[Elecraft] [QRO] KPA500 variation of PA current band-by-band

[david park]

I have recently assembled KPA500 serial number 641.  When testing into a dummy load I find that I get a large variation in the DC power input when driving for 500W CW when I go from band to band. My worst band is 21MHz, where I get the highest current (lowest efficiency).  The drive was adjusted to give 500W as indicated by the LCD display.  Here are my results:-
 
1.8MHz      DC current 10.0A    fan hardly ever comes on
3.5MHz      DC current  10.0A   fan hardly ever comes on
7MHz                             12.0A
10.1MHz                        12.4A
14MHz                           14.3A 
18.1MHz                        13.8A
21MHz                           16.4A   temperature rises rapidly and fan soon gets to full speed
24.9MHz                        13.5A
29.7MHz                        12.3A
 
I have no results for 50MHz band as I have no transceiver covering that band.
 
The readings drift as the heatsink (and the dummy load) warm up, and so I took readings quickly and allowed time for the heatsink to cool down between each band. I find that the KPA500 current sensor tends to give low readings.
I would be interested in knowing how other amplifiers perform.
 
David Park G3PSV