811A's, Dual Grid and Class B triodes

Discussion in 'Electronic Components' started by shoppa@trailing-edge.com, Apr 2, 2007.

  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.
    , Apr 2, 2007
    #1
  2. Ed Engelken Guest

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

    =======================================
    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
    Ed Engelken, Apr 2, 2007
    #2
  3. Tim Wescott Guest

    wrote:
    > 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.
    >

    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
    Tim Wescott, Apr 2, 2007
    #3
  4. tubegarden Guest

    On Apr 2, 6:53�am, "Ed Engelken" <> wrote:
    > > Googling the term, I see that the 52 tube seems to be an example where
    > > both grids are brought out to individual terminals.

    >
    > =======================================
    > 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


    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
    tubegarden, Apr 2, 2007
    #4
  5. Guest

    On Apr 2, 12:14 pm, "tubegarden" <> wrote:
    > On Apr 2, 6:53?am, "Ed Engelken" <> wrote:
    >
    > > > Googling the term, I see that the 52 tube seems to be an example where
    > > > both grids are brought out to individual terminals.

    >
    > > =======================================
    > > 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

    >
    > 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 ...


    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.
    , Apr 2, 2007
    #5
  6. 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.
    Bret Ludwig, Apr 3, 2007
    #6
  7. RapidRonnie Guest

    On Apr 2, 6:27 pm, "Bret Ludwig" <> wrote:
    > 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.


    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.
    >
    > I have never tried it, on the theory 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.


    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.
    RapidRonnie, Apr 3, 2007
    #7
  8. RapidRonnie Guest

    On Apr 2, 8:53 am, "Ed Engelken" <> wrote:
    > > Googling the term, I see that the 52 tube seems to be an example where
    > > both grids are brought out to individual terminals.

    >
    > =======================================
    > 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


    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.
    RapidRonnie, Apr 3, 2007
    #8
  9. 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.
    Ian Iveson, Apr 3, 2007
    #9
  10. 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.

    --
    Jim Mueller

    To get my real email address, replace wrongname with eportiz. Then replace
    nospam with sacbeemail.

    "Tim Wescott" <> wrote in message
    news:...
    > wrote:
    >> 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.
    >>

    > 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
    Jim Mueller, Apr 3, 2007
    #10
  11. Uncle Peter Guest

    "RapidRonnie" <> wrote in message
    news:...
    > On Apr 2, 8:53 am, "Ed Engelken" <> wrote:
    >> > Googling the term, I see that the 52 tube seems to be an example where
    >> > both grids are brought out to individual terminals.

    >>
    >> =======================================
    >> 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

    >
    > 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.
    >
    >

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

    Pete
    Uncle Peter, Apr 4, 2007
    #11
  12. ken scharf Guest

    wrote:
    >
    > 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)?
    >

    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.
    > 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).
    >

    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.
    ken scharf, Apr 5, 2007
    #12

Want to reply to this thread or ask your own question?

It takes just 2 minutes to sign up (and it's free!). Just click the sign up button to choose a username and then you can ask your own questions on the forum.
Similar Threads
  1. rijo1

    811A Tube Amp. Schematic

    rijo1, Feb 14, 2005, in forum: Electronic Design
    Replies:
    3
    Views:
    2,436
    Michael Black
    Feb 14, 2005
  2. Replies:
    2
    Views:
    443
  3. Allan Adler

    triodes

    Allan Adler, Sep 5, 2003, in forum: Misc Electronics
    Replies:
    0
    Views:
    290
    Allan Adler
    Sep 5, 2003
  4. Allan Adler

    3d data for triodes

    Allan Adler, Sep 20, 2003, in forum: Misc Electronics
    Replies:
    1
    Views:
    208
    Fred Abse
    Sep 24, 2003
  5. Mike Payne

    grid connected system w/ no grid power question

    Mike Payne, Mar 8, 2006, in forum: Photovoltaics
    Replies:
    28
    Views:
    576
    wmbjk
    Mar 21, 2006
Loading...

Share This Page