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I would go with a bad audio output tube, or rectifier tube....
Set comes on runs fine, then gets warm 10 - 20 minutes then is seems like the volume goes down, but you can turn down the volume and it plays a little clearer, turn it up, or turn up the bass and it seems to be cutting out on a thump... I had this on an old Emerson record player I had as a kid.
One of the tubes that provides power, or uses power (not the pre-amp stages usually) is using up the reservoir of electrons available. Either the output tube's cathode can't produce enough free electrons for the tube to conduct as much current as needed to provide clear sound. Or the rectifier tube is not passing enough current to provide the set with sufficient B+ at current drain requirements.... So B+ will be dragged down as the set heats up... So when the set begins to degrade in sound quality... What happens to B+ ? You can find it at the rectifier tube, 5U4 or at one of the posts on the multi-section cap.
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Yes you can call me "Squirrel boy"
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