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Just wondering - where is the 6AT6's heater located in relation to ground? Does one leg go to ground? Or is the heater "higher up" in the series string? If higher up, the elevated AC voltage between heater/cathode could transfer hum.
In most series string radios f'rinstance, it's common practice for the 1st audio tube's heater to be at the very bottom (ground) end of the string to minimize hum.
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