I have a schematic published by Eric about 15 yrs. ago that uses a computor switching power supply for the filaments. It is choke filtered DC and he claims no hum. It is an interesting schematic on it's own. It uses a 572 triode as a SE pre-amp circuit. The afoementioned filament PS then a B+ supply from an international linear PS. The tube is a thoriated filament which sounds more clear and detailed than oxided. Interstage transformer coupled. He runs the filaments at 5.04v instead of 6.3. All in all an interesting design that acquired much fanfare.