Lovekraft, you forgot one more mini amp, the ZVex Nano Head. very impressive amp, runs off a 12 volt adaptor. it uses a solid state DC to DC convertor inside it to turn the 12 volts into about 200. its got two sub miniature tubes, both dual triodes. it has 3 as the preamp andthen one that operates in class A and powers the amp through what i believe is an old fender reverb output tranny.
there are pictures of the inside of it on the site tho there's no point in trying to work out a schematic from that, the board is a 4 layer PCB lol.
monkey, if you want an amp thats tiny and sounds amazing then i cannot recomend this more, it can actually run of a 12 volt car battery and would have a decent enough battery life like that (the amp draws about 1.5 amps at startup and then 0.5 for the rest of the time that its on.
if you were gonna build a tiny tube amp then id try to get hold of some sub miniature tubes and investigate the whole DCtoDC convertor (since thats the only way youd be able to run it off a car battery, and i should probablly mention that the electronics for them can be a bit mind boggling lol)
like lovekraft said, id start on something a bit simpler before you start trying to contend with the multiple problems of making tiny tube amps