"Antonio Vernucci" wrote in message
. ..
True pentodes are needed for suppressor grid modulation. The beam
forming plates have much less of an effect on plate current and trying
to use them for modulation will result in a highly nonlinear
modulation waveform. Remember that emission current not going to the
plate will instead go to the screen grid. The power dissipation of
the screen grid is what seriously limits the use of suppressor
modulation.

If you already have the sockets for the 813, I would suggest the use
of 4E27A/5-125B tubes. The 4E27/Heintz & Kaufman 257B has less plate
dissipation, but it too might be suitable.

Please let me know if you need datasheets for either of these tubes
and for the 803.

73, Barry WA4VZQ wa4vzq(-at-)live(-dot-)com

Hi Barry,

thanks for the tips. However, looking at the tubes characteristics, the
main advantage of the 4E27 w.r.t. the 803 is the possibility to work at
much higher frequencies (apart form the socket, which can however be
simply changed with no need to make extra holes on the chassis). For
the rest, it has a lower plate dissipation (65W against 125W),
different filament voltage and it costs more (38$ against 25$).

I appreciate the screen grid dissipation problem but it is not clear
to me why your statement "that emission current not going to the plate
will instead go to the screen grid" would not equally apply if the tube
has a proper suppressor grid instead of bram forming plates. Could you
please clarify?

73

Tony I0JX

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