What problem in Triode vacuum tubes is solved by the screen grid in Tetrode vacuum tubes?

Answer 1

The screen grid shields the control grid from the plate, reducing capacitance and the Miller Effect, improving performance at high frequencies.

It *also* greatly increases the intrinsic ("natural") gain.

This is the amplification factor, "mu".

A triode has a maximum mu of around 100, and as little as 2.0, depending on the valve type.

A tetrode/pentode has a mu in excess of 500, often in the thousands.

However, you won't usually find a specification for mu in tetrode/pentode data sheets, as it's impractical to design circuits with this amount of gain.

Around 300~350 is the practical maximum.

Answer 2

The suppressor grid reduces secondary emissions from the plate to the screen. Thus eliminating "Tetrode Kink" in the conduction curve at high current.