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I can’t figure out what purpose the potentiometer identified by the larger red arrow on this attached schematic. There is already a pot that modifies the tone, located by the output transformer. The pot in question fades to nothing because there is a problem in the circuitry between the IF amp (tube 58) and the 2nd detector (tube 227A) which is right where the pot sits.
Any suggestions are very welcome.
Dan in Calgary worked on the York/Mayfair, so watch this space! 🙂 My guess is it has something to do with varying the DC voltage on the second detector plate, but I don’t know why.
The “2nd Detector” 227A triode is being used as a diode, with the plate shorted to the grid. The potentiometer in question would adjust the DC plate bias current. I don’t have any data on this tube type, but I suspect it would be nonlinear, and the pot would adjust it to the most nonlinear spot, maximizing the detected audio signal. To adjust it properly, I would use an amplitude modulated signal from a generator, and adjust the pot for the maximum audio output.
the 1/2 meg is a vol pot the 50k is the tone pot
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