THE L3 SYSTEM — AMPLIFIERS 905 



DC Feedback 



In addition to the loop feedback at signal frequencies, local dc feed- 

 back is used on each stage. The grid of each tube is returned to a +9- volt 

 potential rather than to ground, and about +10 volts is developed across 

 each cathode resistor. The usual stabilizing effects of self-bias are thus 

 obtained in exaggerated degree, each tube having about 20 db of local 

 dc feedback. Care must then be taken to select the cathode by-pass and 

 interstage coupling condensers so that the transition from low-fre- 

 quency local feedback to in-band loop feedback is accomplished smoothly 

 without the instability which might be caused by a balancing out of these 

 two feedbacks in the transition region. 



For maintenance measurements, the 9-volt bias potential and the dc 

 cathode voltage of each tube are brought out to a multi-pin amplifier 

 test jack through appropriate decoupling filters. Thus the bias on each 

 stage can be measured on an in-service basis. Filament voltage dropping 

 resistors which can be switched in or out of circuit are provided on the 

 repeater panel, so that bias can also be observed for a filament supply 

 voltage 10 per cent below normal. The activity or change in plate current 

 with filament voltage thus measured, or the history of the bias at 

 normal filament voltage, ^vill be used to determine when amplifiers should 

 be taken out of service for tube replacement. 



The upper triode is, of course, a special case: since it is the plate supply 

 path for the lower triode, its cathode is about 160 volts above ground and 

 its grid is returned to a similarly high potential. About 35 db of dc feed- 

 back is obtained on this stage; the same provisions for measurement of 

 bias and activity are made. To avoid excessive filament to cathode 

 voltage, a separate filament winding which floats at the +190-volt 

 supply potential is used for this tube. 



Loop Feedback 



The design of the feedback loops of the input amplifier follows con- 

 ventional practice. The constraints which operate to limit the amount 

 of feedback which can be obtained in the transmitted band are well 

 known.^ Broadly speaking, the figiu*e of merit of the vacuum tubes and 

 the circuit capacities determine the asymototic cutoff, which in any 

 feedback amplifier limits the magnitude of the feedback which can be 

 built up in the band. In multi-loop structures, there are additional 

 limitations. Consider, for example, the formula given on Fig. 10(a) 

 for the feedback on the second tube. If w^e increase without limit the 

 magnitude of the first tube transconductance, we find that the feedback 



