tube

111 Articles

You Too Can Do The Franck-Hertz Experiment

March 15, 2025 by 5 Comments

We talk about quantum states — that is, something can be at one of several discrete values but not in between. For example, a binary digit can be a 1 or a 0, but not 0.3 or 0.5. Atoms have quantum states, but how do we know that? That’s what the Franck-Hertz experiment demonstrates, and [stoppi] shows you how to replicate that famous experiment yourself.

You might need to translate the web page if your German isn’t up to speed, but there’s also a video you can watch below. The basic idea is simple. A gas-filled tube sees a large voltage across the cathode and grid. A smaller voltage connects to the grid and anode. If you increase the grid voltage, you might expect the anode current to increase linearly. However, that doesn’t happen. Instead, you’ll observe dips in the anode current.

When electrons reach a certain energy they excite the gas in the tube. This robs them of the energy they need to overcome the grid/anode voltage, which explains the dips. As the energy increases, the current will again start to rise until it manages to excite the gas to the next quantum level, at which point another dip will occur.

Why not build a whole lab? Quantum stuff, at a certain level, is weird, but this experiment seems understandable enough.

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Convert A Cheap Tube Preamp Into A Headphone Amp With Jenny

February 27, 2025 by 20 Comments

Big-name tube amplifiers often don’t come cheap. Being the preserve of dedicated audiophiles, those delicate hi-fis put their glass components on show to tell you just how pricy they really ought to be. If you just want to dip your toe in the tube world, though, there’s a cheaper and more accessible way to get started. [Jenny List] shows us the way with her neat headphone amp build.

The build starts with an off-the-shelf preamp kit based around two common 6J1 tubes. These Chinese pentode valves come cheap and you can usually get yours hands on this kit for $10 or so. You can use the kit as-is if you just want a pre-amp, but it’s not suitable for headphone use out of the box due to its high-impedance output. That’s where [Jenny] steps in.

You can turn these kits into a pleasing headphone amp with the addition of a few choice components. As per the schematic on Github, a cheap transformer and a handful of passives will get it in the “good enough” range to work. The transformer isn’t perfect, and bass response is a compromise, but it’s a place to start your tinkering journey. Future work from [Jenny] will demonstrate using a MOSFET follower to achieve much the same result.

We’ve seen a great number of headphone amplifiers over the years, including one particularly attractive resin-encased example. Video after the break.

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The Bendix G-15 Runs 75,000 Lines Of Code

December 21, 2024 by 16 Comments

There’s a Blue Bendix in Texas, and thanks to [Usagi Electric] it’s the oldest operating computer in North America.  The Bendix G-15, a vacuum tube computer originally released in 1956, is now booting, and running code from paper tape. [David, aka Usagi] received the G-15 about a year ago from The System Source museum. The goal was to get the computer running so museum patrons could interact with a real tube computer. We’ve been following along since the project began.

[Usagi’s] latest G-15 video covers the last few problems on the road to running code. The biggest hurdle was the fact that the system wasn’t responding properly to the GO button on the typewriter. [Usagi] was able to isolate the issue down to a flip flop and then to a particular signal on an AND gate — the RC signal. The gate appeared to be bad, but swapping the entire circuit card multiple times had no effect. Something else had to be going on.

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Cataract Surgery For An Old TV

October 31, 2024 by 9 Comments

TVs used to be round, and the GE M935AWL is a great example of that. [bandersentv] found one of these ancient sets, but found it had a “cataract”—a large ugly discoloration on the tube. He set about repairing the tube and the set, restoring this grand old piece back to working order.

The video begins with the removal of the round CRT tube. Once it’s extracted from the set, it’s placed in a round garbage can which serves as a handy work stand for the unique device. It’s all delicate work as it’s very easy to damage a picture tube, particularly an old one. Removing the discoloration is quite a job—the problem is caused by adhesive holding the front layer safety glass on, which has going bad over the years. It requires lots of heat to remove. In doing this repair, [bandersentv] notes he’s also giving up the safety of the original extra glass layer on the front of the tube. Worth noting if you’re worried about a given tube’s integrity.

Of course, cleaning the tube is just part of the job. [bandersentv] then gave us a second video in which he returns the tube to its original home and gets the TV back up and running. The quality is surprisingly good given what poor shape the tube was in to begin with.

It’s funny, because modern TV repair is altogether a rather different affair.

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BNCs For An Old Instrument

October 27, 2024 by 5 Comments

Back in the summer our eye was caught by [Jazzy Jane]’s new signal generator, or perhaps we should say her new-to-her signal generator. It’s an Advance E1 from around 1950, and it was particularly interesting from here because it matches the model on the shelf above this bench. She’s back with a new video on the E1, allowing us a further look inside it as she replaces a dead capacitor, gets its audio oscillator working, and upgrades its sockets.

Treating us to a further peek inside the unit, first up is a leaky capacitor. Then a knotty question for old tech enthusiasts, to upgrade or not? The ancient co-ax connectors are out of place on a modern bench, so does originality matter enough to give it a set of BNC sockets? We’d tend to agree; just because we have some adapters for the unit here doesn’t mean it’s convenient. Following on from that is a period variable frequency audio mod which has failed, so out that comes and a little fault-finding is required to get the wiring of the audio transformer.

These instruments are not by any means compact, but they do have the advantage of being exceptionally well-built and above all cheap. We hope readers appreciate videos like the one below the break, and that you’re encouraged not to be scared of diving in to older items like this one to fix them. Meanwhile the first installment is here.

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A Digital Replacement For Your Magic Eye

September 2, 2024 by 20 Comments

Magic Eye tubes were popular as tuning guides on old-school radio gear. However, the tubes, the 6U5 model in particular, have become rare and remarkably hard to come by of late. When the supply dried up, [Bjørner Sandom] decided to build a digital alternative instead.

The build relies on a small round IPS display, measuring an inch in diameter and with a resolution of 128×115 pixels. One can only presume it’s round but not perfectly so. It was then fitted with a 25mm glass lens in order to give it a richer, deeper look more akin to a real Magic Eye tube. In any case, a STM32F103CBT was selected to drive the display, with the 32-bit ARM processor running at a lovely 72 MHz for fast and smooth updates of the screen.

The screen, controller, and supporting circuitry are all built onto a pair of PCBs and installed in a 3D-printed housing that lives atop a tube base. The idea is that the build is a direct replacement for a real 6U5 tube. The STM32 controller receives the automatic gain control voltage from the radio set it’s installed in, and then drives the screen to behave as a real 6U5 tube would under those conditions.

By virtue of the smart design, smooth updates, and that nifty glass lens, the final product is quite a thing to behold. It really does look quite similar to the genuine article. If you’ve got a beloved old set with a beleagured magic eye, you might find this a project worth replicating. Video after the break.

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Making A Solid State 6AK8 Tube

July 22, 2024 by 13 Comments

[M Caldeira] had a project in mind: replacing a common vacuum tube with a solid-state equivalent. The tube in question was an EABC80 or 6AK8 triple diode triode. The key was identifying a high-voltage FET and building it, along with some other components, into a tube base to make a plug-in replacement for the tube. You can see a video about the project below.

These tubes are often used as a detector and preamplifier. Removing the detector tube from a working radio, of course, kills the audio. Replacing the tube with a single diode restores the operation of the radio, although at a disadvantage.

From there, he adds more diodes directly into the socket. Of course, diodes don’t amplify, so he had to break out a LND150 MOSFET with a limit of 500 volts across the device. It takes some additional components, and the whole thing fits in a tube base ready for the socket.

Usually, we see people go the other way using tubes instead of transistors in, say, a computer. If you want real hacking, why not make your own tubes?

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