Posted by
john@kk9a.com on
Feb 07, 2020; 3:16pm
URL: http://elecraft.85.s1.nabble.com/QRO-KPA500-Failure-tp7658244p7658273.html
There may be reasons for running at less than the amplifier max such
as the US 200 watt 30m band limitation. I have no RF engineering
knowledge but after looking at your data I see less heat with lower
output wattage which seems to me like it would be less stress on finals.
John KK9A
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
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