Actually I did _not_ do a good job of explaining this. In a pentode, the
suppressor grid controls the ratio of the emission current going to the screen
to that going to the plate. It also eliminates the problem of secondary
emission. Beam forming plates are specifically designed to control secondary
emission, but they have much less of an effect of controlling plate current.
Rather than going into detail here, allow me to refer you to Karl
Spangenberg's book, "Vacuum Tubes." This book is now available on several
websites. Chapter 11 goes into great detail on how both pentodes and beam
power tubes work and how they differ. I would also refer you to O. H Schade's
classic paper, "Beam Power Tubes," which is also available on many of the same
websites. Pete Millet's site is my favorite with many excellent technical
books. http://www.pmillett.com
One trick to reduce screen grid dissipation in suppressor modulated pentodes
is to power the screen grid with a voltage dropping resistor from the plate
supply rather than use an independent supply.

While not available in power tubes, many receiving pentodes are designed such
that the suppressor grid acts much like the grid in terms of its
transconductance. The 6AS6 is probably the earliest example of such tubes.
They make good mixers.

73, Barry WA4VZQ

Thanks for your answer.

A negative suppressor grid then tends to "isolate" the plate from the rest of
the tube.

So, the effect is similar to when, if the plate voltage of a power audio
amplifier stage is removed by accident, the screen gets quickly destroyed due to
excessive power dissipation.

I shall then carefully check the screen dissipation in the suppressor-modulated
transmitter I am planning to buld.

Thanks and 73

Tony I0JX