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.