CABLE CARRIER TELEPHONE TERMINALS 113 



and thereby avoid developing an undesirable voltage bias. The poling 

 also reduces somewhat the amount by which stray frequencies have to 

 be suppressed in the carrier supply. The demodulator amplifier has a 

 slide wire gain control rheostat to equalize channel levels, which func- 

 tions by changing both the grid bias on the tube and the amount of 

 negative feedback which is introduced by the rheostat. The sliding 

 contact in the slide wire is made practically free from contact trouble 

 by the space current of the tube flowing through it. As the rheostats 

 are only about 1000 ohms and small in size, they can easily be mounted 

 at a distance from the amplifier in the voice-frequency jack field. 



The carrier supply for the twelve channels from 64 to 108 kc, and 

 for the group modems of 120 kc, is derived in the circuit shown in 

 Fig. 4. A regular generator is shown at the top in solid lines and an 

 emergency generator at the bottom is shown in dotted lines. Between 

 the two is an automatic transfer circuit (in dotted lines) which trans- 

 fers to the emergency whenever the regular generator fails to supply 

 the proper amount of 120 kc. to the 120 kc. bus. 



At the upper left-hand corner is shown a 4-kc. tuning fork, of an 

 alloy having a low temperature coefficient driven by the tube to its 

 right to operate as an oscillator of very stable frequency. The next, or 

 control tube, amplifies the 4 kc. to drive the push-pull power stage 

 where a power of about 4 watts is developed. This passes through the 

 4 kc. filter to the non-linear coil where odd harmonics of 4 kc. are pro- 

 duced. The underlying principles of operation of this coil have been 

 published.^ To derive even harmonics of 4 kc, the copper-oxide bridge 

 is used which rectifies about half the energ\^ of the complex wave of odd 

 harmonics but, by balance, greatly reduces the amount of the odd 

 harmonics present in its output. Odd harmonics are obtained at one 

 point and even at the other. This separation into odd and even har- 

 monics by the balance of the copper-oxide bridge provides effective 

 loss of about 30 to 40 db and reduces the requirements on the carrier 

 supply filters which follow. 



The two branches pass through hybrid coils to the banks of channel 

 carrier supply filters. These separate the frequencies and feed them 

 to twelve carrier supply bus bars, one for each channel frequency. 

 From these the individual modems are fed through protective resis- 

 tances so that an accidental short circuit on one of the modems will 

 not cut off the carrier supply to the others. 



The hybrid coils permit the two generators to be connected so that 

 either can feed into the same bank of channel carrier supply filters 

 without being reacted upon by the other. No switching is required 

 when changing from regular to emergency supply. 



