amplifier modulation is unsatisfactory. The analysis leading 



Table III — Modulation Products, in db Below One Milliwatt 



AT Amplifier Output, for Fundamentals 5 db Above One 



Milliwatt at Amplifier Output 



to this result, which is due to F. B. Llewellyn, S. E. Miller and R. W. 

 Ketchledge, is too long to give here. 



Load Carrying Capacity 



The load carrying capacity of an amplifier is difficult to define with 

 exactness. One possible definition is the load at which the modulation 

 coefficients of the amplifier have departed appreciably from the small 

 signal power series values because of loss of feedback as the transcon- 

 ductance is cut off during part of the cycle. The signal carried without 

 serious overload, in terms of a single frequency, is practically constant 

 in the transmitted band as a consequence of the fact that the output 

 voltage and the lower triode grid voltage have nearly the same shape 

 versus frequency, as shown on Fig. 18. The output coupling network 

 shaping approximately compensates for the potentiometer term division 

 of current between the load impedance and parasitic paths to ground. 

 Departure from the small signal power series behaviour just begins to 



