811A's, Dual Grid and Class B triodes

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.

>
> 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

(E-Mail Removed) 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

On Apr 2, 6:53�am, "Ed Engelken" <EdEn...@gvtc.com> 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

On Apr 2, 12:14 pm, "tubegarden" <tubegar...@aol.com> wrote:
> On Apr 2, 6:53?am, "Ed Engelken" <EdEn...@gvtc.com> 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.

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.

On Apr 2, 6:27 pm, "Bret Ludwig" <bretld...@yahoo.com> 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.

On Apr 2, 8:53 am, "Ed Engelken" <EdEn...@gvtc.com> 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.

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.

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 (E-Mail Removed)

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

"Tim Wescott" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> (E-Mail Removed) 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