Artisanal Vacuum Tubes: Hackaday Shows You How

Homemade Vacuum Tube
Homemade Vacuum Tube

About a decade ago I started a strange little journey in my free time that cut a path across electronics manufacturing from over the last century. One morning I decided to find out how the little glowing glass bottles we sometimes call electron tubes worked. Not knowing any better I simply picked up an old copy of the Thomas Register. For those of you generally under 40 that was our version of Google, and resembled a set of 10 yellow pages.

I started calling companies listed under “Electron Tube Manufacturers” until I got a voice on the other end. Most of the numbers would ring to the familiar “this number is no longer in service” message, but in one lucky case I found I was talking to a Mrs. Roni Elsbury, nee Ulmer of M.U. Inc. Her company is one of the only remaining firms still engaged in the production of traditional style vacuum tubes in the U.S. Ever since then I have enjoyed occasional journeys down to her facility to assist her in maintenance of the equipment, work on tooling, and help to solve little engineering challenges that keep this very artisanal process alive. It did not take too many of these trips to realize that this could be distilled down to some very basic tools and processes that could be reproduced in your average garage and that positive, all be it rudimentary results could be had with information widely available on the Internet.

Easy enough to make in your garage

Here is a typical construction process I utilize in my garage. I would like to point out that these processes include glass, thin metal, mica, and sharp tools that all represent cut hazards. Additionally extreme temperature in the form of hand torches, and R.F. induction bombardment are utilized. With all industrial processes extreme care must be taken in any endeavor. I would also like to point out that no mercury or other harmful materials are used in any of these processes. The primary materials are Glass, Nickel, Mica sheet, and tungsten wire. I.P.A and acetone are used as cleaners and a very small amount of barium is used in the getter.

Nearly all connections within a tube are of a welded variety, and spot welding is the quickest and most economical. I simply modified a cheap store-bought spot welder to accept small diameter welding rods and used a 10 amp Variac to control weld temperature. I connected a spring loaded board to use as a make shift jaw closer pedal.

Above on the right is a typical weld. I use Nickel as it has favorable properties for both vacuum use, and tube electronic characteristics. It also is easy to weld and does not oxidize readily under welding conditions in air, as long as the weld time is kept to a minimum. Also shown here is an exothermic evaporable getter ring. This contains a barium/aluminum alloy that evaporates by applying an induced R.F. heating field and adsorbs gas after the tube has been sealed to maintain a reasonable vacuum level.

To the left is what is called a Cage, with one half of the anode not yet attached. The entire structure is composed of Nickel sheet, wire, and mesh. .015 Clear mica, sheared to shape by a standard office scissor shear is punched by means of an arbor press, a block of wood, and a small steel pin. The anode is a sheet of .010″ Nickel formed around a small block of wood. The grid is made of soft temper nickel mesh that has been formed around a block, and welded to two nickel rods for support. Traditionally Molybdenum or nickel plated steel would be used for grid wires, but both materials are hard to work with and weld without special tools. To the right is the completed assembly with the second half of the anode welded to complete the cage.

 Putting it all together

img_2475-croppedHere the Cage has been welded to what is called the Stem and this is turn is called a Mount. The stem is the portion that contains the hermetic glass to metal lead seals and the exhausting tubeulation for connecting the tube to the vacuum system. This particular tube uses a commercially available Stem. They can be made by hand, however is is very difficult to get good results that hold up to use. This is the most demanding and critical part in the tube and there are many electronics glass manufacturers in the U.S. who can provide these parts to drawing, and those who wish to hand craft their own would do well to watch the YouTube videos of Ron Soyland who covers the topic. The tungsten filament is also threaded through the cage and spot welded to the stem leads at this point. This tube uses .0085 diameter 1.5% thoriated tungsten.

Here the fit of the Mount to the glass bulb is checked. The mica spacers should come in contact with the glass bulb to minimize vibration in the cage. The next step is to trim the excess bulb material and seal the bulb and mount together. This is done by placing these parts in a specially made lathe with two headstocks that turn in unison. Many shops would have made their own machines as I have done myself. However, in this picture I am using a commercially available Lathe.

Avoiding breakage from thermal shock

img_2227In large-scale production, carousel style sealing machines were typical. Generally you would anneal the tube after sealing. Sometimes you can flame anneal the glass without the use of an oven, but this is something that can only come with practice and experience. A better solution is to use borosilicate glasses. These types have greater resistance to thermal shock and are much easier to work with. Unfortunately they are far more expensive as they are typically reserved for extreme use. Large transmitting tubes and vapor style lights typically contain these sorts of glasses to handle extreme temperature variation and not damage the hermetic metal lead in seals.

Evacuation of the tube

img_2479

The tube is placed into the vacuum system. This image shows a typical compression port containing a compressible gland. A high vacuum pumping system contains a mechanical rotary vane pump backing either an oil vapor diffusion pump or a turbo molecular pump followed by a refrigerated trap, and then the manifold the tubes are connected to. This will also typically contain a Ionization gauge, pictured on the upper portion of the picture. The analog gauge indicates fore pressure on the diffusion pump. Typically diffusion pumps require a fore pressure of 100 Millitor or micron to begin pumping action. One micron equates to 1.32 X 10-6 atmospheres of pressure. The ultimate pressure of a well built single stage diffusion pump with no leaks and a refrigerated trap is well below 1 X 10-7 TORR.

In vacuum tube practice however this is difficult to reproduce and only attainable in very small batches with long bake out times and extreme care. In large volume production if the actual pressure in the tube at tip off (The melting of the exhaust tube and removal from the pump) occurs at a pressure in the 10-4 range it is considered a good vacuum. It is then the responsibility of the barium getter to attain the final operating vacuum level. Higher levels of vacuum require more pumps and better practices than can be covered here.

Pumped tube waiting to have a base installed
Pumped tube waiting to have a base installed

The skilled do it yourselfer need not fret. It is possible to have acceptable results for experimental tubes using only a dual stage mechanical rotary vane pump with a refrigerated trap. However, care must be taken that every attempt is made to outgas the tube with R.F. bombardment and baking out the tube in an oven placed over it while on the pump for about an hour at more than twice the expected operating temperature. Your final pressure at seal off should be kept to below 20 micron and a large volume of getter material should be used. Additionally the tube should be flooded with nitrogen or argon gas and heated prior to evacuation to reduce the average quantity of oxygen in the tube. Many of the very first amplifier tubes were of this type. The resultant tube will emit, but ionization current will occur as the anode voltage increases. Typically this sort of “Soft” tube has an upper voltage range of 60 to 90 volts. As soon as a vacuum system is on and the tube being pumped is known to be leak free, the filament is lighted and remains lighted through the remainder of the exhaust process. This keeps gas molecules liberated from other metal parts from condensing on the filament and adds additional heat to remove gas from other surfaces. Shown to the left of the tube is a coil for R.F. induction heating of the internal metal components of the tube while on the pump. This also serves as the heat source to flash the getter, producing the mirrored surface and maintain a proper vacuum pressure. Generally a receiving tube will operate in the 1X10-6 range after the getter has had a chance to sorption the remaining gas in a commercially produced tube.

Conclusion

You have to remember that in the early days of electronics many of these processes were developed by observation of empirical evidence. It was several years before a serious scientific study of thermionic emission was begun in earnest and many important discoveries in the production of these devices were made by individuals working with little more than basic tools. Some of these discoveries were made purely by accident. There is no particular reason one cannot simply pick up where the innovators of the teens and twenties left off before tubes became big business. Don’t forget, Menlo Park was nothing more than a ramshackle building, and [de Forest] labored in a dark corner of Federal Telegraph when he developed the Triode. The particular type of tube I build is defiantly crude. Its max plate current is only a few milliamps, it’s gassy, and its gain is pitiful, but it works. It’s a reminder that at one time somebody tried something, they were not really sure how to do it and they had no solid plan of how, but they succeeded. They found a way forward and eventually crafted an entirely new area of science an industry. I think I will go play around in my garage a bit more now.

Resources

There are a few videos and websites on the net that show in excellent detail some of the finer points of tube production.

About Charles Alexanian

Charles is production manager at Alex-Tronix and also in charge of new product development, but he gets bored easily so he moonlights at M.U. Incorporated fixing vacuum pumps and tube test gear. He is also an on call technician for the most downloaded podcast on earth and several recording studios because he hates free time… I mean he really hates it. If he were to ever actually have any free time he would most certainly consume it in one of his many hobbies like: more electronics projects, prospecting, or HAM radio.

66 thoughts on “Artisanal Vacuum Tubes: Hackaday Shows You How

          1. Can’t know if the other person was trolling or not, but they got a reaction. Yes all the work was great, but like all art the finished product isn’t going appeal to all effort if the appreciate the skill, that’s why someone would say “a few”.

    1. This is pretty much the definition of artisanal. This involves far more hand-working, time, and skill than some dude putting store-bought lavender into a jar of epsom salts and calling it “artisanal handmade bath salts”.

    2. According to Mr. Webster, yes really.
      1
      : a worker who practices a trade or handicraft : craftsperson
      2
      : one that produces something (as cheese or wine) in limited quantities often using traditional methods

  1. The level of talent that exists on Hackaday is astounding. Making a working vacuum tube is one of those things I would never imagine an individual could do by themselves in their garage.

    1. I gave up commenting long ago that simply ham is sufficient, no one hears me. While I’d like for someone to give me both a functional Kenwood TS 520 or a Yaesu FT 101, they can keep their tube color TVs. Now that I’d like to pick an all American five or two, none are showing up at the auctions and garage/ yard sales in this area.

  2. I have a vacuum pump that causes water to boil at 80°F is there a way to tell from that how much vacuum it pulls? I have always wanted to make a homemade vacuum tube. I actually made one but never sealed it, I thought about sealing the glass tube with epoxy as I don’t have a way to make a glass plate for the bottom.

  3. It’s there any XRAY risk if you do this wrong? I’d be interested in making some custom vacuum displays, but not of i run the risk of sterilizing observers.
    (I’m a programmer, not a physicist)

    1. I too am a programmer, not a physicist. But I do know that X-rays are produced when high energy electrons suddenly decelerate. To make that happen efficiently requires both high voltage to impart the needed energies, and high vacuum to avoid energy loss to due electron collisions with gas particles before they strike the hard target.

      Tubes intended to produce X-rays typically operate at no less than 20KV. If you’re producing custom displays, you’ll be operating nearly two orders of magnitude lower. The vacuum quality achieved in a typical home shop will also be similarly lower – maybe much lower, if you’re intentionally introducing a fluorescent gas for your display. Any trace of X-rays produced will be so “soft” (meaning little ability to penetrate) that they should be absorbed easily, not escaping the glass of the tube itself. Even if some still escape, exposure is a function of both distance and time; it’s not common practice to view a display for hours on end at point blank range.

      So my opinion is that it’s *very* unlikely you’ll create a hazardous X-ray source by accident. Bordering on impossible, really.

      1. Good to know, thanks! I only knew that xrays were made in vacuums with electricity, never looked into it much deeper than that. I didn’t think it was an issue, but I suppose you should always ask. “But don’t do that” is not something you want to learn the hard way.

    2. I can add to the previous response and say that there is no risk of X-rays.

      There’s a government website (that I can’t find ATM – sorry) where you can type in the medium and X-ray photon energy and see the absorption coefficient.

      When you do this for air and 2000 volts, the absorption is something like a factor of 1000 over 10 inches. Higher absorption rates for lower voltages.

      The X-ray production efficiency at these low voltages is low, so very little of the tube energy will be converted into X-rays. Then the X-ray photons have to get through the glass envelope, then they get diluted by the inverse square law (the energy gets divided over progressive larger areas the further away you are from the tube), then they are absorbed by air at a high rate. Also, metal inside the tube will completely block them.

      In summary: at typical electron tube voltages (well under 2000 volts), there is no risk from generated X-rays at all.

      The original problem with vacuum tube X-rays was the high-voltage TV picture tubes. These were driven at upwards of 15 kV (which are *not* as well absorbed by air), and and kids could be lying in front of the TV for hours on end &c &c.

      This is why there is so much lead in a CRT display and why we have to pay a premium to recycle old TV’s: the lead is in the picture tube glass face (lead glass), and helps shield the X-rays.

      1. I am both a programmer and physicist. X-rays are not easily abosrbed by air. X-rays must either lose energy or decay. The body absorbs certain wavelengths of x-ray extremely well, and you can get 2-5 rads from a single CT scan, yet .002 rads from a gamma infused radiospectroscopy. The important thing is to find both the curries and the possible rads, which takes a lot of testing and measurements. Grab a textbook for physics that is directed towards those going into this type of medical field, and it’ll explain a lot, and help out. Call a good college that offers the course, act your a student taking X class, and you need to find the name of the textbook for X class, they’ll give the name, then you go to amazon buy it, read it, and perform the same experiments, do the homework, take the tests, and when you finish its like you took the class all for the price of the book, and the little tools and such on the side, guarenteed way cheaper though. This is called autodidact learning, and I created this system, but it works exremely well, the only thing lacking is experiencal type knowledge, and body language associated with the expression of said information, but with youtube, youu can usually find that information to acheive a reasonable degree of replicacy.

  4. Some notes about vacuum. A cold trap goes between the device being evacuated and the diffusion pump. It passes though a bath of liquid nitrogen or a dry ice/acetone mixture to help prevent back streaming of pump oil into the tube you are evacuating. If you use a turbo pump you do not need a cold trap, there is no oil to back stream. Another option is a sorption pump, it is a stainless flask filled with zeolite. This is immersed in liquid nitrogen and the gases from the envelope are absorbed in the zeolite. After pumping the sorption pump is heated and a rotary pump is used to remove the trapped gasses. You can find these on eBay pretty regularly.

    If you do use a diffusion pump like this consider using Santovac 5 as your pumping fluid, it is expensive but unlike the DC700 series of pump fluids it will not create a non-conductive coating in the tube if something fail or gets by the cold trap.

    When building a vacuum system you need to be careful of the materials used in the system. In the picture above he is using a copper manifold with the ion gauge attached by a piece of gum rubber vacuum tubing. The copper is OK, not the best but the real problem is the solder. A solder needs to be selected that does not have any components that will outgas in a high vacuum and then there is the issue of any pinhole creating a virtual leak. The rubber hose is not a great idea either, it is intended for roughing level pressures, not UHV. If he went with tig welded stainless he would probably get down to pressures below 10^-6 a whole lot easier than he is. You can get used vacuum fittings off eBay as well.

    I would love to have a glass lathe like he has above, they are terribly expensive, though. A used litton is $5k to $10k used.

    1. Thanks. I tried to keep the article to a reasonable length. I was pushing 1700 words as it was. I tried to use a cobbled together system out of parts laying around for the sake of the article and what somebody might come up with on their own. Old leak testers are a good source for parts, and can typically be easily converted to this sort of use. Particularly the ones from Veeco. The gum rubber tube is far less than ideal, and copper pipe is not much better, but it will do for this sort of thing. A far better option is KF style fittings and parts. It is basically an erector set for vacuum systems and parts are available on Ebay from Taiwan and eastern europe at reasonable prices. Sorption pumps really are the best for cleanliness. Turbo pumps are good but very costly. If I were starting from scratch I would most likely go the used leak tester route. They typically come already mounted to a cart and have most of what you need. As for induction heaters I am awaiting test results from those not nut and bolt heaters. The frequency is a bit low but they think they can make coils that could heat tubes and getters. They are relatively inexpensive and hand held. I have a 2KW Lepel heater (Tube type) mounted to a cart I roll around.

    2. Oh, I forgot to mention. I also have a lathe I built myself for sealing. I got all of the parts for it online and put it together for about $500 over a weekend. Its not precision like that litton but its goof for sealing.

  5. This was a really fascinating story indeed, kudos to the author for writing it! Making a vacuum tube (VT) by hand is definitely a labor of love for the craft.

    My question is though, how come there’s a company that still makes VTs??? In one of the datasheets it mentions that they are approved for use by the DoD. Could that be a relic of times long past when the DoD had equipment with VTs and bought them? Or after so many years there still some piece of equipment, hidden in some lab who knows where, that needs to be up and running and the DoD maintains it? Possibly we’ll never know…

    1. You’d be surprised. :-) The gov’t purchasing structure helps a lot too… there’s some serious inertia in bureaucratic systems when it comes to things like availability and quantity.

      I expect a lot of the continued interest/need comes from the fact that tubes are EMP resistant. Having said that, I expect other commenters will fill in the gaps about the degree of EMP resistance, the need, the options, etc… I’ll leave that discussion to them… that’s how this usually goes when it comes up.

    2. Imagine an old piece of government owned equipment that is still needed today, that requires equipment of the same age to maintain. When faced with redesigning the support equipment at a cost of millions, or supporting a small business who still makes the old parts, the choice is obvious. Also, I thoroughly enjoyed this article. Nice job!

    3. There may still be some smaller airport communications and radar systems using vacuum tubes. I read articles around when the idea was first proposed to transition ATC to using GPS and direct routing that it would take years and many millions of dollars to upgrade all the airport systems in the US for that, mainly because many airports were still using 1960’s and 1970’s systems.

      M.U. Inc could also be making tubes for audio amplifiers, for that warmer, not so hard edged sound.

    4. Don’t forget about guitar amps, this is probably the biggest consumer of VT’s. Tube amps are so sweet sounding that veterans will only play through them. I used to have an Orange Rockerverb and it was the best sounding amp I’ve heard (opinion only), now I play a Vox AC30CC2 and it is amazing. The big named bands that play live shows enough replace their tubes every 2 years.

    5. If I had to lay money on something, it would be RADAR and similar technologies. The real difference between a modern radar set and a legacy radar set isn’t the radar emitters and receivers (though there are some options there that are SOMEWHAT more modern), it’s in the ATTACHED electronics, which are easy enough to replace if you’re an EE. Consequence? There’s a lot of legacy radar equipment that gets maintained and upgraded, but still uses vacuum tube emitters and receivers.

  6. You can also just put a system together using swagelok fitting and stainless tubing. I have had UHV tight connections with standard fittings. You could use an o-ring in the place of the ferrule to seal the ion gauge to.

    My big vacuum system (2000l/s turbo) uses almost all KF fittings, it is good enough to get down to 10^-8 without much issue and still needs to be baked out. I have a little 70l/s turbo pumped RGA/leak checker on a cart that I have considered doing what you are doing with, that is all conflat.

    Turbos can be expensive but it you wait around long enough you can find deals. A couple years ago I picked up two Leybold 50l/s turbos for $99 each as well as the controller (Which I fried.. oops.) Even now there are deals for turbo pumps that are intended for mass specs, they will require a special housing to be made to hold them, but they are pretty cheap.

    Lower frequency for heating the tube is fine, the lower the frequency the deeper the heating. There are some designs over on 4hv.org that might be a bit overkill, myself, I might try to use my 2.5kw AE 450khz supply, that is real overkill.Those bolt heaters do look really cool.

  7. Awesome!

    Years ago, when I wanted to buy giant nixies that I couldnt afford, I thought, I know glassblowing, machining, and some metallurgy, why can’t I make a nixie using heavy walled borosilicate glass?

    Spent months reading on vaccuum tube creation. Came to the conclusion it was not terribly difficult, but the hard part was the glass to metal seals. I believe here you call that disk, with metal bonded through it, including the
    vaccuum port that gets melted shut, the cage.

    All I ever concluded after lots of research was that the coefficient of thermal expansion for the glass type used must be very close to the metal used, ie, the metal and glass should expand and contract nearly at the same rate, so that when melted together, they stay together in basic temperature fluctuation, otherwise, they crack apart, and you lose vaccuum, and your filiments melt.

    I remember that I wanted to absolutely use borosilicate glass, for super heavy duty tubes. I remember a metal called kovar that matched boro glass’s coefficient of expansion close enough that it was used in comercial tubes.
    Problem is, kovar isn’t something you can normally buy, and nothing else I could find would work.

    So if you really want to do it all, every single piece, every one, from scratch, how DO you do the glass to metal seals for a handmade cage? What metal do you use that lasts? How do you get it clean enough that when you melt the glass around it, it will bond, and truly join? How do you do the hardest part then, make the cage?

    Everything else seems pretty straightforward. One of the best things I’ve ever seen here, amazed someone else was crazier than me and really does this!

    1. Kovar is just one type of glass sealin metal. The metal is easy to get. The glass is a different matter. Most of the distributors are the companies who actually make the stems and bulbs themselves and they will dance the craziest dance to sell you finished components rather than the raw glass tubing.

    2. Dumet is used to make feed throughs in soft glasses and that is used in most light bulbs and vacuum tubes.

      For hard glasses you can use tungsten. You heat the tungsten in a flame and it will get an oxide layer. As I was taught, it is Kentucky Fried Chicken brown. UA piece of uranium doped glass is then slid over the rod and heated toll it melts and forms a bead around it. This is then sealed into whatever piece of glasswork you were going to make. U glass is getting a little hard to get but you can still find it. If you look at old power vacuum tubes you will notice the feedthroughs have a greenish yellow tint. You can scavenge the glass and electrodes from these.

      From looking at the link that hyratel posted you can do it without U glass for small wires.

      You can buy lots of different kind of wires here, including kovar:

      http://www.sisweb.com/ms/wire.htm

      1. Thanks to all who replied to me- the materials source sites are amazing- I’ve been looking
        for some of those materials in vain for years. I seen ever Iridium rod is obtainable…
        assuming if I were to make a large nixie, say, a FOOT high, I’d need large leads, and the
        normal sizes of wire wouldn’t handle the current.
        Would using uranium doped glass allow bigger wire diameters to seal for truly large size
        wires (say 1/4 inch)? Tubecrafter’s site doesn’t mention anything with uranium doped
        glass as far as I’ve found. Kovar won’t work with pyrex glass though I see, even though
        I can buy it now.. I’d need specific glass to seal with it.

        1. I dont know how much current you think a bit nixie would take but a 1/4″ tungsten rod would probably handle a couple hundred amps. Even a giant nixie would probably not need more than 30ma per digit at an extreme and for that all you need is just a standard class feedthrough. Considering the filament on one of the most common tube around, the 12ax7 passes 150ma through the heater pins you have nothing to worry about.

          If I were to make a giant nixie I would forget the standard nixie design and have neon tube bent and made and stack those up under a bell jar. I think someone had done that. Building a nixie traditionally in that scale would be incredibly expensive.

        2. The arc current of a Nixie tube is not much. A .100 tungsten pin seal in nonex glass (old glass. Newer types are avaliable now) can handle about 20 amps of current before threat of cracking. Moly pins can handle more. Kovar about the same. I am sure the stems or presses could be made to handle whatever current you would need. It may be very expensive, but for just doing it sake its not a difficult thing to do. The bulb of suitable glass in that size would be very very expensive. Sealable glasses in production quantity are $40 a pound or so and you have to take a whole lot of it.. Thousands of pounds.

          1. To Macona & Charles-

            It’s been so long I’ve forgotten the current math, and quite possibly hadn’t worked
            it out that far as the build research on seals stopped me.
            In essence, I was now ignorant of these facts- thank you both for the education.
            I had wanted to do 1 foot or so tall by 6″ diameter heavy glass wall borosilicate base
            glass nixie tubes, and do the grids with a cnc laser cutter, and the digits with bent iridium
            wire for extreme longevety, as I remember reading somewhere some Russian
            military tubes had done. All the inner cathode & annode I had wanted to make using
            precious metals for ultimate longevity. Yes, hundreds of dollars per tube, but not many
            tubes. The idea was for electro digital readouts in my shop to a lineshafted milling machine,
            for a somewhat retro-victorian scientist’s workshop theme I had wanted to do.

            I know, I’m crazy. It’s ok, I think, because I know I’m crazy. Isn’t that how it works? :)

  8. Took a further look at Ron Soyland’s site- amazing in spades! I see now even Kovar wouldn’t have worked for me, unless I had a specific special boro glass type…this wasn’t around way back when I did my research.

    I cannot believe the things I’ve learned of thanks to Hackaday. I’m utterly speachless at all the coolness people here bring on a daily basis, and I’ve been reading daily here for over 10 years now, since the very beginning.

    Looks like I’m going to get to making my giant nixie tubes afterall :)

    1. Frank. Do you work for Amperite? Last I heard you guys had moved production and were going to set up again. Did this not happen? If this is the case we can propably help. For you I would recomend Richland Glass or Fredricks glass, both in new jersey. If you are still having trouble reply and we will find a way to get in contact.

  9. Wow, this has been a helpful post. I’m trying to make an electron tube (well basically) directly in a vacuum chamber, no fancy glass work. Instead, I’ve got a thoriated tungsten filament (the cathode if I’m understanding this right) glowing at temperatures such that it should be emitting and a sharpened piece of NiCr wire nearby, <1mm. With 1000V on the NiCr, that's my anode. Does anyone here have an idea about how I could measure the emission current (the amount of electrons coming off the tungsten and through the NiCr)?

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