A chunk of 1/2 inch = 12.7 mm brass hex rod looks pretty good as an ersatz heatsink serving as an ersatz plate cap on a halogen bulb standing in for a vacuum tube:
The knockoff Neopixels measure just over 10 mm at their widest points, but some judicious filing rounded it off and brought it down to fit in the 3/8 inch = 0.375 = 9.52 mm hole I drilled in the hex:
I let it run for a day like that to make sure the thing wasn’t going to crap out, then epoxied everything in place. If the WS2812B controller fails, the repair will require drilling out all the electronics and wiring, then rebuilding it in place.
The fins come from the same HSS cutoff tool I used for the Bowl o’ Fire cap, cut at 2.5 mm intervals to produce 0.9 mm fins that IMO better suit the smaller diameter. I stopped cutting when the tool got through the hex flats to produce a continuous ring, cut the hex off a bit above the top fin, rounded the end with a carbide insert cutting tool, then sanded the flats to shine ’em up a bit:
It turns out that 12 inches of wire inside PET braid barely reaches from the cap to the Arduino Pro Mini in the base:
Next time, I’m going to add half a foot more wire than I think it can possibly require, with PET braid to suit.
A thin ring of clear epoxy holds the “heatsink” at the dead center of the bulb. It lights up a bit more than I expected, so opaque epoxy may be in order:
It’s still too big to suit even the big 21HB5A tubes, but brass definitely wins over plastic!
That blue PETG base has become the least-attractive part of the lamp, but it’s survivable for now.
It runs the same TubeMood firmware as the Bowl o’ Fire.
The Smell of Molten Projects in the Morning 
Hide another knockoff Neopixel or two in the base?
I can’t figure out how to get a light path through the base: a lump of tan glop inside the threaded aluminum cup holds everything together. I could disembowl the base, scrape / bash the glop off, and epoxy the bulb into a (3D printed, natch) base-with-LED, but …
For the larger bulbs & that giant 6C21 triode, aiming one of those little laser pointer modules at the innards from a limpet mount on the side of the bulb might work. Run just over the lasing threshold, it might last long enough to be useful, too.