Mike Rieker's PDP-8/V Swaps Digital's Transistors for a Beastly Vacuum Tube Processor

Self-described "hardware and software project enthusiast" Mike Rieker has built a clone of the Digital PDP-8 minicomputer with a difference: its processor is constructed using vacuum tubes, assisted by a Raspberry Pi single-board computer, resulting in a device drawing 1.71kW in order to run at a speed of around 25,000 cycles per second (cps).

"There was no PDP-8/V made by Digital. So I am making one myself, out of electron valves," Rieker writes of the project. "The tubes are just the processor. For memory and I/O [input/output], I use a Raspberry Pi. Transistors are used to do the level conversion between the [Raspberry Pi] and the tubes and to drive the LEDs. At some point, I might change the front panel to neons or incandescents with tube drivers, but LEDs will do for now."

The Digital Equipment Corporation (DEC) Programmed Data Processor PDP-8 family launched in 1965 with the original "Straight-8," constructed from diode-transistor logic flip-chip cards crammed into a roughly fridge-size unit that sold for $18,500 — making it one of the first commercial computers to be sold for under $20,000. The family received a number of redesigns and remodels throughout the years, and remains popular to this day.

While modern clones and emulators of the PDP-8 abound, Rieker's version is unique for having dispensed with transistors in favour of building the machine's 12-bit processor from 6J6A vacuum tubes — a technology that was already old-hat when the original machine launched. "Each tube has two triodes," Rieker explains. "A single triode is required to make an AND-OR-INVERT gate. Three tubes (six triodes) are required to make a D flip-flop (ahem, edge-triggered, bi-stable multivibrator). Two tubes (four triodes) are needed to make a latch."

The finished design, housed in a wooden frame, is made up of five tube-filled boards — the sequencer, the MA register, the PC register, the AC/Link register, and the arithmetic and logic unit (ALU) — with a backplane and a transistor-based front-panel board. The Raspberry Pi provides the driving clock and emulates missing hardware, allowing the machine to run DEC's OS/8 operating system.

Its performance, though, shows just why the world moved on from vacuum tubes in favor of transistorized computers: at its stable peak, the PDP-8/V runs at just 25kcps — 25kHz, translated into a modern processor's equivalent units — with a 1.71kW power draw, including five 20-inch attic fans designed to keep everything from overheating.

Rieker's full project write-up is available on the Outer World Apps website.