I've had my KPA500 for a couple of weeks, now, and I have questions
relating to its efficiency as a function of power output and band. A friend
of mine recommended that I run only 300 watts output most of the time in
order to preserve the life of the FETs. I told him that I didn't spend
$2300 on an amp just to get less than 5 dB of gain in my signal (over my
100-watt exciter) instead of the 7 or 8 dB I paid for. Am I wrong?
I took a lot of data this morning that indicates that there is an inverse
relationship between output power and efficiency - to the extent that, down
to about 300 watts, I find that the amount of heat the finals have to
dissipate is less, the higher the power output. This is rather
anti-intuitive.
And if that is true, then wouldn't it be better for the finals if I
routinely ran the amp at the higher power, so long as voltage, current, and
temperature remain within safe limits? Is it safe to run 600 watts on CW,
especially on 80 and 160 meters, where the efficiency appears to be so good
that the finals run cool at 600 watts - even cooler than at 500 watts?
The following data was taken while transmitting into a dummy load. I
adjusted the drive power to obtain one of several predefined output power
levels and then measured input volts and amps and calculated input power
(HV x Current), power dissipation (Input - Output), and efficiency
(Input/Output). Each measurement was taken after about one second of key
down in CW mode. Doing the calculations in between each test allowed the
finals and dummy load to cool off. I never heard the fan come on. Drive,
Output, Input and Dissipation are in Watts. I have shown voltages falling
slightly below 60.0 in red, but I had no problem getting 600 watts output
with less than 40 watts of drive, even in those cases.
*Band Drive Output Volts Amps Input Dissipation Efficiency*
1.8 23 500 64.3 10.2 654 154 76%
30 600 63.9 10.9 697 97 86%
3.5 23 500 65.1 9.4 612 112 82%
30 600 63.2 10.5 664 64 90%
5 24 500 63.7 11.2 707 207 71%
30 600 62.8 12.0 754 154 80%
7 27 500 62.3 12.5 779 279 64%
35 600 61.2 13.8 845 245 71%
10 32 500 61.2 13.4 820 320 61%
39 600 59.8 14.6 873 273 69%
14 15 225 63.4 9.6 609 384 37%
20 310 62.9 11.5 723 413 43%
24 400 61.5 12.8 787 387 51%
33 500 59.0 14.3 844 344 59%
40 600 58.7 15.8 927 327 65%
18 26 490 60.6 12.9 782 392 63%
33 600 59.6 13.8 822 222 73%
21 26 500 60.0 13.9 834 334 60%
35 600 59.8 15.2 909 309 66%
24 23 500 61.4 12.4 761 261 66%
30 600 61.0 13.5 824 224 73%
28 22 500 61.4 12.7 780 280 64%
27 600 60.6 13.8 836 236 72%
Note that in every case there is lower power dissipation (and therefore
less heat generated), the higher the power. I decided to test several lower
power outputs on 20 meters, and I got the maximum dissipation at 310 watts,
and the dissipation at 225 watts was nearly the same as it was at 400 watts.
I should point out that my HV during standby or idle hovered between 79.5
and 80.0 volts. I am using the Red tap wire on the 115-volt input.
And, yes, I know not to use the amp on 60 or 30 meters.
If anybody is going to yell at me for testing a 500-watt amp at 600 watts –
and for proposing to use that much power routinely on 80 meter CW – let me
know your reasoning and why or how you think that would stress the amp.
Oh, and does anybody know if Elecraft would have adjusted the Power
calibration (per band) of the KPA500 kit version, as mentioned in the User
Manau? I found all the settings to be at their default 100%, FWIW. When
comparing at 100 watts to a cheap external watt meter (which agreed with my
TS-590sg power output setting), I found I had to increase power on almost
every band to 104 watts to trip the 100-watt indication on the KPA500's
wattmeters. I decided to leave the settings at their default.
Rick N6IET
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