30-06-2020, 12:41 PM
Mike Watterson Wrote:All the devices are a sharp cut off, no variable mu possible as that is achieved by varying the grid wire spacing. There are no grid wires at all. They also can ONLY work with a thin filamentary cathode, "indirect" heated versions are technically impossible.
The G1 is a pair of plates either side of the filament, almost gammatron type action. Then two wedges of electrons are attracted from the filament to the two anode plates at opposite sides.
Now that's given me some thought!
The first point, no variable-μ being possible: what happens if the grid rods are slightly curved so their spacing varies along their length? I'm thinking, where the rods are close together will be easier to cut off. Then the remaining regions will be still active, but less responsive to g1 voltage changes owing to the wider spacing.
So, you've made a vari-μ device!
The second point - indirectly-heated being technically impossible: Granted that a point (or line) source of emission is what's needed. But the rod pentodes don't have a line of zero thickness. They have a filament, maybe 30μm diameter. Now 30μ cannot be a 'hard' limit, such that 30μm works but 31μm won't, so just 'grow' the filament. What happens at 50μm? 100μm? 250μm? You'll reach a point where it is possible to drill a hole through the length of the filament, and thread an insulated heater wire through.
There may be no call for doing so, and it may be difficult, But at the moment, I can't accept it to be technically impossible.
