Vacuum Tube LEDS: Ersatz Plate Cap

Lighting up that old voltage regulator tube conclusively demonstrated there’s no point in conjuring high voltages in this day & age. Nay, verily, merely lighting the filament of some tubes would require more power than seems reasonable.

1B3GT high-voltage regulator tube in the Box o’ Hollow State Electronics suggested a different approach:

1B3GT HV tube regulator
1B3GT HV tube regulator

With only a slight loss of historical accuracy, one could light the tube from the top with a Neopixel LED tucked into a similar cap, with power-and-data arriving through a suitably antiqued flying lead. That won’t work on tubes like that 1B3GT with an actual plate terminal  at the top, nor with small Noval / miniature 7-pin tubes topped with an evacuation tip, but it’s fine for tubes like this 6SN7GTB:

6SN7GTB Vacuum Tube
6SN7GTB Vacuum Tube

Obviously, you want a relatively small cap atop the tube, lest the LED visually overwhelm the tube. Some preliminary tests (a.k.a. screwing around) showed that the mica spacer holding the dual triode elements together lights up wonderfully well and diffuses the glow throughout the tube.

Adafruit has relatively large round (and smaller roundish) Neopixel breakout boards, but I bought a bunch of knockoff Neopixels mounted on a 10 mm circular PCB from the usual eBay supplier:

Vacuum Tube LEDs - plate lead - connections
Vacuum Tube LEDs – plate lead – connections

Some PET braid tucked into a snippet of brass tubing dresses up a length of what might once have been audio cable. The braid wants to fray on the ends; confining it with heatstink or brass tubing is mandatory.

That’s a 1 µF ceramic SMD cap soldered between the +5 V and Gnd traces, atop a snippet of Kapton tape, in the hopes that it will help the 100 nF cap (on the other side of the board) tamp down the voltage dunks from PWM current pulses through that long thin wire. The leads come off toward the center to bend neatly upward into the cap.

Duplicating that old plate cap on the 1B3GT would be a fool’s errand, so I went full frontal Vader:

Vacuum Tube Lights - cap solid model - Overview
Vacuum Tube Lights – cap solid model – Overview

The interior recesses the LED far enough to allow for the tube’s top curvature, with a conical adapter to the smaller wiring channel that allows for more plastic supporting the brass tube:

Vacuum Tube Lights - cap solid model - section
Vacuum Tube Lights – cap solid model – section

A glob of epoxy inside the cap anchors the PCB and fuses all the loose ends / floppy wires / braid strands into a solid block that will never come apart again.

It should be printed (or primered and painted) with opaque black or maybe Bakelite Brown, but right now I have cyan PETG and want to see how it plays, soooo:

Vacuum Tube LEDs - plate lead - overview
Vacuum Tube LEDs – plate lead – overview

The cap floats in mid-air over a defunct Philips 60 W halogen bulb that I’ve been saving for just such an occasion. Obviously, you must epoxy / glue the cap in place for a permanent display.

The OpenSCAD source code as a Github gist:

// Vacuum Tube LED Lights
// Ed Nisley KE4ZNU January 2016
Layout = "Cap"; // Show Build Cap Box Octal Noval Mini7
Section = true; // cross-section the object
//- Extrusion parameters must match reality!
ThreadThick = 0.25;
ThreadWidth = 0.40;
HoleWindage = 0.2;
Protrusion = 0.1; // make holes end cleanly
inch = 25.4;
function IntegerMultiple(Size,Unit) = Unit * ceil(Size / Unit);
// Dimensions
T_NAME = 0;
T_NUMPINS = 1; // Socket specifications
TubeBase = [
["Mini7", 8, 9.53, 1.016, 19.0],
["Octal", 8, 17.45, 2.36, 33.0],
["Noval",10, 11.89, 1.1016,20.5],
ID = 0;
OD = 1;
Pixel = [7.0,10.0,3.0]; // ID = contact patch, OD = PCB dia, LENGTH = overall thickness
// Useful routines
module PolyCyl(Dia,Height,ForceSides=0) { // based on nophead's polyholes
Sides = (ForceSides != 0) ? ForceSides : (ceil(Dia) + 2);
FixDia = Dia / cos(180/Sides);
cylinder(r=(FixDia + HoleWindage)/2,
// Tube cap
CapTube = [4.0,3/16 * inch,10.0]; // brass tube for flying lead to cap LED
CapSize = [Pixel[ID],(Pixel[OD] + 3.0),(CapTube[OD] + 2*Pixel[LENGTH])];
CapSides = 6*4;
module Cap() {
difference() {
union() {
cylinder(d=CapSize[OD],h=(CapSize[LENGTH]),$fn=CapSides); // main cap body
translate([0,0,CapSize[LENGTH]]) // rounded top
sphere(d=CapSize[OD]/cos(180/CapSides),$fn=CapSides); // cos() fixes slight undersize vs cylinder
cylinder(d1=(CapSize[OD] + 2*3*ThreadWidth),d2=CapSize[OD],h=1.5*Pixel[LENGTH],$fn=CapSides); // skirt
translate([0,0,-Protrusion]) // bore for wiring to LED
PolyCyl(CapSize[ID],(CapSize[LENGTH] + 3*ThreadThick + Protrusion),CapSides);
translate([0,0,-Protrusion]) // PCB recess with clearance for tube dome
PolyCyl(Pixel[OD],(1.5*Pixel[LENGTH] + Protrusion),CapSides);
translate([0,0,(1.5*Pixel[LENGTH] - Protrusion)]) // step + cone to retain PCB
cylinder(d1=(Pixel[OD]/cos(180/CapSides)),d2=Pixel[ID],h=(Pixel[LENGTH] + Protrusion),$fn=CapSides);
translate([0,0,(CapSize[LENGTH] - CapTube[OD]/(2*cos(180/8)))]) // hole for brass tube holding wire loom
rotate([90,0,0]) rotate(180/8)
// Build it
if (Layout == "Cap") {
if (Section)
difference() {
if (Layout == "Build") {

5 thoughts on “Vacuum Tube LEDS: Ersatz Plate Cap

  1. I’m still having fun playing with high voltages. The VFD projects are just 30V or so, the nixie ones 180, the CRTs 600 – 9000V, and the musical Jacob’s ladder 15kV or so. The exhaust pip on the top of tubes makes a nice anchor point/diffuser for LED lights, if you want to go that way. I’ve found that old mouse cords make good multiconductor ersatz flying leads. Old projector bulbs sometimes contain fancy filaments, reflectors, heat shields, and so forth, so they’re also good candidates for LED illumination.

    1. Old projector bulbs

      Oooh, you’re just gonna love some of the bulbs from the back o’ my heap… [evil grin]

  2. It occurs to me that neopixel type LEDs are perhaps overkill for individual illumination purposes, and you could simply use RGB LEDs directly. This also dodges the issues of remote-powering a PWM controller.

    1. That’s what I thought, but then I came to my senses. Convenient package, integral decoupling / damping, three leads, no ballast resistors, individual addressing with common code when I need two LEDs: what’s not to like?

      Plus ersatz Neopiels in lots of 50 each on round and rectangular PCBs set me back a whopping $10 a pop, so I’m down with digital lighting…

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