PENTODE/TRIODE SWITCH Question

[Otis]

I'm reading up on tube amps and am curious about something regarding amps that  switch between pentode and triode mode with output tubes to reduce power , a very usefule thing that I always use with my seymour duncan convertable which is way more amp than I generaly need.As I understand it, the pentode tube came into being in order to control ocilation in the tube, so how does cutting two grids out decrease power, and why doesn't it introduce the problems that the pentode was designed to prevent?

[Otis]

Ok, I think I've got it. The added sceen grid makes it much more effeicient, the suppresor grid stops unwanted bounce back, so I presume that in the switching they simply disconnect the sceen grid, leaving the suppresor grid be, thereby cutting gain?

[PRR]

> the pentode tube came into being in order to control ocilation

In RADIO-frequency circuits, triode plate-to-grid feedback (and tuned circuits) make oscillation.

In LARGE-signal circuits, triode plate-to-grid feedback makes the tube more linear but also reduces plate swing.

G2 blocks triode plate-to-grid feedback. (G3 keeps G2 effective down to very low plate voltage.)

Historically the first effect was found first.

In audio we are only interested in the second effect.