[Elecraft] QRO - KPA500 Failure

[Bob McGraw K4TAX]

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.