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

[Ray Fantini]

Assuming you are talking about a Heathkit DX-100, I am by no means an expert but I did pull up the schematic of the 100 and that screen clamp 6AQ5 and the two 6146 tubes look like a beast being that when designed they went thru a huge effort to introduce the modulation on the clamp tube, look at the cathode of the clamp and its tied to the cathodes of the two modulators so they had some reason for wanting to introduce audio on the screen circuit. Would assume if not they would have put a filter between two 10 K plate dropping resistors feeding the plate of the clamp tube.
I am use to designs where you want no AC or audio on the screen grid but and you use a clamp to cut of the PA but that’s usually in larger transmitters. Think if it were me I would be inclined to go with the original design of the PA/Modulator and keying circuit assuming they must have had a good reason for doing all that in the first place? Did you read thru the description if there is one in the manual for the DX-100? 
Just like military radios sometimes its best to stay with the original design then start trying Ham modifications. Who knows, maybe the 6146 needed a kick on the screen to make full power or perhaps that’s the way they kept you from going beyond 100 % modulation with that transmitter? Have no idea myself.
The AM Phone site is the source of all knowledge in the AM world, or so they say. Try posting over there under the technical section.


http://www.amfone.net/Amforum/index.php


Ray F/KA3EKH


-----Original Message-----
From: MMRCG at groups.io <MMRCG at groups.io> On Behalf Of David Stinson via groups.io
Sent: Wednesday, October 7, 2020 7:33 AM
To: Boatanchors at mailman <boatanchors at mailman.qth.net>; milsurplus at mailman <milsurplus at mailman.qth.net>
Subject: [MMRCG] Fixed Screen Voltage in Plate Mod. Transmitter PA?

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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




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