The L3 Coaxial System 



Amplifiers 



By L. H. MORRIS, G. H. LOVELL and F. R. DICKINSON 



(Manuscript received April 17, 1953) 



The line ampU£ers for the L3 coaxial system are designed to compensate 

 for the loss of the four miles of cable which separate the repeaters; the flat 

 amplifiers are used to compensate for equalizer loss and as transmitting 

 amplifiers. The two types are basically similar, consisting of two feedback 

 amplifiers in tandem separated by an inter-amplifier network; in the line 

 amplifier this network is variable and is automatically adjusted to com- 

 pensate for variations in cable temperature and for small deviations from 

 the nominal four-mile spax^ing. 



Coupling networks employing high-precision transformers are used to 

 connect the amplifers to the coaxial cable through the required power sepa- 

 ration filters. The low impedance windings of the transformers are center- 

 tapped and a balancing network provided in order to match the cable im- 

 pedance over the transmitted frequency band. The amplifiers are equipped 

 with plug-in tubes of high figure of merit which were developed for this 

 application. A double-triode output stage is used to obtain improved system 

 signal-to-noise performance. Provision is made for preventive maintenance 

 of vacuum tubes and for a controlled adjustment of gain on an in-service 

 basis. 



All important components of the amplifier are subject to quality control 

 procedures to assure that the average gain of groups of amplifiers will be 

 held within narrow limits and that individual amplifiers will form a normal 

 distribution around the average. This approach is essential in order to 

 meet system equalization and signal-to-noise objectives. Careful mechanical 

 design and rigid control of the mechanical aspects of manufacture are neces- 

 sary to minimize gain variations which might be caused by variations of 

 parasitic circuit elements and unwanted feedback effects. Special measures 

 were required to keep the temperature rise within the sealed die-cast housing 

 within tolerable values. 



879 



