Meet The Magic Eye Vacuum Tube

Vacuum tubes ruled electronics for several decades and while you might think of them as simple devices analogous to a transistor or FET, there were many special types. We’re all familiar with nixie tubes that act as numeric displays, and there are other specialty tubes that work as a photomultiplier, to detect radiation, or even generate microwaves. But one of the most peculiar and distinctive specialty tubes has an intriguing name: a magic eye tube. When viewed from the top, you see a visual indication that rotates around a central point, the out ring glowing while the inner is dark, like an iris and pupil.

By [Quark48] – CC BY-SA 2.0

These tubes date back to the RCA 6E5 in 1935. At the time, test equipment that used needles was expensive to make, so there was always a push to replace them with something cheaper.  They were something like a stunted cathode ray tube. In fact, the inventor, Allen DuMont, was well known for innovations in television. An anode held a coating that would glow when hit with electrons — usually green, but sometimes other colors. Later tubes would show a stripe going up and down the tube instead of a circle, but you still call them magic eyes.

The indicator part of this virtual meter took the form of a shadow. Based on the applied signal, the shadow would be larger or smaller. Many tubes also contained a triode which would drive the tube from a signal.

There’s a great web site full of information on these venerable tubes and it has examples of these tubes appearing in plenty of things. They frequently appeared in service equipment, radios, and tape recorders. They even appeared in pro audio equipment like the Binson Echorec echo-delay unit.

Before the Magic Eye

The magic eye actually replaced an even older technology: neon bulbs. Modified neon bulbs appeared in the early 1930s as Tune-A-Lite produced by a company named Duovac. You can read more about them at the Radio Museum. Interestingly, some of these had a mute output that could turn off the radio’s speaker when there was no signal present, like a modern-day squelch circuit.

The operation of the bulb is like a thermometer in that the tube is long and narrow and the glow rises in the tube as the voltage applied increases.

See One on a Monitor Near You

There’s nothing like seeing one of these old gems with your own eyes, but the video below will give you an idea of how a 6E5C eye appeared. The green glow reminds you of a 1950s science fiction film or what Hollywood thinks a nuclear reactor gives off. Some magic eyes like the EM80 had a different shape, check out this teardown video for a closer look.

Get One

Like most old stuff, these are available if you know where to look. eBay and Amazon both have quite a few at various prices. There are even a few kits for things like preamps that use magic eyes as VU meters. Even if you have a magic eye that appears burned out, you might try increasing the supply voltage as [M Caldira] does in this video about reviving weak magic eye tubes .

If you can’t find an actual tube, you might try faking it with persistence of vision using LEDs and a motor.

We’ve only seen a few magic eye projects in the past. There’s the spectrum analyzer. There was also a tear down of an antique capacitor tester.

60 thoughts on “Meet The Magic Eye Vacuum Tube

        1. It’s not a vacuum, it’s filled with very low pressure neon. Almost a vacuum, but not quite. And it’s not a valve, it does not control electron flow by itself, there is external circuitry that is responsible of that.

          1. Well, true, but vacuum tube sort of transcends its definition colloquially. Even gas-filled tubes are still called vacuum tubes the same way we dial phones that no longer have dials. Mercury vapor rectifier tubes, for example. Interestingly, even neon bulbs usually don’t just have neon in them but have a “Penning mixture” which is mostly neon. I think Geiger tubes use the same. Hydrogen and nitrogen are other common culprits. Some other penning mixtures are used to lower the strike voltage of gas discharge tubes using krypton/mercury instead of argon/mercury.

          1. I think it used to come in that form before tubes, in a screw top jar you dabbed your brush into. That was probably Edwardian or Victorian times though.

  1. This was used in a fun high school physics experiment that “measures the mass of an electron” (assuming a bunch of other constants are known): Put this tube inside the bore of a long solenoid, and observe how the electron paths change radius as you change the current through the solenoid.

    The experiment itself was kind of straightforward; I spent all the time figuring out how that tube was constructed!

    1. I think when we did that, we got a result of 0.001 AMU, thus proving that the crusty, old, Bakelite knobbed, lab PSUs were putting out fat electrons, which got stuck in the sharp corners and messed up other experiments.

      1. Heh, 0.001 AMU is within error. Heck, rounded off to the precision you quote, it’s dead on!

        And some things just never die. Here’s the apparatus:
        http://www.diyphysics.com/wp-content/uploads/2012/01/MASS-OF-ELECTRON-APPARATUS-by-IEC-Australia.pdf
        And here’s an example worksheet:
        http://dev.physicslab.org/Document.aspx?doctype=2&filename=AtomicNuclear_MassElectron.xml

        Jeez, where did that half century go? And why do I still remember this stuff but can’t remember what I had for dinner last night?

        1. That’s why you need to stick to the same weekly menu, then just memorise it. Or you can drive your wife nuts by insisting on menu items matching the color bands for the day of the month…. wait.. damn.. I’m doing it subconsciously, mashed red potatoes and ground beef in a tomato sauce tonight. (Not even making this up for the joke… I’m actually scared…. no that bit was a joke.)

    2. I still do this experiment in my physics classes when I teach how a charged particle is deflected in a circular arc due to the magnetic field present. It’s a old PSSC experiment developed by Murray Hagerstrom . Enjoy doing it, but be forewarned the experimental results aren’t great, usually about 200 to 300 percent error!

          1. Close, it looks like South Caldwell is just called Caldwell.

            That seems to be less annoying than Lawn / Upper Lawn, and town names that end in Street near me.

          2. When & where I was in that PSSC class, we had a series of towns nearby called Upper Northampton, Northampton, Lower Northampton, Upper Southampton, Middle Southampton, Southampton, and (wait for it) Upper Queensbury. The inconsistency of the naming scheme bugged me for years.

  2. I am a Metrologist and one of my Universal Measuring Machines a (Carl Zeiss Jena) has a Magic Eye attachment that I use to calibrate plain setting rings with a diameter of less than 8mm. The Magic eye tells me when a ball probe touches the side of the ring and forms a circuit.

  3. I just got rid of a Heathkit capacitor tester with one of those. It sensed the null in a bridge circuit. A linear sensor like that is excellent for things like that. E.g., finding the center of a radio channel. These newfangled digital gadgets don’t have the same resolution. Watching a meter gives you a feeling and it’s faster than trying to interpret changing digits. Seeing a graph is better than a line of numbers. Like, the weather forecast.

    “OK boomer”. I hear ya, I gotta get with the times. Well, I’d be happy if I could find a voltmeter that has a moving-line display. Or how about a little video screen that approximates a 6E5 tube screen? I bet one of those $4 Raspberry Pis would do it. You could make it pin compatible and run it off the 6V filament line.

    1. I think it was the seventies, I saw an announcement for aDMM that included a small analog meter.

      I know when I got the Intersil 7107 DVM “development kit” in 1978, my plan was to add an RCA CA3140 BiMos opamp as a buffer in parallel with the DVM, and then feed a small meter intended as a radio tuning meter. I think I was influenced by something.

      I now have a DMM with LCD bargraph, but it’s sluggish.

      Just get a nice analog meter, add an opamp or two to make it an electronic voltmeter, and you’ve got your peaking tool. No calibration needed, use a pot as a voltage divider on the input.

      1. I got a Craftsmen analog/digital multimeter. Got it for 5 bucks at a pawn shop because battery died. Got a big analog needle with the digital readout under it, nice having both on one meter and can see the lag in the digital because that needle moves pretty quick. Only bad thing on is that theres no banana plugs and got to cram the wires and probes in beside the plastic case.

    2. By, ‘got rid of’ I hope you mean, ‘sold for a handsome price’. Those Heathkit cap testers are getting spendy. I got all mine before the craze it, but I’ve been seeing IT-28’s go for $300 on eBay. As adjustable supplies with limited current and the magic eye as a current meter, they’re great for reforming electrolytics.

      1. Having read capacitor tester enough times, one neuron shakes the dust off and goes “You have a capacitor tester” … dannnng… Once upon a time I had a capacitor tester, of similar vintage and form factor as those Heathkit ones. I just realised I haven’t seen it since my last move, which is a decade and a half back. Kinda funny that, it’s not easy to hide. Now I got it for pennies in the dark days of “The capacitor plague” and made a couple of attempts to use it. Not sure if it was completely non-functional or that I was using it wrong. Youtube wasn’t much of a thing then, that I’d have been able to see one in action there. Anyhoo, it went under the bench, I moved, I forgot about it for this long.

        I don’t know if it had the magic eye on it, I think if it lit up I would have remembered, but we’re stuck in the Schrodinger wave function between, “it was completely busted”, and “the n00b didn’t know what he was doing.” It did I think have a large dial on it. It did have range switching knobs, some jack sockets… I want to think it had a meter at the top, but it’s possible I’m conflating it with a similar size and shape lab PSU that is also kicking around somewhere.

        Now that I come to think about it, cap tester, PSU, and an ancient oscilloscope (The single one of these beasts I’ve got a definite location on atm) are all the same “form factor” and would do great as ballast on the back of a bench to keep it from jumping around, heh, I mean, would look good ranked together on a shelf or on bench. Strangely I’ve never had them in the same area to notice this. Anyway, they are all in need of restoration to definite working condition, or restomod upgrading.

    3. There’s tons of cheap handheld digital scopes that switch to roll chart mode at low speed.

      But magic eye tubes look so cool, like one of the very few bits of vintage tech(Along with Nixie tubes) I’d actually use by choice.

  4. The author states there was magic eyes with other colors than green/green-blueish. What colors and what models and where does this information come from ? I´ve seen many many magic eyes but only from tube radios, and there were ALL green-ish

    1. The picture at the bottom of http://www.magiceyetubes.com/ shows some that look blue. (sorry, that link should be: http://www.magiceyetubes.com/pictures.htm)

      According to https://www.radiomuseum.org/forum/lighting_up_dark_eye_tubes.html:
      I have seen some European indicators from the late 50’s that use a more bluish-green phosphor resembling zinc-oxide (P15) or copper/silver-activated zinc-sulfide such (P2). I suspect the former due to its excellent resistance to Coulomb aging. I believe that zinc-sulfide fluoresces poorly at low-voltage but does have a strong bluish emission at higher currents. It should be evident by some afterglow following removal of excitation. Earlier P2 used only copper activator and was more yellowish-green.

      From Wikipedia: A magic eye tube is a miniature cathode ray tube, usually with a built-in triode signal amplifier. It usually glows bright green, (occasionally yellow in some very old types, e.g., EM4)

      However, I’ve seen EM4s that are green, but that doesn’t mean they are all green or that they didn’t mean something even older.

  5. I’ve just finished restoring a Klemt echolette with an EM84 as a vu meter. Fortunately this one was not burned in so much and works wuite well as a vu meter. And there is a round one in my philips GM 2307 LF generator to set it to 0Hz as you can very clearly see the ultra low frequencies.

  6. My grandfather had a small reel to reel which was probably the Czechoslovakian equivalent to the walkman, but before the cassette tape. So no, it wasn’t being attached to a belt. Anyway, I was mesmerized by its magic eye-like tube, though it was more similar in shape to something like a 12ax7 which was displayed side on rather than from the top, and generated a V style pattern. As with so many things in my childhood years, I had to tear it apart in a way that probably prevented it from ever being reassembled. Well, my grandpa was the exact opposite of really happy when he found it in a pile of pieces. In retrospect, it probably had cost him a substantial sum of money when he bought it. To be fair, I never did see him use it. In the end, I didn’t learn any lesson from opening it up other than not to break grandpa’s stuff.

    1. A relative had a late 50s early 60s Philips reel to reel of similar description, “no larger than a small steamer trunk” LOL I think it was one of the Maestro range from the pics on radiomuseum.org but can’t find an exact match. It had early grille and outer case, and later looking controls, so it was either between the two, or it could have been put together from two duds by one of my other relatives who was into things like that.

      Anyway, that had a magic eye type thing I’d spend hours staring at. Can’t quite recall the function of it, whether it was tracking control or Left and Right balance, because this was the stereo version. Despite being fascinated by the thing, I can’t clearly recall what it looked like. This machine was mostly in use before I reached school age, so that’s maybe why. I think it was somewhat linear and kind of lines up it that wandered left and right and came together in the middle.

  7. I have an Eico Model 950 Resistance Capacitance Comparator Bridge that uses this. It’s probably the second coolest vintage piece I have next to the paper tape punch.

    I fired it up one time to try to measure a resistor, but it puts so much power through the component that my 1/4 watt resistor instantly got too hot to touch.

    But the tube was so cool I immediately embarked on a project to try to reproduce the effect. To no avail.

  8. I have quite a few eye tubes. One of the rarest out there is the 6T5 which looks quite a bit different than the 6E5 and 6U5. Zenith used eye tubes starting in 1938 on their shutter dials. I have several robot shutter dials and I restore them electronically using old tube equipment. My IT-11 capacitor checker and my IT-12 signal tracer both run eye tubes. There were millions of eye tubes made for the military and you can still find them NOS.

  9. There was one tube that had 3 “meters” for SSI and left-right tuning on FM. I knew someone that had one of those tube studded cabinet guts from the pre-stereo days and remember it working. My first proper FM tuner given to me had the side view miniature tube for SSI, it was mono.

  10. Revox reel to reel for the win! My dad has an 36B-ish or along the line, of which he is very fond of. Tube based including a magic eye; additionally a percision mechanics wonder what concerns flutter and wow or rather the lack thereof. Sure: close to trunk size…

  11. I have a multi-band AM and Shortwave tube receiver from 1941 that has a magic eye. When I purchased it from a yard sale outside a farm house, it needed some restoration. On of the items needed was to replace the very faint 6U5 magic eye tube. Fortunately I was easily able to find a new one and it’s bright green glow is hypnotic in a dark room.

  12. I had an Allied stereo receiver in the mid 60’s that used one of these to indicate best tuning on the FM band. It wouldn’t surprise me if it was in a box in the garage somewhere. I know where all the old Dynaco gear is.

    For reference, we used to pore over the Allied Electronics and Lafayette catalogs in my early high school years.

  13. I had a floor model multi-band radio that used a magic eye tube as a signal strength meter. I used to sit there going across the band just to see the eye open and close. I heard Sputnik the first time on that radio. I wish I still had it but it was too much to move around in my younger years.

  14. There were a lot of interesting things they did in the tube days. I have a Zenith shutter dial tube radio from about 1938. Not only does it have a magic eye but it also has a motor drive for tuning (it mutes the audio and you watch the eye to see when you pass over a station) a base reflex speaker (an adjustable cone over the speaker that modifies the base response) and a patented shutter mechanism that mechanically displays the bands which is activated when you change bands. I’ve restored it and this hard to find beauty sounds amazing for an AM / shortwave radio. The cabinet is beautiful too once restored but it’s huge as well. It’s a floor standing unit that was top end in its day and still does well. Just look up shutter dial on YouTube if you want to see one in action

  15. I have a model 12S267. I looked up yours and they both look great as shutter dials do. To fix mine I needed to replace the magic eye, the drive belts, caps, some resistors and replaced some damaged wood. I also installed reproduction period speaker cloth. It looks great and works great. You’re fortunate to have 2.

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