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811A's, Dual Grid and Class B triodes

Discussion in 'Electronic Components' started by [email protected], Apr 2, 2007.

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  1. Guest

    I was browsing through Terman (2nd edition) last night and read a
    paragraph about "Dual Grid and Class B triodes". According to Terman,
    if two concentric (but similar pitch) grids are put in the triode and
    then connected to each other as a single control grid, the result is a
    high-mu triode that needs zero bias for Class B operation.

    I do not see dual-grid tubes mentioned in Terman's 3rd edition in the
    same way.

    Googling the term, I see that the 52 tube seems to be an example where
    both grids are brought out to individual terminals.

    I also see that sometimes "dual grid" is used to describe RF tubes
    where there are two pins for a single grid (to decrease inductance I
    guess), I'm not talking about these tubes.

    The two-grids-connected-together characteristics remind me a lot of,
    for example, the 811A (the most familiar Class B triode I'm familiar
    with), but that only has a single grid terminal. Am I correct that the
    internal grid structures of an 811A are essentially that of two
    connected grids? If not, what inside an 811A makes it zero-bias high-
    mu class B triode, as opposed to say its externally similar non-
    identical-twin the 812A (a low-to-medium-mu triode that needs bias)?

    I also note that Terman claims that the dual-grid structure forms a
    very good electrostatic shield between heater and plate, and see that
    811A's are often used in grounded-grid connection in RF amps. (Must be
    a bitch to neutralize in common-cathode).

    I've been intermittently playing around with SPICE to model 811A
    curves (including grid current at positive grid voltage) and none of
    the conventional triode models work right at all - its curves are more
    like a pentode (in fact it's pretty trivial to fit it this way if you
    let the diode characteristics take over at low plate voltage).

    Tim.
     
  2. Ed Engelken

    Ed Engelken Guest

    =======================================
    The #46 is another. The 46 was popular in a number of Atwater Kent
    radios circa 1932-33. Class B Push-Pull output stages in high-end
    radios had a brief run in the early 1930s, then faded into history. --
    Ed
     
  3. Tim Wescott

    Tim Wescott Guest

    Dual-grid tubes were popular for a short while. They were touted as
    being versatile -- connect grid 2 to grid 1 and you had a high-mu
    triode, connect grid 2 to the plate and you had a low-mu triode.
    Judging from my tube data books, it didn't take folks long to decide
    they just wanted one or the other, not two in one package.

    The curves look pretty triode-like to me, once you take into account the
    fact that the thing is running almost exclusively at positive grid
    voltages. Perhaps the SPICE triode models aren't taking this into account?

    --

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

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

    Do you need to implement control loops in software?
    "Applied Control Theory for Embedded Systems" gives you just what it says.
    See details at http://www.wescottdesign.com/actfes/actfes.html
     
  4. tubegarden

    tubegarden Guest

    Hi RATs!

    I have a huge P-P 46 vintage amp. It is not yet functioning. It is
    clearly very well built :)

    Grow up, Class "A" does not mean good, it means simple ... which,
    despite some journalists' daydreams, is not exactly the same
    thing ...

    Happy Ears!
    Al
     
  5. Guest

    A push-pull pair of zero-bias class B triodes is even more remarkably
    simple.

    When I first saw the schematic of a 811A P-P audio amp, I was
    astonished to find not a single resistor or capacitor in the circuit.
    Two tubes, input and output transformers, and connections to a
    filament supply and a plate supply. That's all there is, folks!

    Tim.
     
  6. Bret Ludwig

    Bret Ludwig Guest

    This is very true and interesting. RCA figured out you could strap
    the control and screen grids of certain power pentodes and use them as
    low or zero bias triodes, and a couple of RCA theater amps do this.
    This is also seen in AM era editions of the W6SAI Radio Handbook for
    modulators. It is simple. It works. Based on long time experiments it
    was concluded it doesn't sound very good at all.

    I have never tried it, on the theoryu if it worked very well it would
    be popular.

    The 811 is certainly an attractive tube economically. It has a mu of
    160 as I remember. The Altec 1570B used them as did the Gotham Audio
    cutter head amp using a GR toroid for output.
     
  7. RapidRonnie

    RapidRonnie Guest

    Several of the last of the RCA MI-series cinema amps did this with
    6L6s and I believe 6146s. None are desireable to the audiophile in
    stock form.
    They showed why toroids are a bad idea for opt's very well, although
    there are still a lot of people not listening. With a conventional or
    C-core transformer and a revised feedback loop the basic design is
    well worth study, though.

    The 811 can be substituted in the MI200 McIntosh if the proper
    intermediate transformer is put in so as to get the filament voltage
    at the 6.3 rather than the 10 volts of the, I think, 8005s. Although
    the amplification factor of the 811 is way too high it works anyway.
    Distortion still meets Mc specs. We wound one, in fact, where I worked
    on a toroid, because we had plenty of scrap surplus transformers we
    could dewind (and take the copper home.) And a Gorman winder, which
    makes winding toroids pretty fast and simple. The 811s pull more
    current but the Mc filament transformers never even got hot.
     
  8. RapidRonnie

    RapidRonnie Guest

    Because using conventional circuits the distortion was terrible. True
    Class B operation works well only at high continuous levels without
    extreme measures such as Wiggins and Mcintosh/Gow/Corderman afforded.
    Al Bereskin at Baldwin designed the "poor man's Mc" for organ use and
    it was published in an extremely good DIY article in the IRE journal
    in '55 or '56. It never got below 2% THD.
     
  9. Ian Iveson

    Ian Iveson Guest

    Bugger... HTML.

    Al said:
    ....
    Grow up, Class "A" does not mean good, it means simple ... which,
    despite some journalists' daydreams, is not exactly the same
    thing

    *Saxophones are really complicated.

    cheers, Ian.
     
  10. Jim Mueller

    Jim Mueller Guest

    The dual grid tubes strike me as a gimmick. Any pentode or beam power tube
    can be connected at least 4 ways. Connect the screen to B+ and it behaves
    as a pentode or beam power tube. Connect the screen to the control grid and
    it becomes a high-mu triode. Connect the screen to the plate and it becomes
    a low-mu triode. Connect the control grid to a low positive voltage and
    drive the screen and it is a space charge triode.

    The 811 has only one grid as do other zero bias tubes like the 6N7 and 1635.
    They just wound the grid with many turns spaced close together to get this
    characteristic. Low-mu tubes like the 812, 2A3, and 12B4 have few turns
    spaced far apart. The only triode I know of with two grids is the 6C5. The
    second grid is internally connected to the plate.

    I have seen circuits using 6V6s with the screen tied to the control grid to
    make a zero bias triode.

    The old ARRL handbooks (50s?, 60s?) had a listing in their tube tables for
    using a 12AX7 as a class B zero bias output tube. They said it could
    produce 7.5W. I never saw a circuit that actually used one this way,
    however.
     
  11. Uncle Peter

    Uncle Peter Guest

    Most of those sets used push-pull class A, not B.

    Pete
     
  12. ken scharf

    ken scharf Guest

    There is more than one way to skin a cat. The 811A is a high mu triode.
    There are several ways to create such a beast.

    One: place the grid closer to the cathode than to the plate. The ratio
    of the distance between grid and cathode vs the distance between cathode
    and plate is one of the factors that determine the mu of a triode.

    Two: construct the grid with close spaced turns. The closer the turns
    of the grid wires are to each other, the higher the mu.

    Three: Use two concentric grids. This is very similar to number two.
    ANY triode works well in grounded grid. A high mu triode will have
    better screening due to the close wound grid. Tetrodes such as the
    4-400 have been used in grounded grid by grounding both grids. Others
    such as the 4cx250 don't have enough heavy enough grids to handle the
    current in class AB2 or B so they are run in cathode driven service with
    normal grid and screen voltages, but with both grid and screen grounded
    for RF.
     
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