| Glow Transfer Counting Tubes |
For technical data, please refer to the Dekatrons of the World reference.
Glow transfer counting tubes, commonly known as dekatrons, are cold-cathode decimal counting devices. Dekatrons pass an ionization glow around a ring of cathodes by sending a single or offset double or triple pulse to intermediate guide electrodes, causing the glow to advance to the next cathode. Though a few dekatrons were manufactured with speeds as fast as 1 MHz, most were used for applications below 100 kHz. Dekatrons double as both counter and display; the count position is viewable through the top of the tube as a glowing dot. This combination of display and computation in a single component was not replicated in a solid state device until the introduction of LED smart displays in the 1970s.
Dekatrons come in three basic types: counters, computer counters and selectors. Counters have a single output cathode, which is pulsed once per full rotation. Computer counters have multiple output cathodes, usually four. Selectors have 10 output cathodes. Contrary to popular misconception, dekatrons were not widely implemented in computers. Only one computer is known to have made significant use of dekatrons, the AERE WITCH. Completed in 1950, each of the WITCH's memory stores contained 90 GC10A dekatron tubes; the WITCH was a major consumer of Ericsson dekatrons until it was decommissioned in 1973.
The world's first production glow transfer counting tube, the short-lived GC10A, was filled with helium, but all other early and common British and US dekatrons are 4 kHz neon-filled devices. Faster dekatrons, in the 10-50 kHz range, are usually filled with argon or a helium-hydrogen mix, popular among hobbyists for the distinctive purple glow. Dekatrons rated at 100 kHz appear to be filled with some sort of Penning mixture which exhibits improved ionization characteristics, allowing for higher counting speeds. A small handful of dekatrons operate at 1 MHz; such tubes use hydrogen as the fill gas and typically have shaped cathodes and other internal complexities.
While most dekatrons are decimal counters, there are also a few base-12 counters, a handful of Soviet base-5 counters, and one binary counter, which operates like a flip-flop.
Invented in 1941 by Joseph Desch of NCR, the Mathetron was the world's first gas-filled ten-stage decade counting tube. More of a multistage thyratron than a glow transfer device, the Mathetron used a method of counting that could best be described as 'indirect glow transfer'. Each tube had a central cathode and 10 anodes, which were separated by a series of baffles. A transfer electrode was placed in the influence of each cathode and was paired with a gate electrode in the next counting stage by an external connection. In this manner, the charge from the active cathode would be transferred to the next, allowing the tube to perform a full base-10 counting operation. Encased in a large tubular envelope with over 30 external connections, mathetrons were extremely complex devices, and were used almost exclusively for the war effort. Desch counters were used in the Manhattan project, and Desch himself went on to head up NCR's Building 26, home of the top secret NCR project to decrypt Enigma-encrypted messages during World War II. The Mathetron was never made available for commercial use, and all known examples were destroyed after the war, along with the rest of NCR's cryptographic equipment.
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Wales Counter / Mnemotron
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In 1947, inventor Nathaniel B. Wales designed and produced a new type of gas counter tube, which consisted of a series of three sheet metal anodes arranged in a vertical stack, each with 10 fingers. The top and bottom plates were bent in such a fashion as to align the tips of the fingers of each anode into a common plane. Two of the anodes acted as guides, and the bottom-most anode acted as the output ring. A ring cathode surrounded the anodes. Counting was performed by an offset double pulse directed into the guides, which would transfer the glow around the counting ring. A single output anode was isolated from the rest, providing a means to send a carry pulse to another tube. The Wales counter was a true top-view glow transfer counting device; all other glow transfer counters are direct descendants of its design. It appears that Wales was never able to obtain funding to mass-produce his tube, and no examples remain in existence. Had the Wales counter been commercially produced, it would have been a very low cost device to manufacture. The entire counting loop contained only five parts, most of which could be made from die-stamped sheet metal. In comparison, the typical Ericsson-style dekatron tube contains over 30 separate parts in its counting loop. Each cathode pin is a separate rod that must be welded or affixed to the support structure.
In 1949, Ericsson Telephone Limited sought to build a better counter. Given the trade name 'Dekatron', the Ericsson counter consisted of a counting loop of 30 cathodes spaced around a central anode. As with the Wales device, the glow was transferred around the counting ring by an offset double pulse that would transfer the glow sequentially from one cathode to another. The earliest Ericsson prototypes were not direct view, as the counting ring was covered by a mica support. However, by the time the tube went into production, this structure was dropped in favor of a direct-view configuration, and the helium filled GC10A counting tube was released upon the market. Unlike previous developers, Ericsson put forth both the funding and research needed to make their glow transfer counting tube into a successful product. Within a matter of only a few years, Ericsson's Dekatron product line had ballooned into over 20 different distinct part numbers, and over a dozen companies jumped into the market with their own devices.
The Ericsson-style dekatrons were, generally speaking, useful and workable solutions to the problem of high speed counting, as their widespread use would attest. However, traditional dekatrons have one significant flaw: if such a tube is left powered on with no input signal for an extended period of time, cathode poisoning from the active cathode will foul the adjacent cathodes, preventing the tube from counting properly. The proposed solution to this problem was the Polyatron, also known as an inverse dekatron. Counter to the dekatron's central anode and many cathodes, the Polyatron has a central cathode surrounded by 30 anodes. The motivation behind this alternate design is that by moving the glow discharge away from the counting ring, fouling of the electrodes could be prevented. Multiple Japanese and Soviet researchers simultaneously developed a Polyatron device in isolation, but the concurrent development of fast, stable transistor-based counters resulted in only one model of Polyatron ever being produced: the Soviet-made A-201. The A-201 has a ring cathode which is separated from its counting loop by a screen electrode, and can drive a Nixie display tube directly.
