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Replacing tubes with FET/transistor cascodes

Discussion in 'Electronic Design' started by Homer J Simpson, Sep 23, 2006.

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  1. Any info out there on replacing tubes with FET/transistor cascodes - a FET
    for input and a high voltage transistor for output?
     
  2. Tim Wescott

    Tim Wescott Guest

    It's been tried, never with great success. In spite of the old saw that
    "A FET looks like a vacuum tube" it doesn't, really -- the bias is
    different, the impedances are way lower, the heater connection really
    sucks in a series string, etc.

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Posting from Google? See http://cfaj.freeshell.org/google/

    "Applied Control Theory for Embedded Systems" came out in April.
    See details at http://www.wescottdesign.com/actfes/actfes.html
     
  3. Eeyore

    Eeyore Guest

    I've been chewing it over. What have you in mind ?

    Many moons ago you could get FETRONS btw. Same idea.

    Graham
     
  4. A large collection of old test equipment all with a multitude of different
    vacuum tubes.
    ISTR an article in Wireless World (UK) along those lines.


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  5. Eeyore

    Eeyore Guest

    A long time ago I'm sure !

    So, had you looked into it at all ?

    Graham
     
  6. John  Larkin

    John Larkin Guest

    The Supertex depletion-mode n-ch mosfets are practically tubes. They
    make great current limiters, too.

    John
     
  7. Eeyore

    Eeyore Guest

    Do they have the same 'square law' characteristic that tubes and jfets are
    supposed to share ?

    Graham
     
  8. John  Larkin

    John Larkin Guest

    Dunno... you could look them up! We use them at Idss, as high-voltage
    current limiters, so we don't care about the transfer curve.

    John
     
  9. I came across a reference to the article but can't track down a copy.


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  10. Eeyore

    Eeyore Guest

    I dare say a Spice model might cast some light on that.

    I suspected as much but thought you might just know anyway.

    Graham
     
  11. Eeyore

    Eeyore Guest

    If you have the date I might be able to help !

    Graham
     
  12. Tim Shoppa

    Tim Shoppa Guest

    Well, there are a LOT of different tubes out there with a lot of
    different characteristics and applications.

    If you have the ability to go in and rework the design to give you all
    the bias sources you want then it's not hard at all.

    If you want a "drop in" replacement that works in a certain application
    then it's not too awful but may require some creative thinking to
    re-bias things where you really want them.

    To be pessimistic, exactly reproducing every characteristic of a given
    tube with solid state stuff isn't trivial. But most tubes are only used
    over a small fairly linear portion of their characteristic range, they
    are often used in circuits that self-adjust their bias, and even when
    they aren't (e.g. AGC action) the exact details are often smooshed into
    words ("sharp-cutoff" vs "remote cutoff") that except for the exponents
    are self-adjusting as well!

    See Motorola application note AN211A (just google for "AN211A") figure
    21 for some optimistic/most favorable examples.

    Tim.
     
  13. Eeyore wrote...
    That's doubtful, usually the common MOSFET models fail in the
    sub-threshold region.
    I use the Supertex depletion-mode MOSFETs in the linear region
    all the time, and have taken careful bench measurements over
    many orders of drain current. At currents below say Idss/25,
    they exactly follow the theory we outline and plot in AoE,
    with an exponential voltage-current relationship, just like
    a BJT transistor, but with a somewhat lower slope (about 1/2
    to 1/5 the transconductance). At higher currents, near Idss,
    they have a square-law characteristic, like tubes, but you
    can rarely use individual FETs in this region for continuous
    linear operation, because for MOSFETs this characteristic is
    at high currents, where Pd = Id * Vd means you have excessive
    power dissipation. Now, if you use a small device, with low
    Idss, just above where you want to operate, and marry it with
    a cascode power MOSFET, to keep Vd down, and provide a large
    heatsink, ahem, that's a FETRON. Compare Teledyne's TS-12AT7
    to a tube, http://www2.famille.ne.jp/~teddy/datalib/fetron.htm
     
  14. Tim Shoppa

    Tim Shoppa Guest

    In which case my reference to AN211A may not have been so helpful.

    Replacement in classic RF/IF/AF stages where exact gain isn't picky and
    the class-A bias is mostly self-adjusting (the examples in AN211A) is
    the easy thing.

    Using them in test or industrial equipment may be entirely different.
    It's extremely unlikely that you'll be able to do a simple drop-in
    replacement in a DC regulator circuit or in a multivibrator or in a
    sweep generator, for example. It's not that semiconductors are
    incapable of any of those jobs it's just that those circuits generally
    depend on cutoff, saturation, and DC characteristics that are not
    self-adjusting, and substitutions of the active component may very well
    require changes in the circuit to get it to do anything at all.

    Tubes aren't exactly hard to come by these days... some are more common
    than others, but if you do a search on the web you WILL find what
    you're looking for.

    Tim.
     
  15. I've looked into that myself, and things look pretty dismal.

    For really low frequency use, like non-picky audio, you can probably
    fake it with a FET and a transistor.

    You can't use high-voltage MOSFETS, they have really high input
    capacitance Tens to hundreds of times higher than tubes.

    And the transconductances are not too hot, and extremely variable. And
    you can't find 400-volt transistors with frequency response above a few
    megahertz.

    You'd need a carefully-crafted bootstrapped input stage to minimize the
    input capacitance, some low-cap diodes to mimic the positive-grid
    region (for schmitt-triggers and multivibrators), then about ten
    well-matched 45 volt transistors in series. A complex and ugly
    arrangement.

    -------

    On the other hand, the old Collins R-392A radio, which was designed to
    run with 24 volts on the tubes, can be retrofitted with MOSFETS, just
    by directly plugging them into all the pentode sockets. The triodes
    can be replaced with most any silicon NPN transistor. Weird but true.
     
  16. colin

    colin Guest

    Ive replaced a couple of tubes directly with just a high voltage T0220
    mosfet,
    these were large expensive high current tubes but just used for power supply
    regulation,
    (the 200v rail got shorted and you could see the wire conections inside the
    tube had fused !)
    and so mosfet was an ideal replacement.

    Colin =^.^=
     
  17. Got it thanks.

    --
    _____ _ _
    |_ _| | | | |
    | | __ _ _ __ ___ | |__| | ___ _ __ ___ ___ _ __
    | | / _` | '_ ` _ \ | __ |/ _ \| '_ ` _ \ / _ \ '__|
    _| |_ | (_| | | | | | | | | | | (_) | | | | | | __/ |
    |_____| \__,_|_| |_| |_| |_| |_|\___/|_| |_| |_|\___|_|
    __ ____
    / _| | _ \
    ___ | |_ | |_) | ___ _ __ __ _
    / _ \| _| | _ < / _ \| '__/ _` |
    | (_) | | | |_) | (_) | | | (_| |_
    \___/|_| |____/ \___/|_| \__, (_)
    __/ |
    |___/











    ....

    ....
     
  18. IN the old days, the replacements seemed to take three forms.

    Creating general replacements that plugged in without any fuss.
    The Fetrons that someone mentioned were this type, but of course
    only a handful of types were made, to replace a handful of extremely
    common tubes. And I recall a detailed letter in QST about them, how
    they worked well in some applications (I seem to recall limiters
    in two-way FM receivers) but less well in more linear applications.
    The exact details may be garbled, but there did seem to be limitations
    of even such a general replacement.

    More common were trying to build to the specific stage. So you look
    at the circuitry around it, and then try to create somethng that fit
    in that socket that worked with the surrounding circuitry. This required
    more fussing, but seemed to be a better choice. I seem to recall that
    the articles I saw often did adjust external resistors to some extent.

    I once solid-stated a surplus Collins PTO by soldering an FET in place
    across the tube socket, and running it off low voltage, and it was fine
    (though it was a 1 to 1.5MHz PTO so was pretty simple). I don't think
    I had to change anything else, but note I wasn't using the PTO in
    the original piece of equipment.

    Then there were the conversions where the chassis and tuned circuits
    and variable capacitors (or whatever) were basically just the foundation
    of a new project. It got the hard work out of the way, but of course
    didn't leave it intact. On the other hand, it was a way of improving
    equipment that had limitations. In that sort of conversion, the audio
    amplifier would be tossed out, and replaced with an audio IC amplifier.
    THe IF strip might be seen more like a function, and then something suitable
    could replace it. The thing about this was you could tamper with external
    components, so if that plate resistor was too high for whatever you were
    doing, you'd just replace it.

    But of course, most of this took place in the late sixties and by the
    end of the seventies it had faded. At the time, the equipment was
    still seen as functional, but the hope of solid state made people want
    to modify them, or maybe it was just for the sake of doing it. The
    equipment was often getting pretty old, as the seventies progressed
    it was all getting replaced with equipment that was not only solid
    state (even using those new-fangled ICs) but that switch to solid
    state allowed for a leap forward in functionality. A tube shortwave
    receiver with single conversion to 455KHz would often still suffer
    from image problems, whether the tubes were left in there or replaced
    with transistors, but a more modern receiver might have moved to
    a single conversion to an HF IF, or tossed in lots of features that
    would require extensive work to add to the old receiver.

    Michael
     
  19. Brad Albing

    Brad Albing Guest

    Tim Shoppa wrote:

    [snip]
    Oh, BTW, I love the way Freescale has reworked all the old Mot. data
    sheets and app-notes. It looks like they paid some junior high school
    kids to take a rubber stamp and stamp their name/URL over the top of
    "Motorola." Nothin' but high class for these guys....

    BA
     
  20. Tim Shoppa

    Tim Shoppa Guest

    Well, all the remaining guys have data sheets that show heritage back
    through several generations of ownership. I still like the CD4000
    series datasheets at TI's website where you can clearly see the
    Harris/Intersil lineage!

    And every time I look at AN211A (every couple of years for the past
    couple of decades) I get completely thrown for a loop by that AND gate
    in Figure 22 making -130V :).

    Tim.
     
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