Re: QRO - KPA500 Failure

Posted by K9MA on Feb 06, 2020; 10:45pm
URL: http://elecraft.85.s1.nabble.com/QRO-KPA500-Failure-tp7658244p7658260.html

Basically, based on Bob's measurements, the power dissipated is fairly
independent of drive level. So, running at reduced power really isn't
stressing anything any more that full output. (Unlike an "ideal" class B
amplifier.)

73,
Scott K9MA



On 2/6/2020 16:06, Bob McGraw K4TAX wrote:

> I've never seen much if any advantage in running a linear below its
> rated output.  I know the regulation state never run more power than
> needed for adequate communications or terms like that.    The
> difference between 500 watts and 100 watts is 6.9 dB or about 1 S unit.
>
> In fact, most all of the time, running at less than rated output is
> less efficient.   It saves nothing.   Same for tube amps where I hear
> hams say they are running at reduced power to "save the tubes".  
> Nope, not the case.
>
> (RF output / DC input) * 100 = % efficiency         DC input watts  =
> DC volts x DC amps
>
> Do the math and you'll see what I mean.
>
> A quick run of the numbers with my KPA500 into a dummy load. Output
> measured with a Bird 43.  DC values taken from the KPA500 display
>
> 450 W out, 819 W input, 64 volts @ 12.8 amps for 55% efficiency Drive
> pwr = 16.5W  14.3 dB gain     819 - 450 = 369 watts = heating factor =
> 0.88
>
> 210 W out, 576 W input, 67 volts @ 8.6 amps for 37% efficiency Drive
> pwr = 7.0 W  14.7 dB gain     576 - 210 = 366 watts = heating factor =
> 1.74
>
> 100 W out, 393 W input, 69 volts @ 5.7 amps for 25% efficiency Drive
> pwr = 3.2 W  14.9 dB gain   393 - 100 = 293 watts = heating factor = 2.93
>
> 73
>
> Bob, K4TAX
>
>
> On 2/6/2020 3:07 PM, Rick Bates, NK7I wrote:
>> Running low output power from it, is far less efficient than pushing
>> it hard (compare wattages consumed to RF watts output delivered at
>> different output levels); which may have stressed the finals. Ideally
>> you run the previous stages at low-moderate power for lower IMD, then
>> punch it up with the amp.
>
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--
Scott  K9MA

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