A Tube AM Transmitter In A Soup Can

A standard early electronics project or kit has for many years been the construction of a small broadcast transmitter with enough power to reach the immediate area, but no further. These days that will almost certainly mean an FM broadcast band transmitter, but in earlier decades it might also have been for the AM broadcast band instead.

The construction of a small AM transmitter presents some interesting problems for an electronic designer. It is extremely easy to make an AM transmitter with a single transistor or tube, but it is rather more difficult to make a good one. The modulation has to be linear across the whole amplitude range, and its effect must not pull the frequency of the oscillator and cause FM distortion.

It’s a task [Joe Sousa] has tackled, with his one tube AM transmitter in a Campbell’s soup can. His write-up of the transmitter contains a full description of the problems he faced, and how his design overcomes them. His oscillator is a cathode follower, with the tube biased in class A mode to ensure as undistorted a sine wave oscillation as possible. Modulation is provided through the suppressor grid of the pentode tube he’s using.

The completed transmitter is mounted inside the iconic soup can, with the mains transformer mounted on a removable bottom plate. There is a provision for both loop and wire antennas to be connected.

It is probable that this transmitter falls under the so-called “Part 15” rules for unlicenced low-power broadcasting in the USA, however it should be borne in mind that not every territory has this provision. If you build this transmitter, make sure you’re not going to attract the interest of your local equivalent of the FCC.

This article should have whetted your appetite for tiny broadcast transmitters. How about comparing the one here with a full-sized model?

Thanks [2ftg] for the tip.

23 thoughts on “A Tube AM Transmitter In A Soup Can

  1. Good to see old friend Joe getting some props. He does some amazing things with tubes, kind of strange for a (probably retired by now) chip designer…he maintains the philbrick archives, a blog about the original tube opamps, is a member of my forums, and shares a love of things that glow. Maybe the smartest engineer I know. Many of the guys who wrote app notes back in the day that taught we old guys electronics for real work or did work for Joe – some are now dead, time gets us all – but a whole lot of my personal heroes from National Semi, Motorola, Analog Devices, LTC etc wound up at Joes place (LTC), as well as a lot of the philbrick alumni. He did a superconducting ratio with NOS Russian wire base tubes where the local oscillator was also the HV supply (after rectification) for the audio output stage, all reflex and in an Altoids tin. He also investigated the fact that some old German linear filament incandescent lights put out amazing amounts of RF in the fm band and made a transmitter out of one (using it as a microphone too). Check Joe out on http://www.radiomuseum.org/collection/joe_sousa.html and the http://www.philbrickarchive.org/ and anywhere else you can find him. A hacker’s hacker.
    Joe paraphrased – “if you can’t do it with fewer parts, you’re not as good an engineer”.
    (try 2 from wordpress login)

  2. Um superhetrodyne reflex in the message above (awaiting moderation as it has links to more of Joe’s work in it). Amazing what this guy can do with a couple tubes.

    1. I was just thinking “surely he means superheterodyne”, but for a brief few wonderful seconds, I was completely in awe of a superconducting radio transmitter. No idea how it’d work or what it’d mean, but it sounded beautiful.

  3. I love everything about this hack down to the design of the can enclosure with tube on top. You really can make something simple beautiful. AM may be old tech but it’s still a great teaching tool for those getting into electronic design.

  4. Ah, the memories this brings back. Long, long ago, when I was in high school, I picked for a few dollars an oscillator for the WWII transmitter, the marvelous Collins ART-13. It was for the naval model and tuned the LF marine bands from 200-600 KHz, so patrol aircraft could talk with ships.

    http://www.collinsradio.org/cca-collins-historical-archives/the-equipment-of-collins-radio/the-war-years/art-13/

    Taking note that the high end brought me into the AM broadcast band, I came up with an interesting idea. To modulate it, I fed the high voltage through a reversed audio output transformer, with the low impedance end fed by audio from a radio tuned to a local AM station. That provided my stealth signal. Estimated power was about a watt.

    For an antenna, I inserted a high-voltage capacitor to keep the 600 volts DC off the antenna, for which I used both sides of the coax feeding a home-made 10 meter beam. To be sure the FCC would not come a knocking, I put it on the air on Christmas Day. Keep in mind this was far from the coast and far from any major city. And if I’d wanted to go for range, I could have used a much higher and longer 80-meter dipole.

    Then I jumped in my parents’s car and drove around. Across a ridge line with dry and sandy soil, the signal faded away after about a half a mile. In another direction, across wet ground, it was still strong when I reached a wood about half a mile away. I was probably broadcasting a mile or so in that direction. That wasn’t for more than about fifteen minutes and my total audience, I am sure, was only me. I’d justified that for-the-heck-of-it purchase, made because on my teen budget I couldn’t afford an actual ART-13. I had to make do with much cheaper Command sets.

    You might want to read the link above about the ART-13. It was beautifully built. The autotune feature meant air crew could switch frequencies and have everything, including the antenna matching, automatically adjust to various presets. The mechanics of that was amazing.

    The 1960s, when I was in high school, were the heyday for the sales of WWII electronics. You can still find them on ebay, although for prices that have gone up ten-fold and more. Then they were “old stuff.” Now they’re collectors items.

    Having proved I could do it, I never tried that again.

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