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What is the minimum number of tubes per bit required to store a CPU register in a vacuum-tube computer?

In particular, have any real vacuum-tube computers ever successfully stored more than 1 bit of a CPU register per tube?

Related, but not quite the same questions:

  • How many 6SN7 tubes did it take to store a bit? asks specifically about the 6SN7 tubes

    • answers seem to imply 2 tubes per bit.
  • Answers to Can I build a working(ish) vacuum tube byte? mention

    • 12AX7 tubes (a dual triode); that answer seems to imply 1 tube per bit, and
    • dekatron tubes which can store 1 decimal digit per tube (and so could theoretically store 3 bits per tube); but it's unclear if this is actually usable as a CPU register storage.
  • The Wikipedia Williams Tube article seems to imply it could store 16*16 = 256 bits, but also seems to imply it was not usable as CPU register storage.

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  • 4
    Not really sure what the question is, as the referenced questions do already answer it. 2 Tiodes or 1 Dual Triode (In the end it does not make any sense to count 'tubes' here as it's about triodes, not how many are packed into a single glass container) or something around 1/3rd using a decatron or 1/256th using a Williams tube. What other answer is there to be expected? (And BTW, as mentioned in the answer, the 6SN7 contains two Triodes, so it's as well only one tube)
    – Raffzahn
    Aug 6 at 21:02
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    @DavidCary Well, there is no logical reason not to make a "hexatron" tube like a dekatron but with 16 electrodes, or whatever - but there were good commercial reasons why nobody actually did it. (And the Williams tube was superseded by magnetic core memory, not by better thermionic devices)
    – alephzero
    Aug 6 at 22:39
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    @DavidCary It feels somehow not fully thought thru. For one, a register is something else than storing a bit. A register contains read and write logic as well as selection and so on. Next, a tube (as in glas tube) is only the container for elements needed, the active elements are triodes. Further how many triodes are packed into a glass tube is not so much a technological issue (As Alephzero already explained very well). And last, even for just a bit storage (aka a Flip Flop) one needs more components than just two triodes. Bottom line, as it stands the question does not make a lot sense.
    – Raffzahn
    Aug 6 at 22:40
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    re: dekatron, but it's unclear if this is actually usable as a CPU register storage -- see Harwell WITCH, which used it for primary storage. You would, I suppose, choose decimal storage iff you were building a decimal machine, and not otherwise. Aug 7 at 0:25
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    You probably need to define "register" as well. In modern use, 'registers" are distinct from "memory". In earlier architectures, "registers" were anything that stored bits, including memory locations identified by "addresses". Aug 7 at 0:32
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The Manchester Small-Scale Experimental Machine used two Williams–Kilburn tubes to hold its registers: one stored a 32-bit accumulator, while another stored the current 32-bit instruction and the current instruction address, or program counter. This was aside from the 32×32 bit Williams–Kilburn tubes used as main memory and display. The Manchester Mark 1 and the derived Ferranti Mark 1 also used the same technology for memory.

The largest proposed thermionic valve memory storage unit was the RCA Selectron at 4096 bits. RCA were never able to produce this reliably in quantity, and by the time the simplified 256 bit Selectron was ready for market, magnetic core had already taken most of the market.

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