[Elecraft] QRO - KPA500 Failure

[Macy monkeys]

I run my KPA500 output just below the level I feel I can get away with...without lighting up the neighborhood. About 350 watts :)

John K7FD

> On Feb 6, 2020, at 2:06 PM, Bob McGraw K4TAX <rmcgraw at blomand.net> 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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