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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