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BELL SYSTEM TECHNICAL JOURNAL 



follows amplifier No. 1 as the final output stage of a Public Address 

 System and as shown consists of one stage with tubes in the push- 

 pull arrangement. To match amplifier No. 1 its input circuit is 

 designed for 4,000 ohms and its output is stepped down by a trans- 

 former to 500 ohms. Amplifier No. 3 shown in Fig. 4, also a push-pull 

 amplifier, is designed as the output stage of an oscillator used in field 

 measurements in the telephone plant. Its input and output im- 

 pedances are 2,000 ohms and 600 ohms, respectively. Amplifier No. 4 

 is intended for use in radio reception, amplifying the signals to loud 

 speaker volume and operating from 110 volt, 60 cycle A.C. supply. 

 As shown there is one rectifying tube for converting the 60 cycle 

 a.c. to d.c. for supplying the anode of the amplifying tube. Its 

 rated input and output impedances are 20,000 ohms and 4,000 ohms, 

 respectively. 



As will be noted from the diagrams there are employed in these 

 amplifiers three types of vacuum tubes. The normal operating volt- 

 ages and average characteristics of these tubes are shown in Table I. 



TABLE I 



The harmonic analyses and load-gain curves of the amplifiers under 

 the conditions noted are shown in Figs. 6, 7, 8, 9 and 10. The gain 

 in transmission units and percentage of each harmonic up to the 

 5th together with the root of the sum of the squares of these per- 

 centages being plotted against watts output on a logarithmic scale. 

 The oscillograph pictures taken at various points along these curves 

 are shown in Figs. 11, 12, 13, 14 and 15. 



As stated above, the oscillograph figure presents the input-output 

 curve of the amplifier. Furthermore, if there is no distortion in the 

 transformers, the horizontal deflection will be proportional to the 

 alternating grid voltage applied to the first stage while the vertical 

 deflection will be proportional to the alternating component of the 

 plate current in the last stage. The figure drawn is then the dynamic 

 Ec — Ib charactertistic of all the tubes combined. That this is sub- 



