25-1: THE BELL SYSTEM TECHNICAL JOURXAL, JANUARY 1957 



Supervisory Unit. 



The supervisory unit comprises a frequency-selection crystal filter of 

 about 100-cycle bandwidth in the range 260-264 kc fed from the low-fre- 

 quency output end of the repeater via a series resistor. This filter feeds a 

 full-wave germanium point-contact crystal-rectifier bridge which acts as 

 a frequency doubler. The second harmonic in the band 520-528 kc is 

 filtered out by a coil-capacitor band-pass filter, and fed back through a 

 resistor to the same point in the repeater. The two series resistors mini- 

 mize the bridging loss of the unit on the repeater and ensure that a faulty 

 supervisory component has negligible effect on the normal working of 

 the repeater. 



DC Path. 



The dc path includes a resistor providing the 90-volt supply and the 

 heater chain of six electron tubes (see Fig. 2). The voltage drop across 

 the heater chain is not utilized for the ampUfier high-voltage supply, as 

 the heaters would then be at a positive potential with respect to the 

 cathodes, thereby increasing the risk of breakdown of heater-cathode 

 insulation. There w^ould also be a complication in maintaining the con- 

 stant heater current, particularly should the high-voltage supply current 

 fail in one path of the amplifier. The normal amplifier high- voltage supply 

 current is 32 ma. 



It is essential to maintain a dc path through the repeater even under 

 fault conditions in order that fault-location methods can be apphed. 

 Special care has therefore been taken to provide parallel paths capable 

 of withstanding the full line current. For example, the high-voltage re- 

 sistor actually consists of a parallel-series combination of ten resistors, 

 and the whole assembly is supported on Sintox (a sintered alumina) 

 blocks which maintain a good insulation at 3 kv dc, even at high tem- 

 peratures. 



Electron tube operation for consistent long life indicates the necessity 

 to maintain a specific constant cathode temperature, and to achieve this, 

 electron tubes are grouped according to heater characteristics into six 

 heater-current groups between 259 and 274 ma and stabilized to ±1 per 

 cent. The appropriate heater-shunt resistor is applied so that the tube 

 operates correctly with 316-ma line current, but for convenience the 

 shunt is taken across each set of three tubes, all in one heater group, 

 forming one amplifier path. Rl is fixed (300 ohms) and R2 is selected to 

 suit the tubes. Rl is the resistance winding of a special short-circuiting 

 fuse; when energized by the full line current should a heater become 



