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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Tubes Have Character With A Tek 570

When tubes were king, you could go to a drugstore with a box full of them from your TV. There would be a tester that would tell you what tubes were bad and, of course, you could buy the replacements for them. That kind of tube tester was pretty simple. If you wanted to really know how to design with a tube or test its parameters, you were much better off with a curve tracer like the Tektronix 570 that [tomtektest] shows off in two recent videos that you can see below.

That piece of kit fell into [Tom’s] lap thanks to an observant delivery driver. The 1955 instrument is very similar to a semiconductor curve tracer but, of course, has the ability to provide much higher voltage for the tubes. The basic idea is that the X axis sweeps from a few volts up to 100s of volts. The vertical scale will show the plate, screen, or grid current. From those curves you can learn a lot about the characteristics of the tube.

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A Stunning Ray Gun Built From Junk

If ever there was a quintessential weapon of science fiction, it would have to be the ray gun. [lonesoulsurfer] built this one-of-a-kind stunner from his impressive collection of junk. It’s centered around a vintage Bakelite soldering gun, a vacuum tube, and a portable stove burner, all of which contribute to the fantastic mid-century look.

Inside is a slightly modified version of a ray gun sound effects circuit from MAKE: that squeezes square waves from a lo-fi synth builder’s favorite IC, the 40106 hex inverting Schmitt trigger. [lonesoulsurfer] was able to reuse the soldering gun’s trigger to start the pew-pew-pew, and he can adjust the death ray’s output with potentiometers. The gun is powered by an old cell phone battery and a combo Li-ion charger/step-up module from the world’s largest virtual auction house. Blast past the break to watch the build video.

If one little green LED isn’t enough for you, maybe you’d prefer this light painting gun.

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A Dancing Cowboy Nixie Tube

If there were four words you never expected to hear in sequence, they would probably be “Dancing cowboy Nixie tube”. But that’s just what [Glasslinger] has made, and it’s exactly what it sounds like – an encapsulated cowboy that dances.

We’ve placed the resulting video below the break, and in it we see a compelling tour through the construction of a Nixie, and the specialist tools required. Little touches such as the need to insulate with glass capillary tube whose wires which shouldn’t glow, the construction of the envelope and stem, and the painstaking layout of the various cowboy components on a sheet of mica are carefully explained.

The tube takes shape in front of us, a driver PCB is etched, and the whole arrangement is placed in a custom wooden box. This is old-school construction at its finest, with the only touch of modernity coming from an Arduino Uno that schedules the various segments. It’s not beyond imagination though to see in time gone by that a Honeywell mechanical sequencer might have been used for the same task.

We’ve brought you [Glasslinger]’s work before of course, but we’ve also seen some more conventional self-made Nixies.

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New Circuits With Old Technology

Before the invention of transistors, vacuum tubes ruled the world. The only way to get amplification or switching (or any electrical control of current) back then was to use tubes. But some tube design limitations were obvious even then. For one, they produce an incredible amount of heat during normal operation, which leads to reliability issues. Tubes were difficult to miniaturize. Thankfully transistors solved all of these issues making vacuum tubes obsolete, but if you want to investigate the past a little bit there are still a few tubes on the market.

[kodera2t] was able to get his hands on a few of these, and they seem to be relatively new. This isn’t too surprising; there are some niche applications where tubes are still used. These have some improvements over their ancestors too, operating at only 30V compared to hundreds of volts for some older equipment. [kodera2t] takes us through a few circuits built with these tubes, from a simple subminiature vacuum tube radio to a more complex reflex radio.

Taking a walk through this history is an interesting exercise, and it’s worth seeing the ways that transistor-based circuits differ from tube-based circuits. If you’re interested enough to move on beyond simple radio circuits, though, you can also start building your own audio equipment with vacuum tubes.

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