We all know that compared with a triode, tetrode valves have an extra "screen" grid between the control grid and anode to reduce the anode/grid capacitance that causes RF feedback and instability. All of us also know that if the anode voltage dips below the screen grid voltage, secondary electrons can be drawn off the plate and onto the screen, causing a kink in the anode curves. Or something like that. Then the pentode was invented with a third, grounded grid between the anode and the screen grid that overcame this secondary emission problem.
My question is - why was it necessary to have the screen grid positively charged? Could it not just be at ground potential like the third suppressor grid? Presumably it would still function as a screen grid but without secondary emission current and a kinked anode curve? Or couldn't it function as screen and suppressor at the same time by being at ground potential?
The above is not rigorously accurate, but good enough to pose the question.