The History and Physics of Triode Vacuum Tubes

The triode vacuum tube might be nearly obsolete today, but it was a technology critical to making radio practical over 100 years ago. [Kathy] has put together a video that tells the story and explains the physics of the device.

The first radio receivers used a device called a Coherer as a detector, relying on two tiny filaments that would stick together when RF was applied, allowing current to pass through. It was a device that worked, but not reliably. It was in 1906 that Lee De Forest came up with a detector device for radios using a vacuum tube containing a plate and a heated filament. This device so strongly resembled the Fleming Valve which John Fleming had patented a year before, that Fleming sued De Forest for patent infringement.

After a bunch of attempts to get around the patent, De Forest decided to add a third element to the tube: the grid. The grid is a piece of metal that sits between the filament and the plate. A signal applied to the grid will control the flow of electrons, allowing this device to operate as an amplifier. The modification created the triode, and got around Fleming’s patent.

[Kathy]’s video does a great job of taking you through the creation of the device, which you can watch after the break. She also has a whole series on the history of electricity, including a video on the Arc Transmitter which we featured previously.

19 thoughts on “The History and Physics of Triode Vacuum Tubes

    1. Back in the 1970s, I used to pass daily by a marker at the corner of Emerson and Channing Streets in Palo Alto, California, that declared this was “The Birthplace of Electronics.”
      “Description: Dedication of California Historical Landmark No. 836 plaque commemorating the “Birthplace of Electronics” at the corner of Emerson Street and Channing Avenue, Palo Alto, 20 June 1970, where Lee de Forest developed the first vacuum tube amplifier and oscillator while working for Federal Telegraph Company.”

      It is still there:
      [facing away from the road]

    2. That’s pretty interesting. Not exactly what we think of as vacuum tubes, but fundamentally the same. Tiny ones etched in silicon cavities.

      Terahertz technology sounds really fascinating. I think the example of using it to enable the TSA to scan for drugs is pretty lackluster, but fair enough. Has anyone considered building a processor with them I wonder? Would that work or would it have major issues?

          1. Naughty bits, or drugs, why is the American TSA even empowered to perform these scans?

            Their mandate is to prevent weapons from being brought onto airplanes, is it not? Why would they bother searching for drugs? Even better question: why do YOU (the American taxpayer, or flying public) even LET them?

    1. I normally ignore such “did you know” or “how stuff works” type videos as they are usually a cookie cutter rehash of another persons work, oft skipping important details due to a lack of understanding on the authors part(like the article authors description of a coherer), but your videos are really good, original and well thought out content!

  1. Triodes are popular with the audiophools / audiophiles. Many of the tubes that go for high prices are triodes. Brands, markings, and shape make significant differences in price of some vacuum tubes.

      1. Funny. The last triode amp I assembled ran quite happily at 434 megahertz (and put out 10 kW while doing it too). Surely, that ought to be enough for all but the very most discerning audiophiles? (The actual bandwidth is only 10 MHz though, so some audiophiles will be disappointed)

        Grounded grid configuration for the win, here, and proper grid neutralization.

        (Of course this is an RF amp, but the principle is the same.)

          1. A single tube, in grounded-grid configuration, does not suffer from the Miller effect, but then you need to think a bit outside the box about input impedance.

            For example, the 10 kW amp I mention above has a 50 ohm impedance at the input (cathode terminal), very convenient for RF work, but maybe not so much for audio.

  2. The relatively new kid on the vacuum tube block is the Inductive Output Tube or Klystrode. It’s the enabling technology for digital TV transmitters: more efficient while still being wideband enough for the HDTV channels.

  3. Not obsolete at all. I build and sell one or two amplifiers a month using 300B’s. With high sensitivity loudspeakers, they sound smoother and don’t clip nearly as harshly as even my McIntosh solid states. Instant power as it relates to distortion? Yes, please.

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