LONG DISTANCE CABLE CIRCUIT 581 



For the purpose of minimizing crosstalk, the loading coils are shielded 

 individually by placing each in a metal container. In addition, the 

 leads to the coils in the stub cable and within the coil case are cabled 

 in individually shielded quads, the "IN" and "OUT" leads of a 

 loading coil being in the same shielded quad. As a result of these 

 precautions, the crosstalk between the loading coils is practically 

 negligible. Even at the highest frequencies involved in program 

 transmission, the crosstalk is only of the order of 2 crosstalk units, 

 corresponding to an attenuation of about 114 db. 



The shielded program circuit coils required on a given cable are 

 potted separately from the loading coils used on the telephone message 

 circuits. These cases are of welded steel construction. A photograph 

 of a 6-coiI case for underground use is shown in Fig. 9. The under- 

 ground type of case has a special protective coating supplemented by 

 a wrapping of heavy paper. 



Amplifiers 



Figure 10 is a schematic of the amplifier circuit as used at non- 

 regulating repeater stations. (At regulating stations an automatic 

 transmission adjustment stage is added, which will be described later.) 

 Front and rear views of the amplifier, which is designed for relay-rack 

 mounting in accordance with present-day telephone practice, are shown 

 in Figs. 11 and 12. The lower panel is the amplifier, the upper the 

 transmission adjustment stage, which will be treated later. 



In the regular amplifier a standard Western Electric 102-F tube 

 is used in the first stage and a 101-F tube in the second. The amplifier 

 uses resistance coupling and the various coils which affect the trans- 

 mission performance have very high inductance so as to give the 

 device very uniform transmission performance at different frequencies. 

 The use of permalloy for the cores of these coils makes it possible to 

 obtain the necessary high inductance without going to unreasonable 

 coil dimensions. The gain is controlled by 5 db and 10 db artificial 

 lines in the input circuit with a slide-wire potentiometer for the fine 

 adjustments. Resistances in the grid circuit of the second tube allow 

 an adjustment of the gain at high frequencies. Increasing the re- 

 sistance causes a decrease in gain at these frequencies. The grid 

 potential of the tubes is obtained from voltage drop in the filament 

 circuit. The condenser in the grid circuit with its associated resistance 

 serves to keep noise which may be present in the filament circuit from 

 entering the grid circuit. 



The ideal amplifier should give a constant gain for all frequencies 

 over the band to be transmitted regardless of variations in magnitude 



