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

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"Tim Wescott" wrote in message
...
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

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