[AMPS] drive derived screen potential

[Lamb]

Hi Skipp:

There are perhaps 2 other reasons to use a circuit which derives screen
voltage from the drive.  The first is that it reduces dissipation in
proportion to the speech, so that it is zero while the operator is not
talking - thus saving up to 750 watts every time the operator shuts his
mouth.  The second is that is simply something different, allowing amateurs
like to experiment.

The effect of a circuit using a screen voltage derived from drive may be
that of a volume expander.  Such devices have been used in audio circuits
for years.  The concern is that such an operation might cause distortion
leading to unfriendly attacks by neighboring hams.  Claims vary widely about
the results.  G2DAF developed one circuit using that scheme. And, I am
advised that he used a spectrum analyzer to analyze his results.  Les
Earnshaw, ZL1AAX, developed a similar circuit.  Others have also explored
the circuitry.  The pioneering that Les has done into electronics would give
his circuitry a great deal of credibility - at least one which I would not
question without doing a lot of investigation.  He designed the Atlas series
of transceivers, developed a number of circuits for military use and more
recently started Kachina.  As I recall from my reading many years ago, he
was quite impressed with his development of this circuitry.

The Radiotron Designer's Handbook, published by the Amalgamated Wireless
Valve Company Pty, Ltd. of Sydney, Australia (1952) and distributed by RCA
states 10 desired characteristics for volume expansion circuits.  Some of
those are:
1. Negligible non-linear distortion.
2.  The expansion should result in the upwards expansion of loud passages
and the downwards expansion of soft passages.
3.  The attack time should be short - times from .2 to 200 milliseconds are
in common use, but the shorter times are preferable.
4.  There should be no audible thump or transient distortion with sudden
large transients.
5.  There should be no appreciable reduction in maximum power output.
6.  The overall gain should not be reduced seriously by the expander.
7.  The shape of the expansion characteristic should provide some expansion
at low output levels, the amount of expansion steadily increasing all the
way to maximum power output.

It also states that "for most purposes an expansion of 10 to 12 db is
satisfactory, although some prefer up to 15 or even 20 db gain".

This definition highlights the dilemma.  If you build an audio amplifier
that distorts, you have to listen to the distortion.  The good news is that
if you build an rf amplifier that distorts, you do not have to listen to the
distortion.

One question is whether amateurs should want to expand their signal after
they have compressed it.

Perhaps this circuit will compensate for too much compression!

I do not know the answers.  As I get older I keep running into more and more
questions.  While the computer age has expanded the knowledge base, it has
also expanded the question base.  I am not sure which is expanding at a
higher rate.

Colin  K7FM


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