THE L3 SYSTEM AMPLIFIERS 891 



coupling network and the parasitic admittances to ground. The portion 

 of the plate current which returns directly to cathode (ground) through 

 the latter path, without passing through the beta circuit, is not effective 

 in producing loop feedback. 



A second and even more important limitation is that the sensitivity 

 of the insertion gain to variations in the coupling network elements is 

 increased as the slope is increased. The same considerations lead us to 

 keep not only the slope but also the gain level or efficiency of these net- 

 works relatively low. The maximum possible coupling network gain 

 which can be obtained over the entire frequency spectrum is limited 

 by the capacity across the circuit, as shown by Bode's Resistance In- 

 tegral Theorem. This capacity cannot be reduced without incurring a 

 more severe potentiometer term penalty and thus limiting feedback, so 

 that the total gain area cannot be profitably increased. The in-band gain 

 can be made greater or less as we concentrate more or less of the total 

 gain area in the transmitted band, but it is found that attempting to get 

 high values of in-band gain area leads to networks which are increasingly 

 sensitive to element deviations. A resistance area efficiency of about 

 50 per cent turns out to be the most acceptable compromise. 



In spite of these steps, the coupling networks are the most important 

 source of manufacturing deviations, but by improved mechanical design 

 and the use of quality control these effects have been reduced by an order 

 of magnitude as compared to previous designs. For example, the end- 

 capacity of each coupling network is a quartz-disc condenser, and the 

 peaking or splitting coils are tension-wound on ceramic forms to give 

 inductances of 1 per cent tolerance with distribution requirements within 

 this range. The windings of the transformers, shown in Fig. 7, are made 

 by plating the turns on threaded forms machined from optical grade 

 quartz or Vycor glass to tolerances of tenths of a thousandth of an inch. 

 Leakage inductance and parasitic capacity deviations are thus held to a 

 minimum. Split ferrite cores, which must also be held to close tolerances, 

 make it possible to use these methods of fabrication, which previously 

 available tape cores would not have permitted. 



Since the amplifier configuration gives series feedback on the tubes 

 adjacent to the cable, and cathode feedback on the tubes adjacent to 

 the regulating network, the high impedance winding of each coupfing 

 network is necessarily off-ground. This leads to considerable difficulty in 

 specifying the equivalent circuit. For an on-ground coupfing network, the 

 equivalent circuit of Fig. 8(b) would be adequate for gain and feedback 

 computations — essentially a reactive equafizer plus an ideal trans- 

 former. Even then the capacities and inductances associated with the 



