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All good points to check listed above. One more: sometimes a first audio tube (12SQ7 or =) will be sensitive to hum coming right through the glass to it. I've found radios that will quiet down using a metal XSQ7. For this to work, octal pin 1 needs to be grounded. To test on any first audio tube, wrap foil around the tube and test clip it to chassis to see if a "real" shield would help.
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Reece
Perfection is hard to reach with a screwdriver.
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