592 



BELL SYSTEM TECHNICAL JOURNAL 



Curves 2 and 3 show the effective transmission gain of transformers 

 having voltage step-ups of 6.5 and 2.5 respectively, when used to 

 connect the first stage of the vacuum tube amplifier to a 600-ohm 

 generator impedance. The envelope delay curves for the output 

 transformer and for the high ratio input transformer are given in 

 Fig. 23. Photographs of the coils are given in Fig. 24. A large 



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Fig. 24 — Transformers used for coupling amplifier circuits to long distance 



telephone lines 



factor in being able to get coils of this type lay in the availability of 

 permalloy for the core material. The output transformer is connected 

 to the amplifier through a blocking condenser in order to avoid 

 possible saturation in the core due to the passage of direct current. 



Measurements made on the several elements of the amplifier system 

 have shown that its overall frequency characteristic is constant to 

 within ± 2 T U from 10 to 20,000 cycles. 



In an amplifier having as much gain as that just described it is 

 apparent that a slight change in the potential of the power supply 

 will cause a considerable change in the overall efficiency. Moreover, 

 variations in the intensity of the light source used with the scanning 

 system will cause corresponding changes in the intensity of the initial 

 luminous signal wave. To insure that the energy level supplied to 

 the line is at all times of the proper magnitude a level indicator has 

 been provided to permit continuous observations of the output of the 

 amplifier. This consists of an amplifier-rectifier circuit so arranged 



