A TWELVE-CHANNEL CARRIER TELEPHONE SYSTEM 139 



The regulating amplifier has two stages of pentode tubes, a high 

 input impedance necessary for the proper operation of the condenser 

 potentiometer, and feedback to stabilize the gain and to prevent inter- 

 modulation of the channels. Its output goes to the line amplifier at 

 repeater stations, and to the first group demodulator at the terminals. 

 At a west terminal there is interposed a high cut-off filter to eliminate 

 frequencies above the upper band which may have been picked up on 

 the open-wire line. 



Pilot Control 



The setting of the condenser which controls the regulating network 

 is determined in accordance with the amount of pilot current flowing 

 in the circuit in the direction concerned. At repeater stations the 

 pilot current is picked off at the output of the line amplifier, being 

 separated from the message transmissions by a quartz filter which 

 has about a 30-cycle pass band. For control of transmission from west 

 to east at the repeater stations, this filter selects 59.9 kilocycles and 

 for control of the oppositely directed transmission, 116.1 kilocycles. 

 At the terminals the pilot channel selecting filter is connected across 

 the output of the auxiliary amplifier following the second group de- 

 modulator where the pilot frequency is 84.1 kilocycles. The pilot 

 current from the pick-off filter is amplified in a single-stage amplifier 

 which has feedback for constancy of operation and input and output 

 circuits tuned to the pilot frequency. After amplification the pilot 

 current is rectified by a temperature compensated copper-oxide 

 rectifier. 



The resulting direct current passes through the operating windings 

 of the control and alarm relays. These Weston Sensitrol relays are, 

 in fact, microammeters with high and low contacts made by the point- 

 ers. The mechanical bias of the moving system is adjusted so that 

 with the normal pilot current the pointer will remain free in the middle 

 between the two contacts. A change of about 0.5 db in this current 

 will cause the pointer of the control relay to make contact with the 

 terminal at the corresponding end of its swing. As the limiting con- 

 tacts are magnetized and the pointer is of magnetic material, good 

 contact is insured. When contact is made on one side a 60-cycle 

 circuit is closed through the motor which controls the regulating con- 

 denser in such a direction as to cause the loss in the regulating network 

 to be increased. Closure of the other contact similarly causes the 

 loss in the regulating network to be decreased. Closure of either con- 

 tact also closes a circuit containing a slow-operate "pulse" relay to 

 release the Sensitrol relays after an interval of about four seconds. 

 During this time the gain of the regulating amplifier will have been 



