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Re: KLM 10-140B Amp and SSB?

> Date:          Sat, 05 Sep 1998 10:46:20 -0500
> To:            amsat-bb@AMSAT.Org
> From:          jjensen <jjensen@mctcnet.net>
> Subject:       [amsat-bb] KLM 10-140B Amp and SSB?

> I found a very old 2 meter brick type amp mixed in a big junk box of mine.
> It's a black box style brick that has "KLM electronics 140 Watt Solid State
> Power Amplifier" on the front panel.  The nomenclature on the back is very
> worn, but it appears that it is a model 10-140B...  It most definately is a
> 2 meter amp.  I'm guessing, however, it is an FM only model, as it has no
> FM/SSB switch, so it's probably not linear???  Anyone familiar with this
> unit and knowledgeable as to any "ssb-ability" for it?
> Please pardon my ignorance on this topic!
> 73 de Joel, W0JDJ
> AMSAT #1795

Dear Joel,

Good question that brings up some interesting issues!

I've recently been doing some work in this area for my own station.
First off, all the "FM/SSB" switch does on the amps I have seen is
switch in a longer time constant for the RF sensing circuit.  That is,
in "SSB" mode, an additional capacitor is switched in to give the 
amplifiers switching relays a longer hang time.  The switch does
absolutly nothing for the linearity (or lack there of) of the amp!

In any case, RF sense switched relays are almost worthless for
serious operation.  Even with the long time constant, they clatter
constantly and are very distracting.  Most amps already have 
plug on the rear that allows you to "hard switch" them from your
transceiver or station squencer.  If they do not have the plug, it
is easy to defeat the RF sensor and add hard switching.

Now, the "linear" part of the story.  Here things get interesting!
The amps I own (Mirage and RF Concepts) use a simple passive 
bias network to bias the power transistors into class AB operation.
You can often find a 50 ohm/5W resistor as part of the network.
The problem with this circuit is that the transistors draw so much 
base current on modulation peaks that the bias voltage drops 
to zero!  The result is that you have class AB operation when your
NOT transmitting and class C operation when you are!

Why the hell are the amps designed this way?  Simple, cost.  Plus
Motorola shows this circuit in their application notes.  da.....

I have seen a simple active bias regulation circuit published.  
I modified it slightly and installed it in my amps.  The result is 
a marrower signal on SSB and less distortion.  That is, linear
operation!  If you want to modify the amp, it can be made to work
very well on SSB.  By the way, KLM products were marketed to
the SSB community so the amp may already be pretty good.
You may want to call them and ask for a schematic and manual.

--73-- David Metz  WA0AUQ
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