Analog Angle Blog

I Still Have a Vacuum Tube in My House?

Mention vacuum tubes in consumer products and it sounds like a time-machine statement; didn’t they go out with disco and bell-bottom pants? Hard to believe but true: you most likely do have a vacuum tube in your home. No, I’m not talking about that now-obsolete glass CRT TV you may have tucked away somewhere even as you replaced it with a flat-screen TV. Nor do I mean those high-end audio amplifiers for audiophiles who feel that vacuum tubes give better sound, as they are only a very small segment of the population.

Instead, I am referring to the resonant cavity magnetron in your microwave oven (see a “how it works” here), Figure 1 . Yes, it’s true, there’s one of these ancient devices in that oven, generating the moderate-power energy at 2.45 GHz that excites the water molecules though resonance and absorption, to heat them and the food.

Figure 1

A basic magnetron schematic and principles of operation; understanding of electromagnetic fields, energy resonance, and energy transfer is needed for even a basic qualitative discussion. (Source:  HyperPhysics from Georgia State University, Department of Physics and Astronomy)

A basic magnetron schematic and principles of operation; understanding of electromagnetic fields, energy resonance, and energy transfer is needed for even a basic qualitative discussion. (Source: HyperPhysics from Georgia State University, Department of Physics and Astronomy)

Why stay with a magnetron, when a large fraction of commercial and industrial RF energy systems of comparable power have gone solid state via LDMOS and GaN technologies. An interesting article “RF Energy Systems: Realizing New Applications” in the December 2015 issue of Microwave Journal (free, but registration required) explains the situation in detail. The gating items are no surprise: performance and price. According to the author, the initial goal is to have a 2.45-GHz solid-state amplifier with output power of 300 W and efficiency of at least 70% and at a cost of $12; the next goal is higher power in the 600-to-1000 W range.

Changing over from the magnetron to a solid-state design is not simple drop-in form/fit/function replacement of one component with another. There are many electromechanical and RF considerations to be understood and resolved, as well as issues of control, reliability, and other factors. Even if you don’t care whether you low-cost home microwave oven has a magnetron or a solid-state source, it’s interesting to see what such a switchover really entails along so many perspectives. Recently, Ampleon (the 2015 spinoff of NXP Semiconductors) and Midea (a Chinese consumer-products company) announced a 200-W oven, the result of a year-long collaboration into the use of solid-state technology for compact oven design (see here for press release), but details are vague.

Should we miss the magnetron, when it's time to go finally arrives? Probably not, this is the nature of progress. But perhaps we should at acknowledge its history and role. The 10-cm (what we now usually refer to as 3 GHz) cavity magnetron was initially developed in the UK during WWII, and hand-carried as a “top secret” to the US for design refinement, system integration, and volume production. (If you are interested in the history of the magnetron and radar's role in WWII, check out this online IEEE article “The Cavity Magnetron: Not Just a British Invention” or Robert Buderi's very readable book “The Invention that Changed the World” (Figure 2 ); of course, there are many other histories available.) It was this highly guarded key component which enabled the construction of high-resolution compact radar, which made such a huge difference to the war effort—especially radar sets which weighed “only” a few hundred pounds and thus could be fitted into aircraft.

Figure 2

'The Invention that Changed the World: How a Small Group of Radar Pioneers Won the Second World War and Launched a Technological Revolution' by Robert Buderi is one of many available on the development of magnetrons, radar, and their vital WWII role--and beyond (Source: Simon & Schuster)

“The Invention that Changed the World: How a Small Group of Radar Pioneers Won the Second World War and Launched a Technological Revolution” by Robert Buderi is one of many available on the development of magnetrons, radar, and their vital WWII role—and beyond (Source: Simon & Schuster)

When the magnetron is replaced, that will pretty much eliminate the last vacuum tube from the typical home, but it doesn't mean that vacuum tubes are totally gone from our industry. They are still used in high-power broadcast stations, of course, although solid-state devices are now standard for transmitters into the tens of kW. A specialized tube called the traveling wave tube (TWT) is still the dominant amplifier used in communications satellites due to its combination of electrical and mechanical performance and reliability/ruggedness characteristics (thermal and radiation). Of course, vacuum tubes are still the only source of many of the specialized particle and energy beams used in high-energy physics.

There's still some serious vacuum tube work being done according to some recent articles in IEEE Spectrum. “The Quest for the Ultimate Vacuum Tube,” explored the fascinating R&D work being done now to enhance the TWT and leverage some of the latest solid-state techniques to improve it further, while “Introducing the Vacuum Transistor: A Device Made of Nothing” looked at merging the vacuum tube and the MOSFET.

Have you ever worked with vacuum tubes? Do you miss their ethereal, primitive warm glow or inherent ruggedness? What do you most like or dislike about them?


My Antenna Dilemma: Preamp or Passive?

Is There Still a Vacuum Tube in Your Future?

Forget about 10 GHz—let’s aim for 100+ GHz

15 comments on “I Still Have a Vacuum Tube in My House?

  1. swarajsoni
    April 23, 2016

    good post

  2. traneus
    April 25, 2016

    Besides the magnetron in the microwave oven, vacuum-fluorescent displays are still reasonably common. These can be recognized by the glowing segments or icons (these are the anodes) being visibly recessed behind meshes (the grids) inside the display glass housing. Sometimes thin orange lines are visible in the front of the display: These are the filaments. Look at one of these displays when it is dark, with all segments off. Turn off the room lights at night and look at it in the dark, and you will see odd points of light where bits of the phosphor-coated anodes are receiving stray electrons.

    I learned electronics on vacuum tubes, starting in the late 1950s, and I still have affection for our thermionic friends.

  3. RadioGraybeard
    April 27, 2016

    I've been following the solid state power industry targeting microwave ovens with interest because it strikes me as an almost absurdly price-sensitive marketplace.  Consumers are used to using a microwave until it breaks down and then tossing it for the newer model, so they might not prefer the longer lifetime of the SSPA.  

    By the way, in this home, there are many vacuum tubes, but that's a deliberate move and I'm perfectly happy with being 6 sigma (i.e., nowhere near normal).


  4. traneus
    April 27, 2016

    RadioGraybeard, how do the solid-state microwave power amplifiers compare to the magnetrons in efficiency?

    I'm sure you remember the 6AF4 miniature triode used as the local oscillator in UHF analog TV tuners. The 6AE4 mini-magnetron was intended for this use, but never made it into production.

    In high school, i made a magnetron by placing a rod permanent magnet next to a 1AG4 subminiature pentode. The magnet clung nicely due the iron in the tube's elements, and greatly reduced the plate current. I don't remember observing the negative-resistance effect.

  5. RickR
    April 27, 2016

    My admiration for the vacuum tube is twofold, one theoretical and the other existential.  The theoretical appeal relates to the sheer beauty of Classical Electrodynamics (referring to the integral form of Maxwell's Equations) which delves into the smooth, infinite, and ethereal world of fields and waves.  The second is the existential labor of love required to produce such a delicate and meticulous construction of grids, plates, cathodes, and getters encased within a crystalline dome.  Compare that to the practical world of matrix math and silicon slabs and you're left with woeful pangs of barren sterility.

    April 27, 2016

    Vacuum Tube = Light Emitting Field Effect Transistor

  7. traneus
    April 27, 2016

    Magnetrons and other vacuum tubes will always be around for high-power high-frequency applications. Higher frequencies require smaller structures and higher electron energies, which lead to higher power densities. The vacuum can survive much higher power densities than crystals can withstand. Free-electron lasers can be thought of as magnetrons generating coherent X-rays by using rows of magnets to wiggle (as in slalom skiing) high-energy electron beams.

    Not mentioned in the illustration about how magnetrons work, is that the magnetic field curves the electrons' paths, so more electrons go to lower potentials. This creates the negative-resistance effect which drives the oscillations.

  8. RadioGraybeard
    April 27, 2016

    traneus, I'm regrettably clueless about magnetron efficiency.  My specialties were receiver and PLL design, but I was around the development of several PAs in that frequency range.  I think 35% is doing good for an SSPA at 2400 MHz. 

    I do recall the 6AF4 and the Nuvistor tubes for the UHF converters, but haven't heard that name in decades.  Those are long gone from this house.  I have two vacuum tube ham rigs and several of the vacuum tube Transoceanics with their famous 1L6 tubes.  (and all low-voltage tube lineup)


  9. jtml
    April 27, 2016

    Industrial and applicance magnetrons have been perfected to reach very high electrical conversion efficiencies in the 80s (%). Solid state amplifiers at S band are not there yet. One example is the new NXP MRF7S24250N LDMOS part at 55%, 250 watts. Not needing linearity, they can be pulsed (like magnetron) on and off to use duty factor to adjust the average heating power. There is a lot of work ahead to make comparable power efficiencies for heating food. But the convenience of potential longer life and simple DC power converters (no HV) are in favor of SSPA eventually.

    Some of us EEs still design and use very high power VED (vacuum electron device) amplifiers in scientific and industrial applications, where it would take several thousand transistors, combiners and power supplies, to develop the same RF power levels as one vacuum tube.

  10. David Ashton
    April 27, 2016

    …as I call them, being of British descent, are great.  I cut my teeth (and set them chattering when I contacted the HT 🙂 on them in the late '60s – I had a series of valve radios to listen to far-away AM and short wave stations. And I had one of the venerable GEC 912 amplifiers with EL84 (6BQ5) output pentodes.  I still want to make a nice stereo amplifier with a pair of EL34 (6CA7) pentodes per channel – 30W. Their warm glow is always reassuring – a kind of “on” light if you will.  Transistors are fine – it's easy to make current flow through something solid.  But Tubes – making current flow through a vacuum?  Much nicer trick!

  11. glen.herrmannsfeldt
    April 27, 2016

    For radio transmitters, with modulated RF output, replacing magnetrons (or klystrons) with solid state devices makes some sense.  You might, for example, be able to modulate faster or in a different way.


     For microwave ovens, I don't see why, unless it is cheaper or a lot more efficient. With normal use, does the magnetron ever wear out?   The microwave oven we have is over 30 years old.  (Yes, I have thought about replacing it with a more powerful one, but otherwise it does what it is supposed to do.)   New ones seem to be 1100W, close to the most you can get from a normal 15A plug. 


    Early microwave ovens had cautions against running them empty, but as well as I know, for years now they have enough protection.  How hard is it to protect solid state RF sources against unfavorable loads? 


    Seems that our library has the book mentioned, but the only copy is checked out.  I put a hold on it. 

  12. Jean-Luc.Suchail
    April 28, 2016

    I still have more than 650 tubes in my private stock. Ranging from old octal to repair vintage radios to several kW RF transmitter tubes. I even bought a few original from Philips some months ago. Their ability to withstande severe mismatch conditions in transmitter systems a rally apprciated !

  13. jimfordbroadcom
    May 18, 2016

    @David Ashton – I'm trying to find your post recently in which you mentioned labor unions.  I suspect we may have a difference of opinion there.  And that's OK!  Could you please point me in the right direction?

    Sorry to digress from the vacuum tubes/valves; I sure love their sound, although the only audio application I have, a tube guitar preamp I built from a kit, got zapped a few years ago while I was probing it with a scope, and I've not had the time to fix it yet.  As I recall, I was trying to find a way to boost the plate voltage of the 12AX7 beyond the “starved” value of 45 V to around 250 V, its maximum rated voltage, to avoid clipping.  Tube clipping sounds just as bad as solid state clipping!

  14. David Ashton
    May 18, 2016

    @Jim…sorry, I'm at the stage where the hair has stopped growing on my head and started growing in my nose and ears….and the memory loss that comes with it.  I am really busy today but will try find it for you.

    Unions…well I am in a union, and with our idiot management we'd be tons worse off if we were not.  It's a long story best discussed over a beer – or 3 – but the electricity industry in which I work is being squeezed (by the politicians of course) at the moment and management have taken the adversarial stance instead of negotiating.  So even when it does get resolved we'l have zero morale. I should say that I came to work for this company because of the great atmosphere it had.  Which is long gone.

    Anyway, until we can get together over a beer, I have to get back to the salt mines….


  15. antedeluvian
    May 18, 2016


    well I am in a union, and with our idiot management we'd be tons worse off if we were not.

    I have a friend who is a professor at a business school. He says: “A company gets the union it deserves”. Sounds about right.


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