870 



THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1953 



403B Gong life 6AK5) tubes. Feedback is taken shunt-shunt, output 

 plate to input grid, to stabilize the input and output tuned circuits 

 against Q changes with temperature. The regulators are designed to 

 operate from —20° F to +160° F and the amplifier gain does not change 

 by more than 0.3 db over this range. 



The beta circuit contains an adjustable condenser divider for field 

 gain adjustment. The tuned sections are heavily damped to get tem- 

 perature stability but nevertheless compensate for the cutoffs of the /x 

 circuit. The resultant loop gain and phase are shown in Fig. 24. It will 

 be noted that the phase margin against singing is rather large, about 60 

 degrees. This permits tube replacement without retuning as well as 

 protection against tuning changes due to time and temperature. 



Grid-plate capacity places a limit on the permissible interstage im- 

 pedance for stability. Thus the output circuit between the third stage 

 and the rectifier is operated as a reactive transformer giving a voltage 

 step-up of 2. This increases the output voltage obtainable without 

 raising the impedance facing the third tube above 12,500 ohms. 



About 16 db of local dc feedback is used on each stage to stabilize the 



28 



24 



20 



ffi 12 



120 



a. 

 160 .9. 



200 K 



IL 



I 



< 



I 

 CL 



280 



320 



360 



6.6 



6.9 



400 



7.6 



7.7 



7.0 7.1 7.2 7.3 7.4 7.5 



FREQUENCY IN MEGACYCLES PER SECOND 



Fig. 24 — Loop gain and phase of the 7266 kc pilot amplifier. 



