Nick Whetstone at Decibel Dungeon, has a great introduction to power supplies for chipamps (Gainclone). Also this is a site with a lot of really accessible intro stuff on any other aspect of the HiFi hobby. Although not directly applicable to tube amps, there is a lot of good, solid advice on understanding, and safely using standard power supply parts in there. Go and check it out!
Image from Decibel Dungeon