608 BELL SYSTEM TECHNICAL JOURNAL 



It is customary to make periodic tests of the equipment. On the 

 longer circuits ^ each day the channels are "lined up" for the required 

 overall transmission equivalents. At less frequent intervals the 

 vacuum tubes are checked for emission, the gains of the sending and 

 receiving branches are measured, the carrier synchronism is checked, 

 etc. 



Vacuum Tubes. Two principal types of vacuum tubes are employed 

 in this system. One of these is the so-called "L" tube. This tube 

 has a filament circuit requiring a current of approximately 0.5 ampere 

 with a voltage drop of 4.0. It has a ju of 6.5, and a normal plate 

 current, when used as an amplifier with a "B" voltage of 130 and a 

 "C" biasing potential of 8, of about 6.5 milliamperes. 



For the output stage of the amplifier a higher capacity tube is 

 employed. This is a so-called "O" tube having a m of about 2.5, 

 a filament current requirement of approximately one ampere at a 

 voltage drop of 4.5. The normal plate current when used as an 

 amplifier with a grid biasing potential of 22 and a plate potential of 

 130 volts is from 17 to 35 milliamperes. 



In addition to the above the pilot channel uses a low-filament 

 current tube. This tube has a /x of 8, a filament current of .060 

 ampere, and a filament voltage drop of 3 volts. The normal plate 

 current when used as an amplifier with a grid biasing potential of 

 7 volts and a plate potential of 130 volts is approximately .003 ampere. 



The tubes employ oxide-coated filaments and have been designed 

 to be especially long lived to meet daily 24-hour service requirements. 



Power Supply. The power required for the carrier equipment is 

 taken from the telephone office supply where this is adequate. Usually 

 the 24-volt central ofifice power plant is suitable for the purpose, and 

 the 130- volt supply for the plate circuits of the tubes is taken from 

 the same batteries provided for telephone repeaters if available. 

 The carrier requirements, however, may amount to a substantial 

 addition to the load on the power plant, particularly where several 

 carrier systems are installed in a relatively small ofiice. 



The amount of power required for a typical carrier telephone 

 system is substantially larger than the usual telephone power require- 

 ments for the same number of facilities. Each system terminal 

 requires approximately 8 amperes at 24 volts and about 400 milli- 

 amperes at 130 volts. The power required for each carrier repeater 

 amounts to about 4 amperes at 24 volts and 250 milliamperes at 

 130 volts. Thus, the total power required for one three-channel 



^ W. H. Harden, "Practices in Telephone Transmission Maintenance Work," 

 Bell System Tech. JL, V. 4, Jan. 1925, pp. 26-51. 



