Restoring A Vintage Tube Tester To Its Former Glory

It can be difficult for modern eyes to make much sense of electronics from the 1960s or earlier. Between the point-to-point soldering, oddball components, and the familiar looking passives blown up to comical proportions like rejected props from “Honey, I Shrunk the Kids”, even experienced hardware hackers may find themselves struggling to understand what a circuit is doing. But that didn’t stop [Cat0Charmer] from taking the time to lovingly restore this Hickok Cardmatic KS-15874-L2 tube tester.

The good news was that the machine had nearly all of its original parts, down to the Hickok branded tubes in the power supply. Unfortunately it looks like a few heavy handed repairs were attempted over the years, with a nest of new wires and components intermixed with what [Cat0Charmer] actually wanted to keep. The before and after shots of individual sections of the machine are particularly enlightening, though again, don’t feel to bad if you still can’t make heads or tails of the cleaned up version.

Hiding new capacitors inside of the old ones.

As you’d expect for a machine of this age, many of the original components were way out of spec. Naturally the capacitors were shot, but even the carbon composition resistors were worthless after all these years; with some measuring 60% away from their original tolerances.

We particularly liked how [Cat0Charmer] hollowed out the old capacitors and installed the new modern ones inside of them, preserving the tester’s vintage look. This trick wasn’t always feasible, but where it was applied, it definitely looks better than seeing a modern capacitor adrift in a sea of 60’s hardware.

After undoing ham-fisted repairs, replacing the dud components, and installing some new old stock tubes, the tester sprung to life with renewed vigor. The previously inoperable internal neon lamps, used by the tester’s voltage regulation system, shone brightly thanks to all the ancillary repairs and changes that went on around them. With a DIY calibration cell built from the schematics in an old Navy manual, [Cat0Charmer] got the tester dialed in and ready for the next phase of its long and storied career.

We love seeing old hardware get restored. It not only keeps useful equipment out of the scrap heap, but because blending new and old technology invariably leads to the kind of innovative problem solving this community is built on.

Should Have Used A Vacuum Tube 555

“You should have used a 555” has become a bit of a meme around these parts lately, and for good reason. There seems to be little that these ubiquitous chips can’t be used for, and in a world where code often substitutes for hardware, it’s easy to point to instances where one could have just used a simple timer chip instead.

Definitely not in the meme category, though, is this overkill vacuum tube 555 timer. It comes to us via [David Lovett], aka [Usagi Electric], who has lately caught the “hollow state” electronics bug and has been experimenting with all sorts of vacuum tube recreations of circuits we’re far more used to seeing rendered in silicon than glass. The urge to replicate the venerable 555 in nothing but vacuum tubes is understandable, as it uses little more than a pair of comparators and a flip-flop, circuits [David] has already built vacuum tube versions of. The only part left was the discharge transistor; a pentode was enlisted to stand in for that vital function, making the circuit complete.

To physically implement the design, [David] built a large PCB to hold the 18 vacuum tubes and the handful of resistors and capacitors needed. Mounted on eight outsized leads made from sheet steel, the circuit pays homage to the original 8-pin DIP form of the 555. The video below shows the design and build process as well as testing of all the common modes of operation for the timer chip.

You can check out more of our coverage of [David]’s vacuum tube adventures, which started with his reverse-engineering of an old IBM logic module. And while he did a great job explaining the inner workings of the 555, you might want to take a deeper dive into how the venerable chip came to be.

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Retrotechtacular: Philips Factory Tour, Circa 1930s

If you’ve got a half-hour or so to spare, you could do worse than this video trip through a Philips factory in the 1930s.

The film is presented without narration, but from the Dutch title cards and the fact that it’s Philips, we gather that this factory of gigantic proportions was somewhere in the Netherlands. In any case, it looks like something right out of [Fritz Lang]’s Metropolis and turned the rawest of materials into finished consumer products.

Much of the film focuses on the making of vacuum tubes; the sheer physicality of the job is what really stands out here. The upper body strength that the glassblowers had to have boggles the mind. Check out the chops — and the soon-to-be very unfashionable mustache — on the glassblower at the 12:00 mark. And it wasn’t just the gents who had mad skills — the fine motor control needed for the delicate assembly of the innards of the tubes, which seems to be mostly staffed by women, is just as impressive. We were also surprised by the amount these manual crafts were assisted by automated systems.

Especially interesting is the section where they build the luidspreker. Without narration or captions, it’s a little hard to tell what’s going on, but it appears that they used an enormous press to form chips of Bakelite into sleek covers for the speakers, which themselves are super-chunky affairs made from scratch in the factory. We’re also treated to assembly of the radios, packaging of finished products, and a group of dockworkers who clearly didn’t read the “Fragile” labels pasted on the boxes.

One can’t help but wonder if these people had the slightest inkling of what was about to sweep over them and the rest of the world. And if they did, would they even begin to comprehend how much the very products that they were making would contribute to both the slaughter of the coming war as well as to the sparing of so many lives? Likely not, but the film is still an interesting glimpse into the creation of an industry, one that relied very much on craftsmanship to get it started.

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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.

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Vacuum Tube Logic Hack Chat

Join us on Wednesday, December 9th at noon Pacific for the Vacuum Tube Logic Hack Chat with David Lovett!

For most of us, circuits based on vacuum tubes are remnants of a technological history that is rapidly fading from our collective memory. To be sure, there are still applications for thermionic emission, especially in power electronics and specialized switching applications. But by and large, progress has left vacuum tubes in a cloud of silicon dust, leaving mainly audiophiles and antique radio enthusiasts to figure out the hows and whys of plates and grids and filaments.

But vacuum tubes aren’t just for the analog world. Some folks like making tubes do tricks they haven’t had to do in a long, long time, at least since the birth of the computer age. Vacuum tube digital electronics seems like a contradiction in terms, but David Lovett, aka Usagi Electric on YouTube, has fallen for it in a big way. His channel is dedicated to working through the analog building blocks of digital logic circuits using tubes almost exclusively. He has come up with unique circuits that don’t require the high bias voltages typically needed, making the circuits easy to work with using equipment likely to be found in any solid-state experimenter’s lab.

David will drop by the Hack Chat to share his enthusiasm for vacuum tube logic and his tips for exploring the sometimes strange world of flying electrons. Join us as we discuss how to set up your own vacuum tube experiments, learn what thermionic emission can teach us about solid-state electronics, and maybe even get a glimpse of what lies ahead in his lab.

join-hack-chatOur Hack Chats are live community events in the Hack Chat group messaging. This week we’ll be sitting down on Wednesday, December 9 at 12:00 PM Pacific time. If time zones have you tied up, we have a handy time zone converter.

Click that speech bubble to the right, and you’ll be taken directly to the Hack Chat group on You don’t have to wait until Wednesday; join whenever you want and you can see what the community is talking about.

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Reverse Engineering A Module From A Vacuum Tube Computer

It’s best to admit upfront that vacuum tubes can be baffling to some of the younger generation of engineers. Yes, we get how electron flow from cathode to anode can be controlled with a grid, and how that can be used to amplify and control current. But there are still some things that just don’t always to click when looking at a schematic for a tube circuit. Maybe we just grew up at the wrong time.

Someone who’s clearly not old enough to have ridden the first wave of electronics but still seems to have mastered the concepts of thermionic emission is [Usagi Electric], who has been doing some great work on reverse engineering modules from old vacuum tube computers. The video below focuses on a two-tube pluggable module from an IBM 650, a machine that dates clear back to 1954. The eBay find was nothing more than two tube sockets and a pair of resistors joined to a plug by a hoop of metal. With almost nothing to go on, [Usagi] was still able to figure out what tubes would have gone in the sockets — the nine-pin socket was a big clue — and determine that the module was likely a dual NAND gate. To test his theory, [Usagi] took some liberties with the original voltages used by IBM and built a breakout PCB. It’s an interesting mix of technologies, but he was able to walk through the truth table and confirm that his module is a dual NAND gate.

The video is a bit long but it’s chock full of tidbits that really help clear up how tubes work. Along with some help from this article about how triodes work, this will put you on the path to thermionic enlightenment.

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Swap Your Microwave For A High Voltage Stereo

When building a new project, common wisdom suggests to avoid “reinventing the wheel”, or doing something simple from scratch that’s easily available already. However, if you can build a high-voltage wheel, so to speak, it might be fun just to see what happens. [Dan] decided to reinvent not the wheel, but the speaker, and instead of any conventional build he decided to make one with parts from a microwave and over 6,000 volts.

The circuit he constructed works essentially like a Tesla coil with a modulated audio signal as an input. The build uses the high voltage transformer from the microwave too, which steps the 240 V input up to around 6 kV. To modulate that kind of voltage, [Dan] sends the audio signal through a GU81M vacuum tube with the support of a fleet of high voltage capacitors. The antenna connected to the magnetron does tend to catch on fire somewhere in the middle of each song, so it’s not the safest device around even if the high voltage can be handled properly, but it does work better than expected as a speaker.

If you want a high-voltage speaker that (probably) won’t burn your house down, though, it might be best to stick to a typical Tesla coil. No promises though, since working with high voltages typically doesn’t come with safety guarantees.

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