Another unlikely vacuum tube implementation: the Korg Nutube
You can find a number of hacker projects making standard Vacuum Fluorescent Display as tube amplifiers, Korg has engineered one specifically for use as an amplifier. At about 30% of the size of a conventional tube, it provides a modern alternative to the steam punk look of tube audiophile gear.
It can run on much lower voltage than traditional vacuum tubes, as low as 5V, it also has a higher impedance and gain of 5, much less than a 12AX7 tube, one might use it as a sound “tubify” effect more than an amplification stage.
Herer’s a hacker introduction to the part.
And the 4th part of the series, building an amp (design files on github)
Korg has an evaluation board for $100
Here’s another open source design with performance graphs, that costs a bit over $100 in parts