Variacs are great tools but I'm thinking of something along the lines of a semiconductor version of a subset of an old Sprague TO-6. Assume I have a rectifier that can supply 600vdc at up to 60ma or so. What I want to do is vary the output voltage from zero (or nearly zero) up to 600 volts with a pot. Regulation is not desired. A Variac seems a bit much for this app.
Although now that you have me thinking about it, perhaps someone makes small ones? A 0.5 amp Variac would be fine and not much bigger than a big pot. Hmm...