back to article Bletchley Park to restore 112-byte* '50s Brit nuke computer

In a project described as "the computing equivalent of the raising of the Mary Rose", engineers at Bletchley Park intend to restore a 1950s-era computer - featuring a magnificent 112.5 bytes of memory* - to working order. The machine in question was built at the Atomic Energy Research Establishment at Harwell in Oxfordshire. …


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  1. micheal
    Thumb Up

    I'm fully supportive of this

    Too many UK achievements get lost in time and get shipped out due to lack of money.

    Bletchley are doing a wonderful job on very little money, I applaud their determination and enthusiasm

  2. Ball boy Silver badge

    Small correction...

    Interesting article but may I make a minor suggestion:

    "but also featured 900 gas-filled tubes" - in the UK, these devices are known as valves (they're only 'tubes' in the US).

    God knows where they're going to locate these damn things; it's hard enough locating them for power transmitters (where transistors / FET, et al still don't cut it ) - getting them for systems that the transistor took over in the 70's will be a lot more fun.

    When they do get it working, is there any chance the UK TAX office could borrow it for their calcs.?

  3. Annihilator
    Paris Hilton

    Half a byte

    "featuring a magnificent 112.5 bytes of memory"

    I first read this as bits, not bytes, and immediately wondered what half a bit was...

    Does this imply that its word size was only a nibble? I thought that way-back-when, a byte was whatever size the hardware defined it to be?

    Still utterly confused, but less so than half a bit.

    But yay to Bletchley. Please build it out of Meccano(c)

  4. Anonymous Coward


    Hear hear!

    ...But the one question I would like answered is... Will it run ping pong? Crysis... not a chance in hell!

    Mines the one with the gargantuan paper roll in the pocket

  5. Moonrajah

    Yes, but...

    Yes, but will it run Crysis?

  6. WonkoTheSane

    @ Ball Boy

    These days, Russia is the main source for valve tech.

  7. Anonymous Coward

    Small correction - Ball boy

    "but also featured 900 gas-filled tubes" - in the UK, these devices are known as valves (they're only 'tubes' in the US).

    Indeed they are known in the UK as Valves. You missed the obvious boner though. They are not 'gas filled' They are also known as (and here's a clue) VACUAM tubes. Right ...they are 'filled' with nothing but vacuam - and plenty of it...(guffaw!).

    Thermionic emission devices. When I were doing techy stuff I made 'em as a project for my OND. All the best Amplifiers used them. I used to have an old cinema amp that you could play bass through and REALLY honk off the neighbours.

    Happy days!

  8. Anonymous Coward

    Bring back ...

    ... the BLMRA (British Leather Manufacturers Research Association) also parked at Bletchley for a few years?

  9. Francis Vaughan

    Pedantry alert @Ball boy

    Curiously the cold cathode tubes, are still called tubes in the UK. They are not switching elements, and are not therefore valves. They are not thermionic either. Even Mullard - who no doubt made the ones used in the computer - call them "tubes" in their own literature.

    One assumes that the computer is largely intact, but given the passage of time probably not safe to power up. No doubt lots of icky perished rubber insulated wire, dried out cpacitors, and the accumulated grime and dust of the ages. Relays will be the same as used in telephone exchanges of the time, and used in Colossus, and relatively easy to rebuild or source. The memory tubes will probably be fine since there is essentially nothing to go wrong, unless they leak. Being cold cathode, they have no heaters, and don't run hot.

    I wonder, did Thommy Flowers design the beast? If ever there was an unsung hero in the history of computing he is it.

  10. sandman

    Worst analogy I've seen today...

    "the computing equivalent of the raising of the Mary Rose". OK, without the giant crane, massive diving and engineering effort, not to mention lousy weather and bloody cold water. Perhaps a little more like reconstructing the Mary Rose...

    Still, a great project and I hope Bletchley gets the funding it deserves. Good luck to them.

  11. Luke McCarthy

    112.5 bytes...

    112.5 bytes = 900 bits

  12. LuMan
    Thumb Up


    Yet another visit to Blechley!

    If you've not yet been, then GO!!

    The workers and volunteers there need to be on the bleedin' Honours List for the amount of effort they put in!

  13. Simon Harris

    112.5 an underestimate?

    As I understood it from the BBC version of the story, storage was 900 dekatron tubes, which could each hold a single digit. Most dekatrons would store 1 of 10 states, so it's a single decimal digit rather a binary one, so the memory could exist in 10^900 states, which in binary terms can be represented in about 2990 bits, or 373.75 of our modern day bytes.

  14. Anonymous Coward

    Gas filled tubes

    I spotted a minor error in your report: according to the BBC story, the tubes are decatrons i.e. they have 10 stable states. Therefore, they store just over 3 bits per tube, not one - those are thyratrons.

    They're also quite rare as they are prone to leaking or going bad in storage. I wonder where such a large quantity of presumably good ones was found.

    - A. Pedant

  15. Steve X

    @Ball boy

    Not necessarily. They're only valves if they control current flow. If they're just ordinary cathode-ray or mercury devices they may well still be tubes, like the fluorescent tubes in my office.

  16. TNMOC peter

    A bigger correction and info

    First the correction....

    The Harwell computer is being restored by the 'engineers' (actually volunteers) of The National Museum of Computing which is based at Bletchley Park, rather than Bletchley park itself.

    Now the info...

    The 'tubes' are Dekatrons and the system has 90 (not 900) of them for it's main storage. Each one is capable of storing a decimal number 0 -> 9. It uses loops of paper tape on 4 readers to load the program and access subroutines and can store intermediate numeric data in the Dekatrons with the final results printed out or punched to paper tape. All the original Dekatrons are still with the system and it also has a number of spares for various parts of the system. How well the Dekatrons have survived is unknown. We already have a stock of Dekatrons so should have enough to replace any that have failed.

    It wasn't particularly quick in operation, taking 2 seconds to add two number and over 15 seconds to do a division but it was reliable, running for over 10 days one Christmas / new year period.

    It is likely to take the best part of a year to restore and will be viewable at TNMOC throughout its restoration period -

  17. TNMOC peter


    I can't count... it is 900 Dekatrons = 900 numbers. The 112.5 appears to have been derived by dividing 900 by 8 (bits) = 112.5, but they are not actually bytes as only numbers can be held.

  18. Richard IV
    Dead Vulture

    Admirable restraint?

    Not one gag about Britain's replacement for Trident. Shame on you!

  19. sandman

    Re: Admirable restraint

    Not a replacement for Trident but the NHS are eyeing it up...

  20. Anonymous Coward


    Yes, Russia and China, in fact. China manufactures and sells exotic valves that we no longer make here or in the rest of Europe. They also tend to be cheaper than the Russian equivalents.

    However, I still have a civilian wartime receiver c.1940 and I can still get valves (Mullard) for that no problem. Pricey though.

    I have a very fine collection of mini/micro hi-speed computer valves from the 50's which still work.

  21. chr0m4t1c

    Will they also be restoring

    The "Vista Capable" sticker?

    And do they realise that the original licence is tied to the original hardware and cannot be transferred?

  22. JS Greenwood


    ...will it blend?

  23. Paul Crawford Silver badge

    Gas filled

    The counting style of valve, along with the massive thyratron switches used in the old high power pulsed radar, are not vaccum "filled" but have a low pressure gas (hydrogen, argon, mercury vapour, etc). Once conducting, they stay on until the current drops to a low level, which is useful for some jobs such as digital counters.

  24. Anonymous Coward
    Anonymous Coward

    Any chance...

    Bletchley Park can get to work restoring XBox 360s to working condition?

  25. Nomen Publicus

    Old Joke

    Wot? No pictures?

  26. Anonymous Coward

    @TNMOC peter

    "it was reliable, running for over 10 days one Christmas / new year period"

    Something more stable than Windows has to be good. When will Dell have one?

  27. jumpyjoe

    They were always valves not tubes

    Take this from a 67 year old. In England they were called "valves"

    not only in my time but also in my father's time. Until recently

    only the US called them "tubes". The advent of the US dominated

    Internet now means everyone under 35 calls them tubes.

    I still have some EL34s and EL85s for a Marshall 100 watt bass

    amplifier which I used in the 1960s. They are still in their

    original boxes and on the ends and sides of the boxes it plainly

    states "electronic valve".

    How about this picture (from an American company)

  28. John Savard

    Fractions of a bit

    Fractions of a bit are indeed possible. Since the unit has 90 Dekatrons, which can store 10 possible values (not 16), its storage capacity can be measured in bits as 90 times the base-2 logarithm of 10. That makes a storage capacity of 298.97352853986... bits.

  29. jake Silver badge


    "OK, without the giant crane, massive diving and engineering effort, not to mention lousy weather and bloody cold water."

    You've never been involved in ancient computer restoration at Bletchley Park, have you?

  30. Richard Porter
    Thumb Up

    Decatrons (sp?)

    "I spotted a minor error in your report: according to the BBC story, the tubes are decatrons i.e. they have 10 stable states. Therefore, they store just over 3 bits per tube, not one - those are thyratrons."

    You know what decatrons are? Looked at end-on you see ten little neon dots in a circle, one of which is glowing at any time. When you apply a pulse the glow moves round to the next cathode. At the end of a complete circuit there's an output pulse to trigger the next decatron. Thus they are capable of counting in decimal and do not in fact store any bits (binary digits) at all.

    Don'd confuse decatrons with numicators which display numerals 0 to 9.

  31. Andus McCoatover

    954 Acorn?

    still have one - bought when I was a nipper for -IIRC- 5 shillings, i.e., two weeks' pocket money.

    Built it into a super-regen 0-V-1. Took it out, but I reckon if I fired it up now, it'd still be OK. Always worried about that plastic bit to the cathode (it's the red bit at the bottom in the piccie in the link) would leak, but after 20 years I tried it - good as new!

    Just like my Sinclair Programmable. Works fine. Now, if I could get my HP-35 calculator working again...

  32. Jean-Luc


    """WITCH (Wolverhampton Instrument for Teaching Computing from Harwell). It was used to teach computing students until 1973."""

    I guess that college is just about to retire the current batch of IBM PC JRs they use to teach Fortran. What's next, BASIC on 386s?

  33. David 45

    Talking of old restored computers

    shows the restoration of a 60's machine which is on display once a year at a charity classic car show I am involved with.

  34. Charles Manning

    @Ball boy: Ya wrong

    Valves, aka tooobs in 'merikanspeak, are not gas filled. They are evacuated (vacuum tubes).

    The gas filled tubes are more likely to be acoustic memory cells.

  35. Adrian Esdaile

    Funny thing, isn't it...

    People totaly wig out when they hear of nuclear power - "ooh, it's nasty, you can make bombs out of it, thats EEEEVIL" Then they do bad street theatre by dressing up as skeletons and Uncle Sam.

    Wierd thing is, you NEVER see a crowd of hippies waving signs saying "NO INTERNAL COMBUSTION ENGINES! NO CHEMICAL EXPLOSIVES!" Both of which have killed way, way more people that nyookyular-anythings.

    Funny old world, aint it?

    Oh, and er, Jean-Luc? Unless they invented time machines, there is NO WAY Wolverhampton College could have been teaching FORTRAN on IMB PCJRs in 1973. I'm just saying.

  36. Anonymous Coward

    Here's one for you...

    Good on ya! Only downside I can see is that it probably needs enough 'leccy to power a small city.

  37. Francis Vaughan

    More pedantry

    Ah, so the tubes are decatrons.

    Web page for the comupter:

    One they are tubes. Mullard would almost certainly have made them (being a UK located company), and they call them tubes. For a set of pictures look here.

    Note the label on the box, Mullard made both valves and tubes. If it doesn't control electron flow it isn't a valve. The fact that our American cousins call everything a tube does not mean that we call everything a valve.

    Also, it is, rather counter intinuively, perfectly reasonable to express the memory capacity in bits. Everyone gets taught that a bit is a binary-digit. But this is only half the story. A bit is more formally defined as the amount of information that is stored in a binary digit. There are other units of information, three are usually defined: the bit, the nat, and the ban. These are the binary, natural, and decimal units of information. One bit equals ln(2) nats = 0.683 nats, and about 0.301 bans. Thus a decimal digit contains about 3.32 bits of information. Since the computer had 900 decatron tubes each capable of storing a single decimal number (and thus held 900 bans), it contained 900 * 3.32 = 2998 bits of information. The mistake is converting that to bytes. That isn't a well defined operation. A bytes does hold exactly 8 bits of information (and thus also holds 5.45 nats or 2.409 bans), so it is forgivable, but still wrong. Anyway, the conversion yields 373.5 equivalent bytes of information. So something may have wrong anyway. However, not all of the tubes will have been used for storage, many will have been used as computational elements, so it may be that the number of tubes devoted to actual data storage was more in line with the computation. But I suspect someone just got the number wrong.

    A little history. The history of the definition of the bit is fully documented. It was first defined and used by Shannon in his seminal paper on information theory: A Mathematical Theory of Communication, The Bell System Technical Journal, Vol. 27, pp. 379–423, 623–656, July, October, 1948. (He gives credit to his college J. W. Tukey for the term.) It is perhaps one of the very few cases where a common term has its entire etymology and history perfectly understood. I get arguments all the time (mostly from IT people, who somehow think that they own the term) that a bit is, and can only be, what a memory element holds. But the guys that invented and defined the term would disagree.

  38. jake Silver badge

    @Richard Porter

    "Don'd confuse decatrons with numicators which display numerals 0 to 9."

    Numicator[tm] was a trade name. The generic[1] is "Nixie Tube", and trust me they came in a lot more varieties than just numeric. I still have a clock, a VOM, and a couple frequency counters that use Nixies ... Yes, it was "Nixie Tube", never "Nixie Valve" ;-)

    Yes, they are gas filled, usually low pressure neon with a little mercury or argon (?? From memory, I could be off on the Penning mixture details, it's been a LONG time since I've even thought about this stuff.)

    [1] Generic in the same sense as "kleenex"; I think it was Burroughs that owned the Nixie trademark, but as far as I know they never defended it in court.

  39. Anonymous Coward
    Anonymous Coward

    Restoring those new fangled computers....

    When a first generation computer is already running in Australia...

  40. Pete 8

    When I was a lad

    We had to use painted rocks for the storage media.

    And they were damn heavy too.

    Kids these days I dont know.

  41. PeterO


    Decatrons are valves and they are gas filled.

    The glow is around the Anode not the Cathode.

    While the only thing they can do is to count, they do hold state equiv. to 3.3 bits. The state can be examined by applying 10 count pulses an watching for the carry output. So if the 7th anode is glowing the carry will come after three pulses. After the carry has pulsed, start pulsing the decatron in the adder until all 10 pulses have been applied. The adder will have moved on 7 positions and may have generated a carry. This is roughly the way the Witch adds decimal numbers together.

  42. Hackbert

    So then...

    this thingy stores 900 decimal digits. Each is the equivalent of log2(10) = 3.32... bits, giving a total of 2989.73... bits or 373.71... bytes. Not bad for these days!

  43. PeterO
    Thumb Down

    CSIRAC, running ?

    " When a first generation computer is already running in Australia.."

    According to Wikipedia "The machine finally found a permanent home in the Melbourne Museum in 2000. It has not been operable since its shutdown, but many of the programs that ran on it have been preserved, and an emulator has been written for it. "

    No mention of it working on the museum web site either....

  44. nikhow

    Erection Unit

    Ah Bletchley......

    When I was there in the late 70's as a Post office Apprentice we nicked a Reliant Robin form the car park and put it on the flat roof of our accommodation. Owner was NOT happy.

    Motorbikes up and down the 100 yard long corridoors.

    Got the Pole Erection Unit and 'planted' a pole in the lawn outside the front door of the mansion.

    The bar in the mansion, well, those hairy arsed welsh jointers drunk it dry every night..

    At 53, I remember it like yesterday.

    Ahhhhhh Happy days.

  45. Anonymous Coward


    Congrats, the closest answer to date, but you rounded too early - you're out in the second dp:

    Essentially solve 900*ln(10)/ln(2)/8 = 373.7169106, or 373.72 bytes

  46. steogede

    Re: Worst analogy I've seen today...

    >> "the computing equivalent of the raising of the Mary Rose". OK, without the giant crane, massive diving and engineering effort, not to mention lousy weather and bloody cold water.

    Did you miss the bit were they said the computer was being stored in Birmingham?

  47. steogede

    Re: More Pedantry

    >> The fact that our American cousins call everything a tube does not mean that we call everything a valve.

    Next you will be telling me that I am wrong when I say "sise doesn't matter".

  48. jw1234

    Brilliant !

    What I'd give for a job at Bletchley Park. I have always been involved with computers and now work for myself but I have always loved the historical side as Microsoft have made computers so boringly predictable

    Anyone from Bletchley park reading this get in touch.

  49. jw1234

    Tubes and Valves - Transistors Yuuuuukkk

    Valves or Tubes in the U.S are very very easy to get. One previous poster thinks they have gone out of fashion. The fact is they are better than the modern equivalent in many cases. Langrex down in Croydon still have and buy in stock from all over the world. Many shortwave transmission station use valves and being a radion amateur myself still use them.

    Apart from dropping them on the floor they are a lot more rugged electrically speaking as they physically move electrons through a near vacuum, with a few gasses as opposed to a solid substrate.

    Listen to any valve radio and then listen to its modern equivalent. Modern ones just just rubbish by comparison.

    Going back to an article done by the late Paul Young for Everyday Electronics in the 80's, when he was lucky enough to meet the boss of Sony (I think), even the boss (in Japanese) agreed.

    Why do you think places like Maplin sell a Valve Amplifier kit ?

    Good luck to those at Bletchley - Keep up the good work !

    John Wheatcroft


  50. jphb


    The main problem with decatrons was comparatively slow switching, the ones used in WITCH are HivAC GC10B's - they glow purple - some high speed (and high voltage) ones are used in the multiplication and division unit, these have a pinkish glow.

    It was a most impressive sight at night !

    A really neat side effect of the use of decatrons was that you could see exactly what number was in every memory location simply by standing in front of the machine and looking at it -

    great for programme debugging.

    You could also stand in front of the multiplication/division unit and watch it doing long multiplication and long division step by step.

  51. jphb

    "The Colossus does not count"

    The reason that the Colossus does not count as the world's oldest working computer

    is simply that the machine on display at Bletchley Park is only about 10 years old.

    There are other replicas and rebuilds of early machines around such as

    the Manchester Baby, the Atanasoff machine and the Zeus Z3, the WITCH

    however is original and is, AFAIK, the 3rd oldest surviving computer, the

    oldest being Australia's CSIRAC and the 2nd oldest the Pilot ACE in the

    London Science Museum.

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