6C21 Triode

Aitch bestowed this gem on me while cleaning out his collection:

6C21 Triode
6C21 Triode

It’s a 6C21 triode, originally used as a radar modulator, atop a letter-size sheet of graph paper. The plate terminal is on top, the grid sticks out to the side, and the filament is common with the cathode through the base pins.

It has impressive specs (datasheet and pictures):

  • 30 kV plate voltage
  • 15 A pulsed plate current, 100 ms max
  • 7.5 V filament at 15 A = 112 W (!)
  • Pulse duty cycle 0.2%

The gray film inside the bulb shows that it’s been used, but the filament still has continuity. Ordinarily, you could turn something like this into a night light by running the filament at a voltage somewhat under its rating, but my bench supply maxed out at @ 3 A without even warming it up; a dim orange night light that burns maybe 75 W is Not A Good Idea.

The base has some intriguing holes, originally used for forced-air cooling, that lead directly to the glass envelope:

6C21 Triode - base
6C21 Triode – base

One could mount discrete LEDs in those holes, maybe a slightly turned-down 10 mm cool-white LED in the middle flanked by red and blue, and run a low-power Arduino-based mood light; by some cosmic coincidence, the hole spacing matches up almost perfectly with those LED strips. Or one could go full analog with three red LEDs driven by the WWVB signal.

I’m thinking a plain black acrylic case, with the tube base sunk into the middle, would be about right. No readouts, no dials, no buttons, just a gently glowing tube.

Maybe a 3D printed socket holding everything in place?

6 thoughts on “6C21 Triode

  1. If you want a more realistic glow, but with color changing, you could use red and yellow LEDs and PWM those. I’ve seen 3-led lashups using red, orange, and yellow, but with two holes, I figure two LEDs is the way to go.

    1. The central hole looks up through the evacuation fitting into that metal cap, but two little apertures let enough light through to illuminate the plate way up on top.

      It cries out for blue-hot illumination up the bore, with orange highlights along the sides as you suggest…

  2. Interesting. If you run the max power at a 0.2% duty cycle, plate dissipation is 90W. I’m guessing the high filament power is to swamp out any variations due to self heating in the ON state.

    If you had two of ’em, you’d have the makings of one heck of a steampunk guitar amplifier… [grin]

    Confession: I wanted a laboratory in the vein of Jules Verne since I was a kid. My second scratch built amp used tubes. The first got the local 50KW AM station better than the crystal radio it was supposed to amplify. [wince]

    1. one heck of a steampunk guitar amplifier…

      And you’d never, ever, have to worry about burning the tube!

      The datasheets give separate specs for amplifier use that are much less aggressive than the pulse ratings, while still far beyond anything reasonable. An audio amp would make no sense at all, but it’d be way cool.

      IIRC, full-throttle output requires 750 W drive from the preamp

      1. 750 W drive from the preamp

        Preamp, eh? My most potent audio amplifier is a 280W Hafler. I would have liked to see the modulators for the 50KW stations near Chicago. The local 5KW station had a nice “little” modulator. Ah, the joys of AM.

        1. It’s sort of like the Chain Home stations desperately peering over the English Channel, pulsing 200 kW at 30 MHz. When Chain Home Low could do a few tens of kW at 300 MHz, they built waveguides you could walk inside, because that was the only way to get enough power to the antennas.

          We lose sight of what we could do when we had to make it work…

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