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