180 



BELL SYSTEM TECHNICAL JOURNAL 



It follows that any variations in the latter will cause signal bias. To 

 minimize this effect, several schemes have been used to stabilize the 

 voltage applied to the level compensator. One of these is shown in 

 Fig. 10^ . In this arrangement the 130-volt battery discharges'through 



ODD-NUMBERED 

 CHANNEL 



EVEN-NUMBERED 

 CHANNEL 



DISCHARGE CIRCUITS ' 

 FOR DRY-CELL BATTERIES 



~4 



TO OTHER , 



detectors] 



[e'even 



(a) STABILIZED WITH DRY CELLS 



I E EVEN 



I — WV 



(B) 



STABILIZED WITH 

 NEON TUBE 



TELEGRAPH 

 BATTERY (130 

 VOLTS nominal) 



■— VvV-rl|lh: 



Fig. 10 — Grid-bias supply circuits, 



a series of resistances Ri, Ri and Rz so proportioned as to provide 

 suitable taps giving the required bias when this battery is at its average 

 voltage. Inasmuch as the filament circuits of two detectors are in 

 series, two different voltages to ground are required. Dry cell batteries 

 Bi and B2 are bridged between the taps which provide the desired 

 biases and ground, of such values that they would give the proper 

 voltages in the absence of the telegraph battery. These dry cells 

 insure constant bias voltages; they supply no current when the tele- 

 graph battery is at its average value; discharge when it is low, and 

 charge when it is high. Resistance Rz is sufficiently large so that these 

 charging and discharging currents are kept down to very small values 

 and the life of the cells is consequently long. Since the bias batteries 

 are part of a rectifying circuit there is a tendency for the signal current 

 passing from grid to filament to charge the dry cells. To compensate 

 for this, adjustable discharge circuits Di and D^ are bridged respec- 

 tively across the two grid-bias taps and ground in the manner shown, 

 and their resistances are varied according to the number of detectors 

 deriving their bias from this source. 



