THE L3 SYSTEM — EQUALIZATION AND REGULATION 875 



because it also introduces a slight compressive action in the amplifica- 

 tion of the tube and thereby prevents rapid wild changes in oscillation 

 level. Bypassed dc feedback on an amplifier causes dc second order dis- 

 tortion to increase the bias and thereby reduce the transconductance. 

 This effect overcomes the tendency of the third order distortion to 

 create expansion in this particular tube. If the thermistor response were 

 fast compared to the reciprocal of the bandwidth of the amplifier the 

 compression action would be unnecessary. However with an audio 

 frequency amplifier and a 100 second thermistor the compression is 

 essential in preventing motorboating. 



The field limits on the compensation of the regulating thermistor over 

 the range -20 to +160 degrees F are ±3 per cent in resistance due to 

 all causes including manufacture and aging. Specific units can be ad- 

 justed to yield compensation to a fraction of a per cent. 



OFFICE REGULATORS 



The L3 office regulators are similar in design to the line regulator. 

 However the office regulators operate their regulating networks via an 

 analog computer and, of course, a variety of pilot frequencies are em- 

 ployed. Because signals are dropped at offices, higher loop feedbacks are 

 used to insure accurate equalization. However, temperature variations 

 are smaller and conventional thermistor ambient temperature com- 

 pensation is adequate. Also the smaller number of office regulators per- 

 mits less isolating loss for nick effect (except for the 7,266-kc oflSce 

 regulator). 



The lower levels of the pilots (except 7,266) are compensated by re- 

 duced isolation loss, (12 instead of 23 db), and reduced detector level 

 (40 instead of 60 volts). Thus the gain required is not sustantially in- 

 creased. The pilot amplifier design is therefore different primarily in 

 the tuning frequency and in the simplifications in the lower frequency 

 units permitted by the higher permissible interstage impedances. 



The diode detector feeds a cathode follower to obtain the dc voltage 

 representing the deviation of the pilot from its assigned value as a low 

 impedance source to feed the computer. The appropriate signals from 

 the computer are fed to the dc amplifier. This amplifier differs from 

 that used in the line regulators in that (1) a push pull input is provided, 

 (2) higher gain is required to produce greater feedback, 30 db, and over- 

 come computer losses, 5 db and (3) the output stage supplies somewhat 

 higher currents, 1 to 30 ma, (except 7,266) because the regulating net- 

 works use a lower impedance thermistor. 



