i recently decided to start monkeying around with some tube amps that i have aquired over the years, namely a couple of bogens, a bell pa head, a couple of small voice of music phono amps, and my big ol traynor yba-3. a great gadget that is a must have for beginning tube amp tweakers is the Lamp Limiter. it allows you to power up the amp while being able to tell if the amplifier is drawing too much current off of the mains. if there is a short in the amp, the lamp will light up brightly. here is a nice article from a UK radio sight about the lamp limiter, and a cool thread about "reading" the action of the bulb.
these UK articles recommend a 100w bulb, where as this article from R.G Keen says that a 25w or larger bulb will do.
a lamp limiter is a great way to help the learning process when working on tube gear, and hopefully can keep you from needlessly blowing anything up!