In this video I quickly cover the electron tubes basics and show a little tube amp project, that uses the 1T4 (DF91). The 1T4 is a low voltage tube that can be operated from 45 (it actually works with 12V too) up to 90 VDC and a heater with only 1,5 V. It is also very cheap and can be as NOS for about 0,5 to 1€.
To make more bang I just put two tubes in parallel. In my configuration the whole set-up makes about 150 mW of sound power.
Find the circuit I followed at: http://www.hobby-bastelecke.de/projekte/roehren_kopfhoererverstaerker.htm (german)