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Put a scope on the B+ line and see if the noise is there. Could be either a bad single-section electrolytic, or a multi-section with one or more sections going bad. I've stirred up both audio and video noise demons by just changing a couple of parts myself. Also check to see if you may have flowed any flux onto the tube pins when replacing parts; a shot of flux remover or De-Oxit/Rid-Ox sometimes helps, it helps, too, if a long-undisturbed tube gets bumped to a dirty spot on one or more pins.
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