I was doing some experiments which involves switching high voltages (>3 kV) under high current (>30 Amps) load. Although solid state devices such as SCRs (Silicon Controlled Rectifier, or Thyristor) or IGBTs are the ideal candidates for this kind of operations, the prices for these devices are through the roof (1, 2) when the breakdown voltage exceeds the 3kV threshold. I did have a couple of QIS6502002 6500V 25A IGBTs I bought on eBay many years ago but as it turned out they were quite fragile, a few minor overloads during my experiments was enough to destroy them unfortunately.

QIS6502002 compared to a TO-220 device

So I turned my eyes to Thyratrons. These are a type of vacuum tubes (to be precise, gas-filled tubes) from the early 1900’s and have been largely replaced by its semiconductor counter part (SCR) nowadays, except for applications where extreme high voltage/high current are needed such as in pulsed radars.

Luckily, many of the medium powered thyratrons can be found on eBay at a fraction of their original cost. And as any vacuum tubes, they are quite forgiving. They can withstand quite a bit of abuse without being destroyed.

The one I picked up is a TGI1-50/5 hydrogen thyratron. As the name suggests, it uses ionized hydrogen gas as the switching medium. Hydrogen thyratron typically utilizes titanium hydride in it’s reservoir and the hydrogen gas is released when the reservoir is heated and recombined into titanium hydride when the temperature cools down. Like many other hydrogen thyratrons, the TGI1-50/5 has a separate heater for the hydrogen reservoir.

TGI1-50/5 Hydrogen Thyratron

The heater element for the hydrogen generator and the cathode filament can be powered independently for fine controlling the hydrogen gas pressure during operation. Some literature suggests that the the triggering sensitivity can be precisely controlled by fine tuning the gas generator voltage. From my limited testing, as far as the TGI1-50/5 tube is concerned, the hydrogen generator voltage can be set to be the same as the filament voltage (6.3V) and the tube can be triggered reliably via a 300V pulse applied to the grid.

Test Circuit Schematics

The schematics above shows my test circuit. A 100 Ω power resistor is connected to the anode to limit the maximum current to be within the specified maximum (50A). The triggering pulse is formed via C1 and R1 with a time constant of roughly 3 ms. The actual values of the RC components is not very critical in this experiment setup as the pulses are triggered manually via a momentary switch. But to achieve the fastest pulse rate, the triggering pulse width should be as short as possible (4-12 µs according to the datasheet). The datasheet does mention the minimum pulse amplitude should be no less than 150V. Again, the actual triggering voltage does not seem to matter much as long as it is above the minimum specified value.

I did not have a proper socket for the thyratron so loose wires were used to wrap around each individual pins. The picture below shows the setup of the experiment. The ~3500V for the anode is doubled from a 1700V tube supply I had using a bridge doubler.

Some thyratrons require the grid to be negatively biased, but for the TGI1-50/5, a zero bias worked just fine which greatly simplified the overall circuit.

Experiment Setup

Here is a video showing the experiment setup and the firing of the hydrogen thyratron.

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10 Thoughts on “Experiments with a Hydrogen Thyratron”

  • A spark gap provides the not inconsiderable advantages of bathing one in purifying ozone and nitrogen oxides, driving away the weak.

  • I think this is a fine use of a thyratron. They don’t get as much love as they deserve, but they are hard to beat in their current handling capabilities at high voltage. It can be quite a pain to get a stack – to get enough standoff – of IGBTs, FETs or other SS devices behaving themselves without blowing up, which it looks like you’ve found… Getting them all to turn on at just the right time without one or more getting smoked is tough.
    One would probably choose a triggered spark gap for “one-shotting” – which is what it sounds like you are doing in this situation. but ignitrons are another thing you might find would work well, except for the fact they are full of liquid mercury and need special handling considerations and probably more expensive. Better suited to crowbar duty, really – actually they would suck for this!
    For pulsed applications where you need high current, fast recovery and aren’t worried about the extra overhead of a reservoir supply, these old timer HT’s are great! I don’t know about this particular tube, but some HT’s are finicky about reservoir voltage in that you can poison them if your not careful.
    Dr. A.S. “Scott” Gilmour Jr. is a guy you need to know, if your not familiar with him already, and you like rabbit holes, or tubes…

    • I worked with Scott Gilmour a while back. I haven’t heard from him in, um, maybe 20 years though and last I talked with his son (who works at AFRL Rome NY) he was not in good health.

      I’ve used ignitrons for crowbars (40KV or so) in radar transmitters and they usually work fine, but the mercury problem made me go to triggered vacuum gaps. Of course those were single shots. For higher PRFs, stacked SCRs and FETs are the way to go (with much attention given to balancing and protection). For higher powers, hundreds of amps at tens of kilovolts, you can get bigger thyratrons a couple feet tall. Well, you could, anyhow.

  • Years ago worked on UNIVAC VIC and VIIIC 1/2″ computer tape drives that had high power SCR’s that switched power to the tape reel motors. The prior generation of these used Thyratrons to control the drive to the motors. I never worked on those tape drives but the guys that did said that the tubes were very reliable.


  • Hello, I recently purchsed a TGI1-50/5 off ebay and I was just wondering which pins correspond to which functions in the tube.

    (there are no markings on the tube besides the model and year of manufacture, the datasheet doesn’t number the pins either so i’m a bit lost.)

    I don’t want to try myself and fry the poor thing. so i’m hoping you could help me out.

    Kind regards
    Cian Smith

    • Hi Cian, there should be a “bump” on the base and if you look from the bottom immediately to the left of the bump (key) is pin 1. And the pins go clockwise. You can match the pins up with what I have shown in the schematics. You can verify the pinout here http://www.155la3.ru/datafiles/tgi1_50_5.pdf. I have a link in the article, but it is a bit hard to tell as the link is the same color as the text only it’s bolded.

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