"Antonio Vernucci" wrote in message
...
Hello Tony, de Piero I5SPO.

Quite a ( very ) long time ago, i've done an A.M. TX with a
Geloso VFO and a class C RL12P35 Wehrmacht pentode, whose
characteristics are similar to 1625 ( a.k.a. 807 with
12.6 v filament ), using a carbon mike and a step-up
audio transformer.

I believe that, using beam tetrodes, there's no way... :-(

I know also 813 and 814 ( have 2 pcs ) and these, like 807,
are useful for controlled carrier A.M. by modulating screen grid.

Do not know 803, but if i remember well, are low power tubes.

Used with suppressor modulation, their output will be one fourth
of CW rated output, and efficiency is consequently low... :-(

HTH,

73, Piero.

Hi Piero,

the 803 is about the same as the 813, though it has a proper suppressor
screen. Other differences are socket (5 pin vs. 7 pin) and maximum
frequency (20 MHz against 30 MHz).

What I need to know is if any one has ever tried to vary the 813 beam
forming plate voltage, so as to verify whether the plate current can so
be controlled. If so, I could avoid to purchase the 803s and use the
813s I already have.

I know that with suppressor screen modulation output power is about one
fourth, but efficiency will not be so low because, in absence of
modulation, also the input power decreases (by somewhat less than one
fourth). With a pair of 803s (or 813s I hope!) I should obtain about
200W of unmodulated carrier power, peaking at 800W under 100%
modulation.

I used to have a few RL12P35s, though I never actually used them. A
Radio Rivista article by Dante I1DC describes a transmitter using the
RL12P50 (a bigger brother of the RL12P35). But all those tubes are too
small for well exploiting the potential advantages of suppressor screen
modulation. As a matter of fact the advantage of not having to build a
powerful modulator becomes more evident when the needed modulator would
be real big!

73

Tony I0JX

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