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"Hum" pickup is often not hum but buzz ... 180 or 240 Hz and up, up, up.
This is often pickup from wires connected directly to the power transformer
or direct to the line.
If the filter capacitors are grounded will away from the
transformer it may well be unfixable keeping the radio near
original. I've fix it by completely redoing the power supply with
1N4007s and two modest electrolytics and two modest series
resistors, right at the transformer, and letting the resultant somewhat
filtered DC go around the radio to the tube rectifier and filter caps.
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