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metal tubes

Discussion in 'Misc Electronics' started by Allan Adler, Sep 22, 2003.

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  1. Allan Adler

    Allan Adler Guest

    Are metal electron tubes made entirely out of metal or does the metal
    merely cap some glass which constitutes the real tube? Are metal electron
    tubes easier to make than glass ones? I would think they are, since one
    avoids the problems of making a glass-to-metal seal. But maybe there are
    other problems, such as getting a good vacuum, avoiding unuathorized circuit
    paths, etc. Which is actually cheaper to make? I realize the answer might
    depend on whether you are only making one or making 10,000, but in either case.

    Ignorantly,
    Allan Adler


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  2. Fred Abse

    Fred Abse Guest

    In a word, both.

    There are (or were) all metal tubes, some of which were available in
    glass, too. The 6V6 was one of those, IIRC.

    Nuvistors were all metal, too.

    On the other hand, there were tubes such as the Mullard EF50, which looked
    like all metal, and even had a screw locking ring, but were really an
    all-glass tube inside an aluminum can.

    All-glass tubes are (or were) much cheaper to make than all-metal, one
    you'd got the plant installed.

    As to the question of making one or 10,000, most widely used tubes were
    made in their *hundreds of thousands*. remember that the replacement
    market was bigger than the OEM market, at least for consumer types.

    One off tubes, such as large transmitter tubes were largely made by hand,
    and cost thousands of bucks each. The really big ones were demountable.
    You could take them apart and fit new cathodes (filaments, really, though
    they looked like a bent six-inch nail). Such tubes were run pumped all the
    time, with a diffusion vacuum pump pack nearly as big as the transmitter
    cage. Big transmitter types were a mixture of metal and ceramic, with
    either a forced-air or water cooled anode. There were some ingenious ways
    of stopping the cooling water circuit from loading the output, such as
    winding the pipes to form an RF choke.

    The BBC in the UK were running 200KW tube transmitters in the MF broadcast
    band into the 1980s.
    May still be, for all I know.
     
  3. A E

    A E Guest

    I don't see how. The base was ceramic. You gotta have the pins isolated from
    each other somehow, so you always need to make a metal-to-something seal.
     
  4. Fred Abse

    Fred Abse Guest

    Quite true. I was using "all metal" as it was in common usage back then,
    meaning metal envelope. Erroneous, but nevertheless common.

    On another tack, there were things called "metal insulators". Run your
    mind past that one. (clue: transmission lines)
     
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