A tinkerer built an 8-bit computer from 460 vacuum tubes that runs software but needs a stepladder for repairs

The wall-mounted machine uses 1950s technology and needs frequent tube replacements

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The takeaway: A homemade computer built with 460 vacuum tubes puts early computing technology to work in a modern project. The wall-mounted 8-bit system runs custom software, accepts user input, and drives a flip-digit display without a microprocessor, microcontroller, or modern integrated circuit. Its design highlights both the capabilities of tube-based computing and the limitations that led the industry to adopt semiconductor technology.

The project, called The Tube Computer MK3, was built by Mike, a self-taught electronics tinkerer who recently documented the machine on Hackaday. It is the third vacuum-tube computer he has developed this decade.

Vacuum tubes powered early electronic computers before transistors and integrated circuits replaced them. Tubes are large and fragile, generate heat, and have shorter working lives than semiconductor components. Mike's computer shows why the industry moved on from them, while also proving they can still power a working computer.

Mike describes the system as "a modern 8-bit design, built with recycled 1950s vacuum tubes." It is not a replica of a particular historic computer. Instead, it applies older electronic components to a contemporary 8-bit design built from discrete logic.

The wall-mounted system uses 460 recycled Soviet-era 6N3P double-triode vacuum tubes dating to the 1950s, along with germanium diodes. The tubes are arranged on 46 circuit boards, with 10 tubes per board, and linked by five backplane PCBs. The boards sit on a 190-centimeter-by-130-centimeter acrylic panel supported by an aluminum box-section frame.

Its logic is deliberately limited. The arithmetic logic design carries out five operations used by the instruction set: sum, carry, NOT, increment, and XNOR. The computer has no pipelining. Each instruction goes through a six-step fetch cycle followed by one execute cycle.

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By modern standards, the design is slow and unwieldy. But unlike a silicon chip, its computing logic is spread across hundreds of visible tubes and circuit boards. Getting the computer ready to run takes time. After power-on, the tubes need between 10 and 15 minutes to warm up. Mike then resets it to bring the components into a common operational state.

Reliability is an ongoing concern. The used and old-stock tubes have a working life of about 500 hours, so replacements and repairs are part of running the system. Because the computer is mounted on a wall, Mike sometimes needs a stepladder to reach a failed tube.

Heat is another challenge for the tube-based system. The tubes glow during operation and keep the computer room warm. Mike keeps a fire extinguisher nearby, and the running system leaves behind the smell of burning dust. Earlier tube-computer iterations have sometimes failed explosively.

The computer currently runs an Airship Simulator that lets users pilot a British R80 airship from Brighton to Paris. Membrane controls placed over an image of the R80's bridge send commands directly to the computer's input board.

"By pressing controls on the image of the bridge of the R80, you can direct the airship software," Mike said. "These membrane buttons simply put 5 volts directly to the input board of The Tube Computer. You can control the ballast, gas release, engine power, elevators, rudder and the bow mooring gear, which is used to release the airship from the tower."

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