THE L3 SYSTEM AMPLIFIERS 909 



but in this case the remaining margin is sufficient, since the circuit ele- 

 ments are still under good control at this frequency. Because the plate- 

 cathode impedance of VT2 is very high, the similar potentiometer term 

 at the output of the input amplifier causes only negligible changes in 

 input amplifier stability margins as the thermistor changes. 



In the 70-mc region there are two almost equally important feedback 

 loops in the output amplifier — one through the transconductance of 

 the lower triode, the other through the grid-plate capacity of this tube. 

 Balances between these feedback paths are observed in the 70 to 100-mc 

 region in the course of measuring the feedback, sometimes accompanied 

 by 180° shifts in the phase of the loop transmission at frequencies 

 above the balance point, an effect which theoretically depends on just 

 how the two vectors go through the balance point. The occurrence of 

 these balances is accompanied by a few degrees loss of phase margin 

 in the 30-mc cut-off region, which must also be allowed for in setting 

 the 30-mc stability margins, since sufficient control of parasitics to 

 prevent these 70-mc effects is out of the question. 



Parasitic resonances between the lead inductances and the capacities 

 of the circuit, which tend to cause instabilities in the very high-fre- 

 quency region about 200 mc, are damped by small resistors in the leads, 

 and the lead inductances are kept small by careful mechanical design. 

 In this frequency region, neither measurement nor computation of 

 stability margins can be trusted as anything but a rough guide. On the 

 other hand, adding damping resistors in grid leads and other critical 

 points to prevent 200-mc sings causes a phase margin penalty in the 

 30-mc region, so a nice judgment of how much damping to add is called 

 for. Final values of damping were chosen so that typical amplifiers could 

 not be made to sing by increasing critical lead lengths or by substantial 

 increases in parasitic capacity, thus assuring that manufacturing varia- 

 tions of elements and wiring will not cause high-frequency sings. The 

 return ratio of VT4: is shown on Fig. 16, the mu-beta effects of both 

 amplifiers on Fig. 17. 



Signal Levels, Modulation, and Noise 



Fig. 18 shows the signal levels within the amplifier in db relative to 

 one volt from grid to cathode of VT4:, which is a convenient point to use 

 as a reference for system signal-to-noise studies. It mil be noted that as 

 a result of using the input beta circuit to give so much of the shaping of 

 amplifier gain, the input amplifier has little gain at low frequencies. In 

 consequence the input tube of the output amplifier and the regulating 

 network are important thermal noise sources at low frequencies. The 



