Steve Bench published an excellant cure for the AC DHT filament hum problem more than 10 years ago. See it at this link-
http://diyaudioprojects.com/mirror/m...ch/humbal.html
Have to say I'm not in favor of running the filaments on DC. Just another complication to get in the way of reliability. Altho in some instances we gotta do to.
Cheers, John