http://elecraft.85.s1.nabble.com/KPA-1500-Efficiency-As-A-Function-of-Input-Power-tp7644435p7644453.html
Not sure why that's a surprise. It's a linear amp ... the bias point is
> Thanks for the replies.
>
> Upon further testing, I have found that the efficiency of the KPA-1500
> is varies greatly with driving power. The following table illustrates
> the relationships between exciter power, voltage, current,
> power-in(voltage x amps), power-out, efficiency, and dissipated power.
> The following test was made with the KPA-1500 into a dummy load and
> readings from the KPA-1500 utility software.
>
> (Exciter)(Amps) (Voltage) (Power-In) (Power-Out) (Efficiency Pout/Pin)
> (Dissipated Pwr)
> 10w 29a 52.7v 1528w 375W 375/1528 =
> 24.5% 1205w
> 15w 37a 52.7v 1950w 620w 620/1950 =
> 31.7% 1335w
> 20w 43a 52.6v 2262w 861w 861/2262 =
> 38.0% 1413w
> 25w 47a 52.6v 2472w 1060w 1060/2472 =
> 42.8% 1451w
> 30w 51a 52.6v 2682w 1227w 1227/2682 =
> 45.7% 1420w
> 35w 53a 52.5v 2782w 1380w 1380/2782 =
> 49.5% 1412w
> 40w 55a 52.5v 2887w 1497w 1497/2887 =
> 51.9% 1382w
> 45w 56a 52.5v 2940w 1600w 1600/2940 =
> 54.4% 1313w
> 50w 57a 52.5v 2992w 1703w 1703/2992 =
> 56.9% 1315w
> 53w 59a 52.5v 3097w 1825w 1825/3097 =
> 58.9% 1306w
>
> The remarkable thing is that DISSIPATED POWER appears to be relatively
> constant from 375w output through 1825w output. This implies the
> KPA-1500 will need to dissipate nearly the same amount of heat at low
> power output as at high power output, if I am interpreting the data
> correctly. I would like to know from Elecraft if this is normal behavior.
>
> Charles K5UA
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