[Milsurplus] Fixed Screen Voltage in Plate Mod. Transmitter PA?

[David Stinson]

While working on reviving a plate-modulated
AM transmitter,
I came-upon some "expert" advice that has
me doubting.  "Experts" are often well-meaning
people who have their sometimes-misguided
or premature "advice" accepted without
question, simply because a friend said
"he's an expert."

The word of mistaken "experts"
often gets passed-around until it's taken
as gospel.  One well-known expert, who
otherwise has many good ideas,
surprised me by willy-nilly increasing
the Hi B+ filtering in a transmitter
from 8 mFd to 220 mFd, saying:
"What the heck; it can't hurt!"
It most certainly CAN hurt, depending
on the robustness of one's rectifiers
and transformers.  50s and 60s ham
rigs- especially kits like Heath and
Johnson- didn't leave a lot of extra
head-room in their power supply
designs and parts.  I'm not a fan of
the DX-100 because it runs everything
at "the hairy edge."

Now before I get started, let me confess that
I'm no "expert."  My concerns may be nothing
and I may not understand the principle involved.
That's why I ask you- it's a blessing to know so
many people who are smarter than me. :-)

In this case, the advice was to remove
the PA screen buss from the modulated
side of the mod transformer and move
it to the fixed B+ buss side,
fixing the screen voltage at Hi-B+ minus
dividers.  This is supposed to make the
modulator "more linear"
and achieve a higher mod level.

If we consider what's happening during
typical plate-and-screen modulation,
both the plate and the screen are at
high potential at the peak of the
modulation waveform, and both are
at zero potential at the zero-crossing.

However, if we remove the screen from
the modulated voltage and fix it at B+
minus dividers, then as we approach
zero-crossing, the plate is at zero
potential, but the screen is at full B+.
We thus draw high screen current
and may also encourage current from
secondary emission.  This can't be a
good thing for either the PA tube or
our transformer, which is now forced
to supply current at a higher duty cycle.

This bugs me.  Someone smarter than
me please tell me why it's OK to do this.

Thanks!
GL OM ES 73 DE Dave AB5S




-- 
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus