I was browsing though some space charge tubes (these were tubes designed to operate at low voltages -- usually for car radios) recently and happened upon the 12K5 which is designated as a power output tetrode. I thought it might be useful for driving some headphones, and when I went looking, it turned out that others had been here before me.
A number of people, (here and here) it seems, have used these for small, low power, guitar amps. This seemed like a fun little project, so I threw one together.
Below is a schematic (click it for a larger version) as well as some construction pictures.
For the power supply, I used an old 12V wall wart from a dead internet router. It is unregulated, so I powered the heaters from the unregulated part, and built a RCRC filter using 4.7 ohm resistors and 2200uF caps to cut down on hum. The speaker, which is 5cm in diameter, is taken from an old computer case. It works pretty well, though I did have to try a couple of speakers before I found one that was loud enough.
Speaking of which, into 32 ohm Grado headphoes, with the amp turned all the way up, it is loud, but not painfully so. This means that less sensitive headphones might need more voltage on the output. With the speaker, it is loud enough to play and hear, but it doesn't carry much into the next room with a door closed. It is set up such that plugging in headphones turns off the speaker.
The only other note is that other tubes will work as the driver -- a 12AX7 for instance -- but the 12U7 seems to sound a little better. Otherwise, there is nothing magical about the parts selection. I just used whatever fell out of the proverbial coffee can.