Kenneth Scharf wrote:
Antonio Vernucci wrote:
There are two other ways of handling the screen voltage in a
modulated stage. The screen can be feed by a regulated voltage supply
and a separate winding on the modulation transformer is used to
modulate the screen.

Unfortunately, as I had written, my transformer has a single secondary
winding.

A variation on this is to put an audio choke in series with the
screen supply and to optionally couple the screen to the hot side of
the plate modulator winding via a capacitor.

Yes, I had seen that method too on the ARRL Handbook (as well as in
the EIMAC booklet), but I was trying to avoid the bulk of an extra choke.

Nevertheless, browsing the Internet, I found a dealer who sells 20H
50mA chokes of reasonable size at a reasonable price. So, I am getting
more and more convinced that that is the way to go.

Still, I would be interested to hear from someone who has actually
tried that method.

Thanks & 73.

Tony I0JX
You can also just ignore the issue and only apply modulation to the
plate of the pentode/beam power tube. In theory you won't reach 100%
modulation, however given that the screen input is a tiny fraction of
the plate input you'll probably achieve somewhere between 95-99%
modulation. You don't want to go over 100%, and unless you are using a
speech compressor you won't ever hit 100% modulation peaks when properly
set up anyway.
Another idea would be to use a second low power modulation transformer
with a separate amplifier feed off the same mike amp and with it's own
level adjustment to only modulate the screen. You'd need two level
adjustments in addition to the main mike gain control to set the proper
levels for the high power plate modulator and the low power screen
modulator. The screen modulator would only need to be a few watts,
maybe single class A 6L6 or 6V6. I realize this is getting more
complicated than just the choke in the screen circuit, but that's the
beauty of home brew, trying things out just for the heck of it.

Another idea would be to use a second low power modulation transformer with a
separate amplifier feed off the same mike amp and with it's own level
adjustment to only modulate the screen. You'd need two level adjustments in
addition to the main mike gain control to set the proper levels for the high
power plate modulator and the low power screen modulator. The screen
modulator would only need to be a few watts, maybe single class A 6L6 or 6V6.
I realize this is getting more complicated than just the choke in the screen
circuit, but that's the beauty of home brew, trying things out just for the
heck of it.

I sometimes tend to be perfectionist, and I would then not like to start a
project pursuing a compromise solution from the very beginning. So, I would like
to have the screen modulated too, not just the plate.

With regard to the separate audio amplifier for the screen, it should work but I
am not sure whether some phase shift could occur between the two amplified audio
signals (apart from the 180-degree phase ambiguity that can be easily resolved
crossing the wires of one of the two modulation transformers).

I have come to the conclusion that using a filter choke on the screen probably
is the simplest way to go, and the safest for the final tube too.

73

Tony I0JX

Antonio Vernucci wrote:
I sometimes tend to be perfectionist, and I would then not like to start a
project pursuing a compromise solution from the very beginning. So, I would like
to have the screen modulated too, not just the plate.

I think that if you do this, you have higher audio distortion even though you
have a slightly higher modulation level.

With regard to the separate audio amplifier for the screen, it should work but I
am not sure whether some phase shift could occur between the two amplified audio
signals (apart from the 180-degree phase ambiguity that can be easily resolved
crossing the wires of one of the two modulation transformers).

It depends mostly on how well matched the modulation transformers are. Most
of the group delay will be the result of the transformers.

Better to use a transformer with two secondaries than two individual amps.

I have come to the conclusion that using a filter choke on the screen probably
is the simplest way to go, and the safest for the final tube too.

Is the best possible fidelity important to you? Is having a tiny little bit
more modulation important to you? I think in the manuals to the Bauer 707
broadcast transmitters there is a paragraph or two about the compromise.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."