Tom Hobson posted an enticing Instructables HOWTO on building a stereo tube amplifier. I haven't built the project, but I love the bonus video he linked to, a 1942 RCA promo for vacuum tubes titled "Electrons on Parade." From Tom's Instructable intro:
Ever wanted to build a highly dangerous, inefficient, and essentially obsolete piece of electronics? Well, I have. That's pretty much what a tube amp is. Vacuum tubes are old electronic components that act like transistors, controlling a lot of current with a little current. You usually hear about tubes being used in guitar amplifiers, because they distort in a way that suits guitar playing. However, tubes can also be used to amplify a stereo signal from another audio source such as a CD or MP3 player. Tube amps, unfortunately, aren't the most practical things in the world; they consume a great deal of power, get very hot, and are big. That being said, they look damn cool, and some people seem to think they sound pretty nice too.
You can learn a lot about electricity and electronics from a project such as this.
