THE L3 SYSTEM — AMPLIFIERS 



887 



Fig. 5 — Input interstage, illustrating use of case resin frameworks. 



are available on the panel to plug into the unit after it is in place on the 

 slides. 



Vacuum Tubes 



The tubes have been fully described in an earlier paper ;^ their charac- 

 teristics are summarized in Table I. They are plugged into conventional 

 sockets, and single rather than parallel tubes are used in each stage. 

 These are departures from the practice of the LI coaxial system, in 

 which two tubes in parallel were soldered in each stage. The use of 

 sockets increases the parasitic capacities and reduces the obtainable 

 feedback by one or two db, but it was felt that the resulting maintenance 

 economy was worth this sacrifice. With single tubes, the failure of one 

 tube results in a line failure and a switch to the protection line. At first 

 sight, it would appear that using parallel tubes in each stage should 

 greatly decrease the probability of line failure. A study of LI experience, 

 however, showed that most tube failures could either be forestalled by 

 preventive maintenance, or else were of such a nature (for example, 

 shorts within the tube) that the parallel tube would not afford protec- 

 tion against line failure. The reliability advantage of parallel tubes, then, 

 turns out to be small; their use, on the other hand, increases the wiring 



