I am planning to build a suppressor-grid modulated AM transmitter.

In all the diagrams I have seen they use proper pentodes, i.e. tubes that have a
real suppressor grid (e.g. 803).

I have seen no diagram using beam power tubes (e.g. 813 or 814) which have beam
forming plates instead of the suppressor grid .

Before purchasing 803 tubes, I would like to be sure that 813s or 814s are
really unsuitable for suppressor-grid modulation.

My question is whether, with 813s or 814s, the plate current can be actually
controlled by varying the beam forming plates voltage.

Does anyone have experience on that issue?

73

Tony I0JX

Rome, Italy

Il 16/01/2011 19.19, Antonio Vernucci ha scritto:
I am planning to build a suppressor-grid modulated AM transmitter.

In all the diagrams I have seen they use proper pentodes, i.e. tubes
that have a real suppressor grid (e.g. 803).

I have seen no diagram using beam power tubes (e.g. 813 or 814) which
have beam forming plates instead of the suppressor grid .

Before purchasing 803 tubes, I would like to be sure that 813s or 814s
are really unsuitable for suppressor-grid modulation.

My question is whether, with 813s or 814s, the plate current can be
actually controlled by varying the beam forming plates voltage.

Does anyone have experience on that issue?

73

Tony I0JX

Rome, Italy


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.

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

"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

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

"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

"Antonio Vernucci" wrote in message
...
I am planning to build a suppressor-grid modulated AM transmitter.

In all the diagrams I have seen they use proper pentodes, i.e. tubes that
have a real suppressor grid (e.g. 803).

I have seen no diagram using beam power tubes (e.g. 813 or 814) which have
beam forming plates instead of the suppressor grid .

Before purchasing 803 tubes, I would like to be sure that 813s or 814s are
really unsuitable for suppressor-grid modulation.

My question is whether, with 813s or 814s, the plate current can be
actually controlled by varying the beam forming plates voltage.

Does anyone have experience on that issue?

This does not answer the question, but is offered as a bit of homebrew
lore.
After WWII, the 1625's were plentiful and cheap. Some Ham found that with
certain brands of 1625's, the suppressor grid was not tied to the cathode
inside the glass envelope. The two were tied together inside the tube base.
So, after removing the base, the two elements could be separated from each
other.
The aim was, guess what? Suppressor-grid modulation! I can remember
hearing some of these rigs on the air. I don't know just what percentage of
modulation was possible, but I remember it being pretty weak. Good strong
signal with very low modulation level.
The aim had been to use what parts one had, and it surely was an
inexpensive
way to join the AM crowd!

Old Chief Lynn, W7LTQ

"coffelt2" wrote in message
...
This does not answer the question, but is offered as a bit of
homebrew lore.
After WWII, the 1625's were plentiful and cheap. Some Ham found that
with
certain brands of 1625's, the suppressor grid was not tied to the
cathode
inside the glass envelope. The two were tied together inside the tube
base.
So, after removing the base, the two elements could be separated from
each
other.
The aim was, guess what? Suppressor-grid modulation! I can remember
hearing some of these rigs on the air. I don't know just what
percentage of
modulation was possible, but I remember it being pretty weak. Good
strong
signal with very low modulation level.
The aim had been to use what parts one had, and it surely was an
inexpensive
way to join the AM crowd!

Old Chief Lynn, W7LTQ

In the 1957 ARRL Handbook, these particular tubes were also used in a
cathode driven (grounded grid) amplifier.

73, Barry WA4VZQ

The aim was, guess what? Suppressor-grid modulation! I can remember
hearing some of these rigs on the air. I don't know just what percentage of
modulation was possible, but I remember it being pretty weak. Good strong
signal with very low modulation level.
The aim had been to use what parts one had, and it surely was an
inexpensive
way to join the AM crowd!

Old Chief Lynn, W7LTQ

In the 1957 ARRL Handbook, these particular tubes were also used in a cathode
driven (grounded grid) amplifier.

73, Barry WA4VZQ

Yes, I was aware of the "modified 1625" but for linear amplifier usage. Now I
know that they were also used for suppressor-grid modulated stages.

But now a doubt come to my mind: the 1625 is a beam-power tube, not a pentode
(actually it is an 807 with 12.6V filament). So, how could it work in
suppressor-grid modulated stages?

73

Tony I0JX

"Antonio Vernucci" wrote in message
...
The aim was, guess what? Suppressor-grid modulation! I can
remember
hearing some of these rigs on the air. I don't know just what
percentage of
modulation was possible, but I remember it being pretty weak. Good
strong
signal with very low modulation level.
The aim had been to use what parts one had, and it surely was an
inexpensive
way to join the AM crowd!

Old Chief Lynn, W7LTQ

In the 1957 ARRL Handbook, these particular tubes were also used in a
cathode driven (grounded grid) amplifier.

73, Barry WA4VZQ

Yes, I was aware of the "modified 1625" but for linear amplifier usage.
Now I know that they were also used for suppressor-grid modulated
stages.

But now a doubt come to my mind: the 1625 is a beam-power tube, not a
pentode (actually it is an 807 with 12.6V filament). So, how could it
work in suppressor-grid modulated stages?

73

Tony I0JX

Read Lynn's post again. He said the carrier was strong but the audio was
quite weak. It is quite difficult to get over 95% modulation with a
suppressor modulator. With beam power tubes, you get far less than this.
Also you get lots of distortion because the beam forming plates do not
linearly control plate current, so you have to lower the modulation level
even further to make the audio readable by listeners. Hence the "strong
signal with very low modulation level" makes sense. Fortunately with
pentodes, it is easy to drive the output to zero (negative modulation).
The real problem occurs when the suppressor is driven very far into the
positive voltage region and it starts drawing current. Typically the
suppressor grid can only dissipate a few watts. Since current is drawn
during the positive peaks, an audio driver must present a low impedance.

73, Barry WA4VZQ