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ken scharf

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.