1950’s AM Transmitter is Fun but Dangerous

[Mr. Carlson] bought a Globe Scout Model 40A ham radio transmitter at a hamfest. The 40A was a grand old transmitter full of tubes, high voltage, and a giant transformer. It is really interesting to see how much things have changed over the years. The transmitter is huge but has comparatively few parts. You needed a crystal for the frequency you wanted to talk. There were two little modules that were precursors to hybrid circuits (which were precursors to ICs) that were often called PECs or couplates (not couplets) but other than those, it is all tubes and discrete components beautifully wired point-to-point.

The really surprising part, though, is the back panel. There’s a screw terminal to drive the coil of an external coaxial relay that has line voltage on it. There’s also a plug on the back with exposed terminals that has plate voltage on it which is considerable. In the 1950s, you assumed people operating equipment like this would be careful not to touch exposed high voltage.

[Mr Carlson] does a great job of walking through the schematic and, of course, also fires the radio up and looks at the output with a communication monitor. It has been a long time since we’ve loaded up a tube transmitter and watching it done while looking at the output was very nostalgic.

We were surprised the little transmitter only managed about 20 watts out. A single 6146 should be able to get to 50 watts, but perhaps the final is worn out. The ad to the right is from a 1957 issue of Popular Electronics is for a similar model. It claims 60 watts out. You should note that the $100 price didn’t include coils which you could buy for each band, nor did it include crystals, a microphone, or a key. A 1955 article referenced this exact model:

Globe Scout Model 40A: This is a general-purpose 50 watt c.w., 40 watt phone bandswitching transmitter, working on 10 through 160 meters. It is intended to fill the need for a compact unit in the low power field for either fixed station or mobile use. The unit contains six tubes,
including rectifier. It is crystal controlled, or can be driven by any external variable frequency oscillator (v.f.o.). Built-in antenna tuner permits use of any standard type antenna. For mobile use, a suitable dynamotor or vibrator power supply is connected through an auxiliary socket.
Dimensions are 8 x 16 x 8 inches. In kit form, $89.95; factory wired, $99.95.

Compact indeed. The power supply connection is the one on the back panel with the exposed high voltage. The radio already had its electrolytics replaced — a topic of hot contention lately. If you want to build your own tube-based AM transmitter, you might want to have soup for lunch.

[Main image Globe Scout photo via Boatanchor Pix]

39 thoughts on “1950’s AM Transmitter is Fun but Dangerous

  1. Well there was a time when men were men and expected to exercise common sense…those who failed to do so were removed from the gene pool.

    Alas those days are behind us…

      1. Heh-heh. My wife the quality engineer often had to promote herself to de-facto safety engineer, and quit one job because her safety instructions were routinely ignored. Two days after she quit two guys got hit in the face with molten plastic when a poorly maintained extruder exploded.

        I myself learned about 600VDC by getting it across the arms. I managed to let go, somehow.

        The major safety device is between the ears. But we know that already, don’t we?

      2. Well my point was really that under all circumstances common sense should be applied. Just because the ancient radio doesn’t have a three part safety interlock and an 8×12 warning sticker, it is not automatically unsafe if a little common sense is applied.

        As an example power tools are all capable of causing serious damage, but applying common sense like “keep you hand after from the spinning parts” and “clamp your work down” will keep you more safe than any guard.

      1. The idea is that the hand behind your back is tucked into a pocket or a belt, so that grabbing with two hands takes deliberate effort. Also, you won’t somehow have that unused hand resting on something at ground potential.

    1. Aren’t those old radios heavy as fuck too? So you tend to reach behind them to adjust stuff rather than turning them over, meaning you can’t even see what you are doing.

  2. Ah, our freshly bubble-wrapped lives…Getting shocked is unpleasant and a learning experience. I’d bet more people did and do die slipping in the bathroom – for which you don’t have to demonstrate any particular skill, like taking a fairly tough test to get a ham license…
    Lots of ways to win a Darwin award, then and still. I’m not sure the world is a better place when made safe for stupid.
    It’s not like this jumped out of nowhere and hurt random innocent strangers.

      1. Or get persistent issues. Sometimes people don’t die from shock but do get life-long issues, including heart issues and sometimes nervous system ones.
        I mention it because II think for some reason some people are much more easy about shrugging off a possibility of death than the thought of constant annoying issues.

  3. Prior to the 1960’s, electric plugs and outlets had two identical prongs. (Today, the “neutral” prong is larger to keep you from plugging it into the “hot” side of the socket.) That permitted the “neutral/ground” or cabinet side of a plug to be inadvertently inserted into the “hot” side of the socket. If you touched an appliance with a cabinet that was neutral/grounded to the earth (or sat on a steam radiator) and then touched another appliance that was plugged in “cabinet to hot side,” it was the equivalent of sticking your fingers in a socket. You’d get 120 volts across your arms (did it one time, learned my lesson). In fact, we once had a TV set that had a sheet metal cabinet with a wood grain paint job.

    For those wondering “how much is too much,” the whole thing comes down to moisture and amperage. If your hands are moist or wet (perspiration or water), you’re dead. Keeping your left hand in your pocket removes the path through your heart, dramatically improving your odds. Extremely high voltages (cattle fence charger, spark plugs, etc.) are usually very low amperage (current) so they catch your attention but do no real harm (other than banging your elbow on something when you jerk your arm back).

    1. We were always taught to touch a chassis with the back of our hands first so that if it was hot your muscles would contract and pull your hand away. If you touched it normally and your muscles contracted you could get caught in a death grip.

          1. Crazy to carry a multimeter around cow pastures with fences that may or may not be energized depending on where they are feeding when you are 8 years old. Sheesh man.

    2. We had an old stand-up freezer in our basement right on the concrete. If you went down in bare feet and touched the door handle you’d get a nice tingle in your forearm. Nothing painful, just annoying. My brother and I learned to quickly grab the handle just enough to pop the seal and open the door.

    3. In my neck of the woods electrical outlets don’t do the live/neutral thing and plugs have no polarity.
      It makes it odd to see people from other countries having that stuff be very important to them.
      You still have the color coding and standard of the actual wiring though, but in terms of devices you have power and earth and there is no assumption of live and neutral at all.

  4. Back in those days, people had this thing called common sense. Nowadays, people seem to be missing this critical survival skill. For example, the moron who killed himself shooting a firework from atop his head! Then, his equally intelligent mother tried to sue the fireworks manufacturer… because there wasn’t a warning label on the firework!

    1. Till your wife one day thinks she needs the power strip and somehow touches the back of the ham device and drops dead. You, as the manly man would surely just scoff and get a beer while saying “Darwin award deserved”.

      1. No, because see… my GIRLFRIEND has the same common sense bestowed upon me. She knows better than to go touching shit she doesn’t understand and through me she has learned the ways of electricity,,,

  5. Keep in mind that too much emphasis on safety can have two conflicting results.

    1. People who should know better assume safety and get harmed because they’d not learned to spot danger themselves.

    2. Never having faced genuine dangers before, people become paralyzed when facing one that is unavoidable.

    1. People also assume that the safety devices will protect them and they can throw common sense out the window. Like the guy in my hunter’s safety course who said it was okay to point a gun at someone if the safety was on since you could pull the trigger and it wouldn’t go off, so why was it important? Because the safety is a mechanical device and can therefore fail, and should not be the primary means of ensuring safety – that’s the role of not pointing the gun at your buddy! (Not to mention that in some firearms, the safety simply prevents the trigger from being pulled (or makes the trigger pull not do anything), it does not prevent the sear from releasing or the firing pin from contacting the cartridge, in other guns it locks the firing pin or locks the sear such that if it gets dropped etc.. it’s a lot less likely to go off, and then you have guns like my Mosin-Nagant where the “safety” is twisting the end of the bolt, which locks the firing pin in place, but it’s cumbersome to engage/disengage, and you can’t open the bolt with the safety engaged, and most firearm safety manuals dictate engaging the safety when loading/unloading)
      That’s the problem I have with a lot of the electronic nannies and other safety devices used today, not that they are a bad thing, but for example, cars with automatic braking while it’s a great thing for those times where it’s the difference between an accident and not, it also allows people to put their trust fully in the system – “I don’t have to drive safely because my car will take over and brake on its own, so I can sit here and watch a movie on my phone and my car will do the rest” until the time it doesn’t…

      1. Safety devices occasionally seem to have a sense of irony.

        Worker: “I cut my hand open on the safety device.”
        Boss: “That’s too bad. Fill out this accident report. Remember to put in the suggestion to avoid the accident in future.”

        Worker writes suggestion: Remove safety device.

        Boss: “Can’t say that. Take it back and do it over.”

    1. I snip off ground lugs on extension cords. Peel labels off high voltage equipment. Disable door interlocks on CNC milling centers. Replace fuses with copper slugs. Screw down adjustment screws on relief valves.

      I’m what happens if you don’t pay attention.

      The above may be a bit extreme, but the effects are the same if you don’t keep your head in the game. Pay attention if it ain’t all puppies and butterflies. Murphy tends to a maximum. I’ve been working with high energy, high voltage/current, high explosives and nuclear isotope energetic jobs for more than forty years.

      Still suck wind, have ten fingers, can see and hear. Don’t be stupid. Don’t let your buddy do something stupid.

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