I suspect this is not what you want to hear, but I would really have to suggest you immerse yourself in basic DC electronics theory, followed by basic AC electronics. I imagine you can find plenty of both, including some military-type courses, on the web.
IMO, there is really no good way around learning this stuff first. Armed with that knowledge, though, it becomes infinitely easier to analyze existing designs, and begin to understand things like power supply design software parameters.
Since you mentioned tubes, I will add that some of the old tube manuals, many of which are available as re-prints and .pdf files on line, have excellent information on the basics, particularly regarding vacuum tube rectifiers and chokes in power supplies.