134 BELL SYSTEM TECHNICAL JOURNAL 



Repeaters 



At each repeater point line filters and directional filters are required 

 on both sides of the amplifying equipment to separate type J currents 

 from those of lower-frequency services on the line and to separate 

 oppositely directed groups for separate amplification in one-way line 

 amplifiers. These filters have been described in connection with the 

 terminals where they perform similar functions. Two regulating 

 amplifiers, one for each direction of transmission, properly controlled 

 to compensate for variations in the attenuation of the preceding line 

 section, are also needed at each repeater point. These are described 

 later under "Regulation." 



Figure 8 shows the circuit of one of the line repeaters and indicates 

 the location of the directional filters, and certain supplementary 

 filters for suppressing frequencies outside the transmitted range; also 

 the regulating amplifier circuit, and the pick-off of the pilot channel 

 which controls the gain. 



The line amplifier has three stages of pentodes. The first two 

 stages use single tubes of high voltage amplification and low power 

 capacity while the third stage has four power pentodes in parallel to 

 increase the output capacity. Because of considerable heat developed 

 by these power tubes, special precautions are necessary to dissipate the 

 heat and to protect condensers and other elements mounted near 

 them. 



Negative feedback to improve the operation of the amplifier is 

 supplied over two paths. The inner feedback, from the plates of the 

 output tubes over a properly designed network to the grid of the input 

 tube, reduces the gain at frequencies outside the transmitted band and 

 so prevents singing at those frequencies. It has little effect at fre- 

 quencies within the type J range. The outer feedback path includes 

 the input and output transformers, which are made as hybrid coils. 

 In each of these one pair of the conjugate windings is connected to the 

 incoming or outgoing circuit of the amplifier while the other pair is 

 used for the feedback connection. By feeding back through the trans- 

 formers in this way, they benefit by feedback in much the same way 

 as the tubes, and the overall characteristic of the amplifier is practically 

 independent of the transformer characteristics. This feedback re- 

 duces the amplifier gain by over 40 db and correspondingly reduces 

 modulation effects within the amplifier, and gives exceptionally stable 

 transmission with respect to tube and voltage changes. It is also 

 designed to improve and stabilize the input and output impedances. 



