890 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1953 



ducting path is thus provided from vacuum tube envelope to amplifier 

 housing. This design. resulted in a temperature drop in the output am- 

 plifier tubes of about 70° C without any temperature rise in the chassis 

 or circuit elements over that produced when ordinary types of shields 

 were used. The smaller vacuum tubes in the input amplifier do not 

 ordinarily run as hot as those in the output amplifier and a temperature 

 drop of only about 55° C was attained by these methods. 



Coupling Networks 



The input and output coupling networks are essentially identical. 

 The low side of each transformer is a balanced center-tapped winding 

 which together with the balancing network acts as a hybrid, to produce a 

 good 75-ohm impedance facing the cable. The use of this type of con- 

 nection gives a signal-to-noise advantage over the use of a brute-force 

 high-side terminating network. The advantage is theoretically 3 db in 

 the case of the output coupling network and would approach the same 

 figure in the case of the input network if tube noise were dominant over 

 the resistance noise of the cable. ^ Aside from the fact that the design of a 

 high-side shunt termination network for an off-ground peaked tran- 

 former is well-nigh impossible, the use of a balancing network in a 

 hybrid connection has the important additional advantage that the 

 adjustment of this network to obtain a good reflection coefficient has 

 negligible effect on the insertion gain of the circuit. 



A relatively modest share of the total shaping required has been 

 allocated to the coupling networks: 5.5 db each, or 11 db total. One 

 reason for this is that although these networks are outside the feedback 

 loops in the usual sense, nevertheless the impedances which they present 

 to the amplifiers are important factors in the feedback design, and the 

 effects which they produce must not be allowed to become so severe as 

 to limit the feedback to too low a value. It is obvious from inspection 

 of Fig. 1 that only a part of the voltage developed across the input 

 beta circuit by the plate current of the second tube will appear as a grid- 

 cathode voltage to drive the first stage. The proportion of the beta circuit 

 voltage which will be thus effective in producing feedback around the 

 loop will be dependent on the potentiometer division between the 

 impedance of the coupHng network and the grid-cathode impedance of 

 the first tube. The greater the peaking of the input coupling network, 

 the greater its impedance at high frequencies where the grid-cathode 

 capacitive impedance Ls already decreasing, and hence the greater the 

 potentiometer term loss. A similar loss occurs in the output amplifier. 

 The plate current of the output stage divides between the output 



