http://elecraft.85.s1.nabble.com/KPA500-Questions-about-power-and-efficiency-tp7637476p7637478.html
> 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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