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Re: Need Recommendation for 2M Amp



> From:          Ke4yyddave@aol.com
> Date:          Tue, 12 Sep 2000 07:38:44 EDT
> Subject:       Re: [amsat-bb] Need Recommendation for 2M Amp
> To:            bruninga@nadn.navy.mil, k6ccc@AMSAT.Org
> Cc:            greg@valkyrie.net, k5hv@hilconet.com, amsat-bb@AMSAT.Org,
>                rlubash@poco.mv.com

> I think Jim's problem was the nonservice which TE Systems is famous.
> 
> Te Systems has repeater series amps designed for continuos service.  If they 
> are not suitable for that service why advertise such?
> 
> David

Dear Dave,

They advertise them as such in order to sell them:-)!  Seriously, "ham" type
as opposed to commercial amps can be used in repeater service.  It
just takes some modification and care.  Heat is the great enemy of amps.
And not just the heat sink temperature.  You have to consider the heating
of the board components as well.

Some amps have poor thermal design to start with.  That is, the PA
transistors are poorly mounted in thermal terms.  Some have components
such as power resistors in bias networks and RF chokes that are
physically too small for the amount of current they are suppose to carry.

For my repeater applications, I start by derating the amp by 50%.  For
50 watts out, I use a 100 watt amp.  Then bench test the amp for at least
an hour on the air checking the heat rise on the heat sink and the 
transistor cases (this is the important one).  Look for smoking resistors,
exploding RF chokes, etc.

You will have to use a cooling fan.  I use two or more in case one fails.
The fans are controlled by an electronic thermostat circuit.  If the amp
overheats, the fan controller shuts it down.  You may have to ventilate
the underside of the chassis as well to cool the board components.

Now getting back to Amsat/SSB issues.  The amps I own are not
linear or even close to it.  The "SSB/FM" switch on the front merely
changes the time constant for the RF detection switching circuit!
A resistive bias supply to the base of the transistors is always in
the circuit.  In theory, the transistors are biased class AB for SSB
operation.

In practice, the resistive bias networks do not supply enough current
to keep the transistor in class AB during voice peaks.  The bases
drop to zero volts (or even go negative!).  In other words, your amp
runs class AB when its not transmitting and class C when it is!

Somewhere I read of a modification to a amp for an active bias
regulator.  I tried the circuit and have been well pleased by the 
result.  Many ops have commented on the clarity of my signal
on 2M and 6M.  The amps draw more current as expected since
the bases can draw all the current they need to keep biased
on.

BTW, when I modified my RF Concepts amp, the RF chokes in
it exploded due to the high current peaks.  Replacing them with
handwound ones solved the problem.  Yes, the modified amps
have worked perfectly for the last three years.

The bottom line is that our currently available amps are built
to price.  We hams are cheap.  You can extend the life of a amp
by derating it and paying attention to cooling.  I have a small 12V
muffin fan that I sit on top of my hard working 170W 2M amp and
it never gets warm.  That fan is cheap insurance.  An active
regulated bias supply will greatly improve linearity and your
on the air audio quality.

Pay attention to drive levels.  I've heard many operators try to get
the last watt out of their amp.  Getting 180 watts out of your 170 watt
amp will not get you any more points in the next contest.  If your amp
is rated at 10 watts input, that means 10 watts, not 20!  

I own three amps that I use in my SSB stations.  My OLD Mirage D1010
100 watt UHF amp is very well made, it has been rugged and trouble
free.  My 6M 100 watt amp is home brew from a artical in QST.  It has
also been trouble free.  My RF Concepts amps construction is best
described as "haywire" with tiny little cheap RF chokes and RF relays
held to the PC board with glue.  After modification it also works well.
Today, with Mirage gone, I would be hard pressed to decide what
to buy myself.

--73-- David WA0AUQ

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