Eric, et & al:
I have a KPA500 on the desk. It is about 1 month old. As to
efficiency which is related to power output vs. power input, a simple
ohms law calculation is all that's needed to see what is occurring.
During transmit, note the value of HV and the value of CURRENT. Also
note the power output in watts. All of these values are on the LCD
display. Per Ohms law, Volts x Current = watts which is DC watts
input. Then Power output ÷ DC watts input X 100 = % efficiency.
With my amp in CW mode key down on 80M: Voltage = 63.0, Current = 12.1
amps, RF power output is 575 watts. Thus 63.0 x 12.1 = 762.3 watts DC
power input. Then 575/763.3 = .7533 x 100 = 75.33%. In this
configuration 762.3 - 575 = 187.3 watts of required heat dissipation.
Now at a lower power output on 80M: Voltage is 64.6, Current is 8.8
amps, RF power output is 300 watts. Thus 64.6 x 8.8 = 568.5 watts DC
power input. Then 300/568.5 = .5277 x 100 = 52.77% In this
configuration 268.5 watts of required heat dissipation.
My power measurements are made using my Bird 43 and a 50.5 ohm dummy
load. I have not done a calibration on the KPA500 power meter,
although I find it relatively accurate.
Now in each case, the difference between RF power output and DC power
input is the watts of heat that will be dissipated. Yes, it is clear
that running the amp at higher power, but never to exceed the rating nor
temperature limits is the more efficient method. By the way, those hams
that run their tube amps at reduced power "to save the tubes" are in
fact doing just the opposite in as much as the efficiency at lower power
is less than at higher power. The heat has to go somewhere and it ends
up in the amp or being exhausted by the cooling system.
As to running the amp at 500 watts, or 600 watts or 700 watts, thus just
before fault power, I would need to examine the IMD products to be
confident the amp is "clean". A solid state amp which is being over
driven just a wee bit can get really nasty on the air and real fast.
The product is splatter on either side of the operating frequency.
Avoid this and sacrifice a few watts of output power.
As to the KPA1500, I have no experience or knowledge. But I would say
it follows the same rational of operating and efficiency.
73
Bob, K4TAX
On 5/24/2018 7:54 PM, eric norris via Elecraft wrote:
While trying to run my KPA500 at full-tilt--but as cool as possible--in digital
modes, I have noticed over the years that:
1). The KPA500 actually runs cooler at just-below-fault power output than it
does at either 500 or even 400 watts output.
2) The KPA500 runs substanitally cooler at a 1:1 SWR than it does at even
1.2:1.
Does the KPA1500 also do this? I'd appreciate actual experience more than
conjecture.
Thanks and 73,
Eric WD6DBM
Sent from Yahoo Mail on Android
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