[Elecraft] KPA500

Tue Sep 6 18:00:16 EDT 2011

It's not just SSB that needs a good linear. As you move the amp out of its
linear range, special keying shaping becomes more important if you don't to
generate clicks heard across the band. Typically, it's necessary to soften
up the keyed driving rig since Class C will drastically steepen both the
make and break. 

Another issue is that modern rigs and amps like the KPA500 (and K3) have
fixed-tuned output filters that become increasingly inefficient when the
power is pushed above or below the design value because the impedance of the
amplifier changes. Older rigs that required tuning the output (dipping the
final plate current/adjusting loading for desired current at the dip)
allowed the operator to adjust for the impedance change. 

It's not often noticed when reducing power below the design value because
few will notice if the current demand of the finals only drops 20% or 30%
when the power is reduced by 50%. But going higher in power quickly pushes
the finals out of their linear range toward saturation. Vacuum tubes would
do the same is pushed hard enough, but have a much wider working range that
is usually limited only by the heat it has to dissipate. Solid state amps
more quickly saturate, unable to draw the current on peaks needed to stay
linear. That's one reason why rigs running >100 watts invariably use a
higher voltage than the common 13VDC on the finals. 

Free 1625's in the 1950's?? Sheesh, I had to pay 10 cents each for them! 

73, 

Ron AC7AC

-----Original Message-----
From: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Guy Olinger K2AV
Sent: Tuesday, September 06, 2011 10:25 AM
To: Jon K Hellan
Cc: elecraft at mailman.qth.net
Subject: Re: [Elecraft] KPA500

There ya go, the drive-it-until-it-smokes instinct.

That WORKS on CW .....  as long as it doesn't fault, or burn something,
because class C and CW get along well together.  BUT SSB cannot exceed the
** LINEAR ** range of the amplifier without splatter, which means
drive-it-until-it-smokes always generates splatter.  ANY amplifier can
generate CW power well beyond the linear range, 1) if the power supply will
supply it, 2) if the device does not burn up because of inability to
dissipate all the heat (and remember that solid state devices can have
runaway failures where they lose control of current before the current
carrying elements burn), and 3) if the amp does not have a fault system to
prevent the previous.

I can tune up my 3-1000Z monster to 2500 watts, too, (power supply is 5 kV,
2 amps CCS move-it-with-a-hand-truck monster) but if I do, it's gone WAY,
WAY, WAY off linear and spreads SSB up and down 10.  The huge range overkill
for a 1500 watt legal limit is to have the 1500 watt signal WELL down into
the linear range, it's not to run 2500 watts.  The double overkill 2 amp CCS
is to make sure that contesting does not burn it up, make it wheeze, or ever
come anywhere near close.  And the original owner may have been running more
than one amp with it.

I would have been quite happy enough with 1 amp CCS, but the total overkill
came with it when I bought it.

The KPA500 comes with internal over-range, that you can't get at because of
the fault system.  Elecraft is very well aware of ham radio's queer
self-destructive predilections and trying for maximum smoke (what a telling
Freudian clue in that idiom!).  AND also well aware that SSB does not
tolerate that.  They have their technical reputation to mind, and also know
that the outcome of "maximum smoke" usually gets shamelessly blamed on the
manufacturer.

In high school days we used to do drive-it-until-it-smokes with WWII surplus
ARC-5 transmitters, and could get 500 watts out of a pair of 1625's (two
twelve volt filament 807's in series to run off military 24 volt battery
systems).  We had a near inexhaustible supply of FREE 1625's and enjoyed
watching the tube envelopes and anodes melt holes in them.  Kind of like
fireworks without the explosion.

But the 1625's were free.  KPA500 and innards not exactly free...

Melting down 1625's WAS fun, but when it comes down to trying to hear
someone in an SSB contest, one could wish that every amp had the shutdowns
so people could NOT drive their amps into non-linearity and put all those
unnecessarily broad signals on the band.  Thank you Elecraft for putting
some linear range in the amp that people can't get at.

73, Guy.

On Tue, Sep 6, 2011 at 4:31 AM, Jon K Hellan <hellan at acm.org> wrote:

> On 09/05/2011 11:34 PM, Lyle Johnson wrote:
> > Yes, 500 watts PEP regardless of mode.  This is thus 500W RTTY/FM/CW
> > (key down :-) etc.
>
> Excellent, but then why limit it to 500 W in low duty cycle modes?
> Shouldn't it be capable of more? Are the intermod issues?
>
> Jon
>
>
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