Rick,
I think you are trying to "outwit" the designers.
One thing I can say is that the KPA500 is quite robust and
conservatively rated. One or more of the Field Testers was asked to run
an RTTY contest at "full out" power, and I know one who ran the whole
weekend contest at 700 watts and no problems were detected.
Typical of Elecraft's conservative designs, there are 2 devices, but the
ratings for each device is equal to the stated power output for the
amplifier. Other manufacturers may specify the power for the maximum
ratings for the devices, but not Elecraft.
The FETs in the KPA500 are *each* rated for greater than 500 watts -
that is a total of 1000 watts, so they are "loafing along" at 500 watts.
Similarly with the KPA1500 which uses 2 devices each rated at 1500
watts (3000 watt rating total), the KPA1500 will do legal limit with the
devices loafing along.
Bottom line, the Elecraft amplifiers will do maximum power at a 50% duty
cycle all day long. You should reduce the power for long winded AM
transmissions, but otherwise you can run maximum power.
As far as the efficiency, that is a design compromise. Maximum
efficiency is achieved at somewhere near the maximum rated output. At
levels less than the rated power (or greater than), you can expect the
efficiency to decrease.
Even below the rated power, the devices are running well below their
rated levels, so you are not stressing the FETs by running below 500 watts.
In other words, the designer will set the parameters for the maximum
power rating, and above and below that point, the efficiency will be
lower because the optimal impedance will not be present.
73,
Don W3FPR
On 1/16/2018 6:14 PM, Richard Stutsman wrote:
I've had my KPA500 for a couple of weeks, now, and I have questions
relating to its efficiency as a function of power output and band. A friend
of mine recommended that I run only 300 watts output most of the time in
order to preserve the life of the FETs. I told him that I didn't spend
$2300 on an amp just to get less than 5 dB of gain in my signal (over my
100-watt exciter) instead of the 7 or 8 dB I paid for. Am I wrong?
I took a lot of data this morning that indicates that there is an inverse
relationship between output power and efficiency - to the extent that, down
to about 300 watts, I find that the amount of heat the finals have to
dissipate is less, the higher the power output. This is rather
anti-intuitive.
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