Quote:
Originally Posted by Tom9589
Any idea why RCA put that 5.1 ohm resistor in the filament circuit of the discriminator tube?
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That is a good question. I thought about this from the different aspects of cathode temperature and space charge or possibly improving discriminator linearity.
I think the clue may be gathered from the Radio Designers Handbook by F. Langford-Smith Fourth Edition (1954) on page 1095 under FM Detectors. RCA was using the Foster-Seeley discriminator which according to Langford-Smith was subject to introducing troublesome heater-cathode hum with a miniature vacuum tube such as the 6AL5. The later Ratio Detector was not subject to the hum problem.
I believe that the introduction of the 5.1 ohms reduced the a.c. heater potential and lifted one leg of the filament from ground to help balance and reduce the hum potential between heater and cathode. Because the 6AL5 is a signal detector in this application, the reduced cathode temperature and resultant space charge reduction would not significantly affect its operation.
This is my opinion and I am open to other thoughts.