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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Vintage Monoscope Tubes Generate Classic TV Test Patterns Once Again

Night creatures and insomniacs of a bygone era may fondly recall a TV test pattern appearing once [Jack Parr] or [Steve Allen] had had their say and the local TV station’s regular broadcast day had concluded. It was affectionately known as the Indian Head test pattern, for the stylized Native American, resplendent in a feathered headdress, that featured prominently in the graphic.

Unknown to most viewers was exactly how that test pattern and others like it were generated. But thanks to [Rich “The Lab Guy” Diehl] and his monoscope restoration project, we can all share in the retro details. It turns out that while some test patterns were merely a studio camera trained on a printed card, most were generated by a special tube called a monoscope. It functioned in basically the same manner as a studio camera, but rather than scanning the incident light of a scene with an electron beam, the image was permanently etched into a thin aluminum plate. [Rich] laid hands on two vintage monoscope tubes, one containing the Indian Head test pattern, and set about building a device to use them. “The Chief” can hold either tube in a Faraday cage of thin, flexible PCB material and 3D-printed parts, with supporting electronics like the power supply and video amplifiers in an aluminum chassis below.

It’s a nice piece of work and a great lesson in how it used to be done, and the lithophane of the Indian head is a nice touch. Hats off to [The Lab Guy] for build quality and great documentation, including a detailed video series that starts with the video below. If you need a little more background on how video came to be, [Philo Farnsworth]’s story is a good place to start.

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Retrotechtacular: How Not To Design With Transistors

Consider the plight of a mid-career or even freshly minted electrical engineer in 1960. He or she was perched precariously between two worlds – the proven, practical, and well-supported world of vacuum tube electronics, and the exciting, new but as yet unproven world of the transistor. The solid-state devices had only started making inroads into electronic products relatively recently, and mass production techniques were starting to drive the cost per unit down enough to start including them in your designs. But, your company has a long history with hot glass and no experience with flecks of silicon. What to do?

To answer that question, you might have turned to this helpful guide, “Tubes and Transistors: A Comparative Guide” (PDF link). The fancy booklet, with a great graphic design that our own [Joe Kim] would absolutely love, was the product of the Electron Tube Information Council, an apparently defunct group representing the interests of the vacuum tube manufacturers. Just reading the introduction of this propaganda piece reveals just how worried companies like RCA, General Electric, and Westinghouse must have been as the 1950s turned into the 1960s. The booklet was clearly aimed directly at engineers and sought to persuade them of the vacuum tube’s continued relevance and long-term viability. They helpfully explain that tubes are a reliable, proven technology that had powered decades of designs, and that innovations such as heaterless cathodes and miniaturization were just around the corner. Transistors, we’re told, suffer from “spread of characteristics” that correctly describes the state of materials engineering of silicon and germanium at the time, a thornier problem than dealing with glass and wires but that they had to know would be solved within a few years.

With cherry-picked facts and figures, the booklet makes what was probably in 1960 a persuasive case for sticking with tubes. But the Electron Tube Information Council was fighting a losing battle, and within a decade of swamping engineers with this book, the industry had largely shifted to the transistor. Careers were disrupted, jobs disappeared, and fortunes were lost, but the industry pressed forward as it always does. Still, it’s understandable why they tried so hard to stem the tide with a book like this. The whole PDF is worth a look, and we’d love to have a hard copy just for nostalgia’s sake.

Thanks to [David Gustafik] for the tip.

Rebuilding The First Vocal Encryption System

Back in the early days of radio, it was quickly apparent that the technology would revolutionize warfare, but only if some way could be found to prevent enemies from hearing what was said. During World War II, the Allies put a considerable amount of effort into securing vocal transmissions, resulting in a system called SIGSALY – 50 tons of gear developed by Bell Laboratories with the help of Alan Turing that successfully secured communications between the likes of Churchill and Roosevelt during the war.

Now, a small piece of the SIGSALY system lives again, in the form of a period-faithful reproduction of the vocal quantizer used in the system. It’s the work of [Jon D. Paul], who undertook the build to better understand how the SIGSALY system worked. [Jon] also wanted to honor the original builders, who developed a surprisingly sophisticated system given the technology of the day.

SIGSALY was seriously Top Secret in the day, and most of the documentation was destroyed when the system was decommissioned. Working from scant information, [Jon] was able to recreate the quantizer from period parts, including five vintage VT-109/2051 thyratrons scrounged from eBay. The vacuum tubes are similar in operation to silicon-controlled rectifiers (SCRs) and form the core of the ADC, along with a resistor divider ladder network. Almost every component is period correct, and everything is housed in a nice acrylic case. It’s a beautiful piece of work and a great homage to a nearly forgotten piece of cryptographic history.

Interestingly, Bell Labs had a bit of a head start on the technology that went into SIGSALY, by virtue of their work on the first voice synthesizer in the 1930s.

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A Dozen Tubes Make An Educational Amplifier

If you asked [Hans_Daniel] what he learned by building a tube audio amplifier with a dozen tubes that he found, the answer might just be, “don’t wind your own transformers.” We were impressed, though, that he went from not knowing much about tubes to a good looking amplifier build. We also like the name — NASS II-12 which apparently stands for “not a single semiconductor.”

Even the chassis looked really good. We didn’t know textolite was still a thing, but apparently, the retro laminate is still around somewhere. It looks like a high-end audio component and with the tubes proudly on display on the top, it should be a lot of fun to use.

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Philo Farnsworth, RCA, And The Battle For Television

The parenthood of any invention of consequence is almost never cut and dried. The natural tendency to want a simple story that’s easy to tell — Edison invented the light bulb, Bell invented the telephone — often belies the more complex tale: that most inventions have uncertain origins, and their back stories are often far more interesting as a result.

Inventing is a rough business. It is said that a patent is just a license to get sued, and it’s true that the determination of priority of invention often falls to the courts. Such battles often pit the little guy against a corporate behemoth, the latter with buckets of money to spend in making the former’s life miserable for months or years. The odds are rarely in the favor of the little guy, but in few cases was the deck so stacked against someone as it was for a young man barely out of high school, Philo Farnsworth, when he went up against one of the largest companies in the United States to settle a simple but critical question: who invented television?

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Tesla Coil Uses Vacuum Tube

What do you do when you find a 5 kW transmitting tube in your local electronics store? If you are [TannerTech], you build a vacuum tube tesla coil. This isn’t the usual little wimpy coil, but a big bad boy that would look at home in an old horror movie.

The first power up was a bit anticlimactic, although it was working, it wasn’t very spectacular other than the tube glowing brightly. A few adjustments and some mineral oil did the trick.

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