Saturday, December 17, 2011

Festive Meets Nerdy

I like vacuum tubes. Have I mentioned this before?

I have a mental registry of small projects that involve vacuum tubes and one long-standing idea is a Christmas tree ornament. Here's how it goes.

(This involves fire, broken glass, and probably all of the chemical horrors of the 1950s. Say your prayers, wear eye protection, and Don't Be Stupid.)

The picture above is a good choice for a tube (though it's not the exact one I used). I have a bunch that I salvaged from old radios and televisions I've gutted over the last couple of years. The plan was to remove the guts, which is a pretty tricky operation and requires a couple of things when choosing a tube: it's got to have a big plastic boot on the bottom (to cover and reinforce the broken glass we'll deal with shortly), and the adhesive holding the glass tube to the plastic boot has to be weak. This way the adhesive gives way before the tube shatters (leaving you with a fistful of broken glass and a story to tell the ER physician).

The pins at the bottom of the tube connect to elements inside the glass envelope and generally also hold the whole thing together. There is a solder cup at the end of each pin, so if the tube is suitable (i.e. not too much adhesive), it's possible to pull the boot away from the glass while the solder is melted.

I did this by wrapping the plastic part of the base in tin foil:

...and sparking up the blowtorch:

I heated the tips of the pins, trying not to destroy the plastic. Luck was with me and I ended up with the desired number of pieces:

You can see from this picture that the tube is pretty horribly filthy. You can also see the adhesive or foam that was between the boot and the glass. Some tubes are impossible to separate this way because of the adhesive. This one was easy.

Here's another look:

OK, so the next thing to do is break the glass envelope at the bottom -- carefully, it's old glass -- and remove the guts. I used a Dremel with a diamond saw:

This blade isn't cheap but I tried a number of other things -- heat and regular cutting blades -- and discovered a lot of really good ways of shattering tubes but little else. A quick buzz around the outside with the diamond saw, first to score the surface, then to cut through, works pretty cleanly:

I did take one chip out of the glass but nothing to worry about. There's lots of room for mistakes under the boot -- but as the glass does taper slightly, cutting too close to the bottom means it's difficult to pull the guts out.I had to chop up the glass bottom with a pair of pliers in order to get everything out.

As much as I like the internals of tubes -- they're like little skyscrapers -- I haven't figured out anything to do with them. Yet.

Next step is to clean out the bulb -- carefully, both because it's fragile and because the cut edge is like the rim of a tin can, but a thousand times more murderous. Toilet paper to the rescue. The carbon deposits inside the tube are easy to remove.

I'm making this for my nephew, who is too young to read this (and I trust his mom, if she's subscribed, will have the courtesy to feign surprise and delight on Christmas). I took apart a turtle light that I had kicking around, and with the LED and some wire, made this:

You can see the LED a little better from the back:

I soldered the two wires coming through the bottom back into the solder cups on the plastic boot, epoxied the glass envelope back over top, and connected the rest of the turtle light (the battery, switch, and flasher) to the base. Here's how it looks, fully assembled:

And from the front:

And with the lights off:

Not bad! Not only that, but I totally failed to burn, cut, or blind myself despite a dozen really good opportunities.


  1. Nice work ... However just one correction. Propane torch does not equal "blowtorch". Blowtorches are the old and rarely used versions of torches from yesteryear. See following pic:

  2. Nice, but I would have used a slightly more useless TV tube, as the 35L6 you used for this is a popular output tube for radios. (unless this particular tube was bad to begin with)

  3. While we're picking on you, the black substance inside tubes isn't carbon, it's a barium "getter". It's intentionally reactive material intended to soak up trace impurities in the tube's vacuum. When it goes white, that means it's fully consumed and the tube is probably flooded with air.

    Incidentally, in cryogenic vacuum systems, activated charcoal often fulfills the same role. It's cold enough in these systems that any trace molecules simply need something to stick to, no chemical reaction required.

  4. @Keith, @Graeme: Neat, thanks.

    @Hemingray, agreed -- I'd prefer not to destroy good vintage stuff, and to that end I've got a stack of dead Mesa Boogie tubes from a guitar shop lined up for this kind of project. Unfortunately I haven't figured out how to open them up yet. The other problem is what to do with old but probably functional tubes; I don't need them but I doubt they'd get any attention on ebay and locally there's a store that has quite a large stock, brand new and warehoused for probably 40 years.

  5. You could recycle the innards by putting SMD orange LEDs up where the heater used to go, that would be neat.

    Also, make a "USB vacuum pendrive" but with orange LEDs and blue for when when its really being thrashed :-)
    For extra points have a little bargraph so the amount of glowiness changes depending on usage.


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