Replacing the original Noval socket in the string with the platter-friendly version, bracing the wiring with duct tape, balancing it on my desk, and firing it up:
The green phase looks nice, too:
Those screws are too big.
The getter flash covers the entire top of the tube; shining an LED down through the evacuation tip won’t work and even a laser doesn’t do much. That saves me the trouble of trying to create a cap that doesn’t wreck the tube’s good looks.
I originally planned to use white / natural PETG for the socket, but the more I see of those things, the more I think black is the new white. The sockets should vanish into the background, to let the tubes (and their reflections) carry the show.
The (yet to be designed) base must vanish under the platter edge, too, which puts a real crimp on its overall height. I’m not sure how to fit an Arduino Pro Mini and an FTDI board beside the existing socket; perhaps this calls for a unified socket-base design held on by those screws, rather than a separate socket inside a base enclosure.
Even though I know the tubes are inert and cool, I still hesitate before removing them from their sockets with the Neopixels running: you simply do not unplug a hot, powered device!
The Smell of Molten Projects in the Morning 
Looks like a nice twin triode.
A genuine USA 12AT7, with a date code that might be 5-72: it’s almost new! Apparently you can get more-or-less new tubes, too; IMO if you must inquire about the price, you don’t really need one.
I can state with absolute certainty that this tube never saw service in a guitar amp!
I much prefer the USA/Western Europe tubes from 1950-1980. That overpriced EH one is built in the Reflektor factory, and is known to have short life and variable quality. On the other hand, I was restoring a Danish copy of an HP counter module, and found out that cheap Chinese 12AU7 tubes work fine in it (an Eccles-Jordan multivibrator doesn’t give a fig about linearity).
Perhaps snooty audiophiles will buy anything with nothing inside, as long as it features gold-flashed pins and produces sticker shock on sight? [wince]
That’s very pretty. If you are considering colors, I’ve always thought that the orange heater glow and blue plasma of an active tube is mesmerizingly beautiful. I haven’t fired up my Williamson Amp in a long time. That might be therapeutic. My HuanZu headphone amp doesn’t produce the plasma or it is getting washed out by the blue LEDs they put under the 12AX7’s. Speaking of which, do you know how to get rid of heater generated hum in an audio amplifier?
Most tubes don’t have a blue plasma, that normally occurs in gas tubes like voltage regulators and rectifiers. Some beam power tubes will show a pretty blue fluorescence in the glass as errant electrons strike it, but 12AX7s are unlikely to show any of these effects. My favorite way to get rid of heater generated hum is to run the heaters on DC. Failing that, make sure heater wiring is twisted to cancel out the fields it generates, and add a “hum balance” pot to null out the hum.
Yeah, the Williamson amplifier has 6L6GC beam power tetrodes and they have a pretty blue glow. I never knew they were unusual.