They vary in spec according to the rating of the lamp fixture. Quite a few people have used them both in PSUs and as output-stage plate loads (more so with Hi-fi builds) with varying results. The main issue is space - they're quite large for their rated inductance. Insulation is excellent and they often run hot in their intended application. A guy I know used a large lamp ballast in an SE parafeed amp along with a P-P transformer that had one side of the primary open. That way he got a cheap amp and used scrap parts.