A quartet of ceramic octal tube sockets arrived from halfway around the planet and matched up nicely with the business end of a 0D3 voltage regulator tube from the Hollow State Electronics box:
If the 1-48 on the side of the tube base (facing away in the picture) means anything, then General Electric built it in January 1948.
The pinout view in the datasheet assumes you’re looking at the bottom of the socket, which makes perfect sense given the hand-wired chassis construction techniques of the day:
So the view is backwards when seen from the top, not that you’d ever need it:
The internal jumper across pins 3-7 allows you to disconnect the downstream circuit when the regulator isn’t in the socket, which is a Very Good Idea with a shunt regulator.
Not having a 200 V power supply ready to hand, but having recently restocked the 9 V alkaline battery box, this actually worked:
That’s 16 x 9-ish V = 150 V across the battery terminals, plus a 50 V adjustable bench power supply coming in on clip leads from the upper right, with current shown on a digital panel meter across a 1 Ω sense resistor. The classic 1.5 kΩ carbon resistor emerged from from a coffee can of parts that Came With The House™ and seemed appropriate for the occasion.
The tube conducts a few milliamps through a small plasma filament discharge at 150 V. The current ramps up to about 10 mA as the supply voltage increases to 180 V, whereupon the tube fires and the current jumps to 30 mA (which is less than the spec, but I ran the power supply in constant-current mode to avoid whoopsies).
Reducing the current to 10 mA slightly reduces the area involved in the plasma discharge, but the tube still produces a nice display through the mica spacer / insulator atop the plate:
That isn’t quite in focus, but should give you the general idea.
I didn’t measure the operating voltages across the tube, mostly because I didn’t want more cheap clip leads cluttering the bench.
It’d make a very low intensity nightlight that dissipates a watt or two. Boosting the current to the absolute maximum 40 mA would brighten it up a bit, but dissipating 6 W in the tube probably won’t do it any good.
This obviously calls for an Arduino monitoring the tube current with a Hall-effect sensor and regulating it with a hulking MOSFET…
The Smell of Molten Projects in the Morning 
I know just the FET: STQ1NK80ZR-AP, comes in a cute little TO-92 package, good for 800V and 300mA (not at the same time, obviously), costs about a quarter apiece. Hardly “hulking”, but I think it would do the job. I was using one of these to test the transient response of a 600V CRT power supply after I got frustrated trying to do it with a beam triode tube. A hall-effect sensor, however, is just silly.
I’ll take that as encouragement… [grin]
The datasheet says 225 V will fire the tube in total darkness (at no more than 100 mA for 10 seconds), after which the tube regulates around 150 V at 5 to 40 mA. With a big 1.8 kΩ dropping resistor, the transistor must dissipate maybe half a watt at 10 mA; a tad much for a TO-92 package. Given that I need only one or two of these things, matching the supply to the tube wouldn’t be totally absurd.
Interestingly, a 0D3 produces (up to) 15 mVRMS of noise at 40 mA. Surely, random numbers lurk therein, but that’s probably not worth the effort.
Oh yes, encouragement was intended. There are plenty of 1kV TO-220 packaged FETs that could do the job (they’re a little closer to “hulking”, but still not in the class with that ET227). Amusingly, for high voltages, gas tubes are quieter than zeners. That is, unless you have too much parallel capacitance, at which point they become relaxation oscillators. On further thought, “for high voltages” is the only place you’d use gas tube regulators. I have a little “corotron” good for 700V, but it’s not worth beans as a night light.