Retrotechtacular: Hallicrafters Goes To War

When the USA entered World War Two, they lacked a powerful mobile communications unit. To plug this gap they engaged Hallicrafters, prewar manufacturers of amateur radio transmitters and receivers, who adapted and ruggedized one of their existing products for the application.

The resulting transmitter was something of a success, with production running into many thousands of units. Hallicrafters were justifiably proud of it, so commissioned a short two-part film on its development which is the subject of this article.

The transmitter itself was a very high quality device for the era, but even with the film’s brief insight into operating back in the AM era the radio aspect is not what should capture your interest. Instead of the radio it is the in-depth tour of an electronics manufacturing plant in the war years that makes this film, from the development process of a military product from a civilian one through all the stages of production to the units finally being fitted to Chevrolet K-51 panel vans and shipped to the front. Chassis-based electronics requiring electric hoists to move from bench to bench are a world away from today’s surface-mount micro-circuitry.

So sit back and enjoy the film, both parts are below the break.

[archiveorg VoiceofV1944 width=640 height=480 frameborder=0 webkitallowfullscreen=true mozallowfullscreen=true]

[archiveorg VoiceofV1944_2 width=640 height=480 frameborder=0 webkitallowfullscreen=true mozallowfullscreen=true]

If your appetite has been whetted for World War II-era technology in glorious monochrome by this film, maybe we can help. There is this film on the post-war testing of V2 rockets in the Hackaday archive, this film on the jet engine, or how about this film on experiments with derailing trains?

Thanks [K8MHZ], via this thread.

30 thoughts on “Retrotechtacular: Hallicrafters Goes To War

  1. Iron, man. You just can’t get around iron in transformers. For all the advancements that science and engineering make, you still need an iron-core transformer, and are limited by the magnetic saturation in that core.

    As I understand it, power supplies like PC ATX supplies which can handle a surprising amount of power for their size, use a small transformer, but run the transformation with a synthesized signal of > 20 khz, so you can use a teeny transformer for low current, and all the energy is transferred in the high frequency.

    1. This is why aircraft power since there WAS aircraft power was 400 Hz – this allows the power transformers and filters for airborne electronics to be much smaller and lighter. In the formulas used for power transformer design, solving for cross-sectional area or number of turns, both are inversely proportional to the lowest frequency the transformer must be able to handle. (

      1. 400 Hz in itself isn’t going to save weight, I don’t believe. Dad picked up at a local action a DC to 400 VAC 400 HZ dynamo. An Air Force tech told me 400 VAC because less current is needed to produce the need power. Less current means smaller conductors equaling less weight 400 HS means better filtering after rectifying the AC with smaller lighter capacitors. The referenced web transformer calculator is for line/mains transformers. With a 400 volt power source vacuum tube electronic equipment most likely will not need power transformers That raise the question why not generate and deliver DC to the electronic equipment?. That wasn’t something that occur to me to ask that of the AF tech.

        1. Sorry, but wrong. Yes, using higher voltage means smaller wires, but it doesn’t reduce the size or weight of transformers because doubling the voltage doubles the number of turns you need on the transformer windings, so it’s a push. Conversely, increasing frequency DOES reduce transformer size and weight. It looks like the link I provided wasn’t the best – I thought it had calculations for VA rating as well as turns/volt. The actual formulas I’ve used when designing transformers come from the ITT Radio Engineering Manual, or something like that. But it’s been a while since I’ve needed to do that, so I was trying to find an online calculator, and I thought the one in the link was equivalent. I didn’t realize that the core size calculator part assumed 50 Hz power.

          Anyway, it goes like this: you first calculate the size (cross-sectional area) of core needed according to your volt*amp requirement, which varies inversely proportionately with frequency, then once you have the core cross-sectional area, the formula on the link tells you how many turns/volt you need, and this ALSO is inversely proportional to frequency.

          According to, flux density = E / (4.44N * f * A), where E is the applied AC voltage, N is number of turns, f is frequency, and A is cross-sectional area of the core in square meters. This means that for a given flux density, increasing the frequency decreases the cross-sectional area. The maximum flux density varies by core material, so this is the formula you solve for A in order to determine what size core you need to use.

    2. Transformers have to handle the frequency applied to them. 60Hz, from the wall outlet, is very low so the transformer has to be big.

      Run the transformer at a higher frequency, and the windings can be smaller. So a lot of surplus from airplanes runs at 400Hz, the transformers can be smaller and lighter. Since a generator is needed to supply AC in the first place, they can run it at 400Hz rather than the standard at home 60Hz.

      The transformers in the switching supplies can be even smaller since they operate at an even higher frequency. The actual iron doesn’t have to be as big as at 60Hz.

      But “big iron” means something else in an AM transmitter of this vintage. You have to modulate the output tube(s). For voice, they don’t have to handle anything below 300Hz (hifi would be a different matter), but it has to handle the voltage going to the transmitter output stage, and the current needed. This can be significant for a kilowatt transmitter, I vaguely remember that as the sort of power level here.

      You can do screen or cathode modulation, not needing the large modulation transformer, but the results aren’t as great. With solid state, they must be doing something different, but in tube days it’s hard to eliminate that plate transformer.

      Which is why in big AM transmitters the modulator would be separate from the rest of the transmitter, as would the power supply.


      1. There was a “Retrotechtacular” video featured on HAD a while back (, of a tour of a 500 KW AM radio station transmitter. The modulator tubes were as big as the RF finals, and the plate transformers were HUGE. Like man-sized.

        I think that most modern transmitters in the 1 KW range modulate the exciter (at a few watts) and run that through a linear amplifier, which simplifies things a great deal. But even that’s “old school” – probably all of the latest broadcast transmitter finals are class D or class E switching amplifiers.

        1. The “latest broadcast transmitter finals” are a usually a new class of tube: (new in the sense of modern and commercially available: the concept is old). Datasheets available here: It’s a single output device, not class D or E. It’s more-or-less an analog signal going through the amp itself, so you could consider it a class A amplifier, but it’s fairly narrowband.

          FM transmitters have been running switchmode (class C) with a single output device pretty much forever.

    3. Back in the day an AM transmitter applied the audio to the power feed on the final amplifier. The modulator thus needed an audio amplifier of the same power as the RF amplifier, and an audio transformer capable of taking all that power. So yes, pretty big iron.

      With respect to comments about supply frequency & size of transformers I was reminded of the Swiss electric steam trains with their 16Hz supply that meant they had HUGE transformers.

  2. Regarding the linked video on V2 testing:

    “V2 Rocket – Assemblin…” The YouTube account associated with this video has been terminated due to multiple third-party notifications of copyright infringement.

  3. There is almost not a single day where an article from HaD that does not talk about war, weapons, terrorism, WWII, bombs, or rockets.
    What´s wrong with you, HaD, seriously ?
    It was not like this before Supplyframe bought you. Why this editorial choice ?
    Why such ideology has to bleed over an engineering website ? Why editors here don´t stick to tech and constantly bring things related to war ?

    W H Y ?

    1. Maybe because wars are defining points in history. They also drive some of the biggest technological/engineering breakthroughs. It’s a shame that it took killing each other to get these advancements – but it has happened.

      As for Weapons, I don’t think I see too much of them on here? I’ve seen quite a few “props” and perhaps laser weapons, which quite frankly I’m okay with as they show great projects people have created. HaD also covered 3D printed weapons not too long ago IIRC, and they covered it cautiously. The author didn’t point where to download to the models, and also warned of the dangerous (firearms exploding etc).

      Rockets – I don’t think I need to say much here? The range of engineering and technology that goes into rockets is extensive; from the manufacturing process, to the electrical systems on board.

    2. I’m sorry you see that as the dominant angle of this piece, it certainly wasn’t the intention and has nothing to do with Supplyframe or any other editorial input. I featured it because of its tour of a 1940s electronics assembly plant.

      1. I agree it is not the dominant angle, fortunately. But it´s already too much. Enough.
        Take this example: :
        You´re writing about resilient electronic components that are still used. You introduce your article with “As a fresh-faced electronic engineering student while the first Gulf War was raging in a far-off desert” :
        It is absolutely OFF TOPIC. Why bring that ?
        Like the article yesterday “REMOTE SENSING BOMBS COULD STEM TERRORISM” ??? WTF ???
        Seen from overseas (yes, you have readers with different sensibility, culture, and personal history), it´s very much like a nation-wide obsession, propaganda and ideology, and it´s sad to see a website dedicaced to hack and engineering vectoring (consciously or not) that.

        1. War has been raging constantly on this planet for the last 100 years at least. You can stick your head in a hole and rail at every mention of it, or you can accept it as a sad part of reality and get on with enjoying your day living in a part of the world where for the moment peace is available.

    3. Like it or not, much of the innovation in electronics came from wars (including the space program, which was one front of the Cold War), just as much innovation in biology and chemistry has come from disease. Extreme situations bring out the best in some people.

      In the current case, this particular article is really about a transmitter designed and built for amateur radio operators. But Hallicrafters didn’t make a film about that, so the best record of it was this film on what it took (very little, really) to adapt this to military use. So if you prefer, you can think of this as a film about the manufacturing of an amateur radio transmitter in the 1940s. It’s a rare look into how things were done 70 years ago. Except that bit about people doing all the various steps required to build the power supply section, all right next to each other on the same bench at the same time. That was pretty clearly staged.

      1. >a transmitter designed and built for amateur radio operators.

        True that. I don’t have anything in the way of tanks or artillery, but I’ve collected some detritus surrounding military radios. The first shortwave-only radio I ever saw was something a friend brought to school, an eighty-plus pound Farnsworth BC-342-N often paired with the Hallicrafter’s BC-610 in the Signal Corps … SCR-399 and -499 vehicular sets, IIRC. Bought my own sometime after that (32 years ago) and marveled at the completeness of the documentation and the heft of the build, compared to the civilian production, solid state radios of the 1980’s.

        I’m interested in the vintage manufacturing pieces that HAD publishes, whether they be on planetary gear manufacturing from an automotive group (even if it was once used on a Military Jeep), phonograph technology (even if it was once used for AFRS), bakelite manufacture (even if once used on field telephone handsets or missile nose cones), or radio, even military radio, theory and manufacture. There’s always something that I’ve not seen before in there.

        ((There are many, many websites where cabals can enjoy discussing their indignation and outrage that the United States ever had a fully functioning military, and imply the U.S. is the only nation that concerns itself with defense and weaponry, and does so both inexplicably and one-sidedly. I recently deleted my four year old account on a website which has become shot through with ‘Social Justice Warriors’ full of their own privilege at being “triggered”. Since we are here instead, can we just agree to disagree, and take it ‘as read’, that certain individuals are forever going to be in that piteous group, and get back to discussing technology?))

    4. War causes technological progress, think of the state of aircraft in 1914 vs. 1918 or computers and nuclear technology in 1939 vs. 1945, if you like the history of technology you’ll run into the war every once in a while, sorry if that offends your delicate sensibilities.

    5. t Just in case you didn’t notice this post and the videos where about technology. Like it not, war does drive technology, that’s not the fault of Hackaday. Beyond that you are overstating the number of war related posts

  4. This is why I read Hackaday. Great article, and then I learn something new about a subject I thought I was somewhat knowledgeable about in the comments (turns out I wasn’t). Awesome!

    And yeah, a lot (but not all, of course!) of the best stories are about wartime feats of aptitude. Nothing like a bit of motivation, even if it’s for all the wrong reasons. Compare what these guys did to the latest software startup, spending 10’s of millions of dollars rehashing yet another video chat service that may or may not be sorta secure. :)

    Another rabbit hole to explore is crystal oscillators during WWII – the economics around crystal hunters in the jungles of Brazil took an interesting turn when suddenly everyone in the world needed quartz crystals for their war radios. GPS receiver oscillators had a similar quest in the 90’s; everyone trying to find a stable oscillator. Cool beans.

  5. Heck, I owned a BC-610 in 1968. It w as a real beast – a big black hulking thing the size of an industrial grade washing machine. With tubes the size of your head and coils the size of a cat, you really got the feeling that there was some serious physics going on. Great sounding signal. Changing frequencies was a real chore though – you had to flip open access doors and replace and retune plug in units and fiddle with big coils. Frequency agile, it was not. And it was designed in a time when TV and radio interference was not an issue, so it tended to wipe out TV and radio in the area.

  6. I worked at Northrup who bought the old Hallicrafters in Rolling Meadows Il. There were cabinets full of old parts, many of them most of the new kids would have to ask “what the hell is this”. Names like “variometer” and cabinets full of LItz wire, any size you could imagine. Hallicrafters made portable radios, TVs, radios and I even found a portable EKG machine one day. From what I understand they made a pretty good TV. The magnetics group had an area to themselves, transformers hung from the walls. If you had a general input/output and requirement the manager would look around the room and point at one and say ” that’s the size yours will be”

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