Excellent replies so far.
What we start with is a ten watt radio. We add the KPA3, which provides
a jump in output by 10 db. We can add the KPA500 which provides another
~12 db; because you're now loafing the KPA3, keeping linearity
'manageable'. (Yes, you can push the KPA500 to 600 watts on many bands,
but the IMD gives less return on that power investment.)
This has been very successful as a model for Elecraft. I don't see any
need for it to change (but without restarting another thread, a KPA1500
with a KAT1500 matching tuner *would* be awesome).
If the user wants to run at 25 watts, use a K3(s)/100. It's a simple
macro, computer command or knob twist to reach 25 watts and the signal
will be sparkling clean. But I also note that if 25 watts was a hard
limit, my KPA500 could not be driven to full output on all bands, a
couple require more input.
Let's not mess with success... This is one case that fewer options (not
having a 25 watt amp at max OR having it biased into class A, using more
energy) is a good thing.
73,
Rick wa6nhc
On 2/22/2017 8:30 PM, Cameron Francey wrote:
Thumbs up here to this reply!
Also, if you have 25 watts max from the rig then you limit your ability to
reduce to 100 watts or less where its more than enough.
Remembering that we use the least amount of power needed to maintain the
contact. Just because the gas peddle goes as far as the floor doesn't mean it
needs to be pushed that far to get to the shops!
Best regards,
Cameron -AF7DK/GM7LQR
________________________________
From: Elecraft <[email protected]> on behalf of Matthew Cook
<[email protected]>
Sent: Wednesday, February 22, 2017 7:32 PM
To: Harry Yingst
Cc: Elecraft Reflector
Subject: Re: [Elecraft] Product Idea - 25 Watt PA for the K3
To make this idea work you need to make a 25Wpep amplifier that is kept in
it's linear region so that the IMD from this amplifier does not cause
problems in the output of the KPA500.
To make a class A-B 25W amplifier such as this with enough linearity will
result in you designing an amplifier capable of 100Wpep at full smoke. The
non-linear nature of HF PA's is easily found on the net. Having designed
PA's for HF manpacks a number of years ago we simply turned down our
100Wpep design to 25Wpep and pocketed the performance increase at the
expense of cost.
The K3/100 at 25-35W has excellent linearity (highly likely by design, pat
designers on back for this one) which allows a more aggressive approach
within the KPA500 for grunt at the expense of "some" linearity. You're
not increasing the cost of the KPA500 trying to eeek out every last bit of
linearity to get around the rubbish coming from the transceiver.
I'm keen to do some measurements in the near future of the SPE 1.3k-fa
amplifier being driven by a K3/10. Having measured the IMD of my the
K3/100 at 10W (KPA3 disabled) recently I was a little surprised at the
result, it was not as clean as when driven at 30W with the KPA3 enabled.
I'd not be surprised to see some output degradation in the SPE from the
K3/10 c.f K3/100 driven at 20W with a -3dB inline attenuator. This would
certainly explain some of the "wide" local signals that I've been seeing in
my SDR recently.
Engineering is always about compromise, and cascaded amplifiers are one of
the more trickier things to strike the right balance with.
73
Matthew
VK5ZM
On 23 February 2017 at 07:55, Harry Yingst via Elecraft <
[email protected]> wrote:
Seeing how the KPA- 500 tends to need less than 25 watts to drive it to
full output (500 watts) It got me thinking about a 25 watt PA Option.
Would it be possible to replace the existing 10 watt PA board with a 25
watt PA board that would use a larger heatsink instead of the case.
If the stock Filters and Relays can handle the 25 watts it would seem that
it would be a more inexpensive option for those who plan on going with the
KPA500.
Just a thought...
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