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

[n5ge at n5ge.com]

Is your dummy load absolutely a flat 1.0:1 from 1.0MHz to 30MHz?

73,
Tom
Amateur Radio Operator N5GE
ARRL Lifetime Member
QCWA Lifetime Member

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On Tue, 07 Feb 2012 13:35:47 +0000, Brian Alsop <alsopb at nc.rr.com> wrote:

>Guys,
>
>I'm also curious about this-- especially what's going on on 15M.
>My values at 500 W out into a dummy load are:
>             (amp), [volts], {efficiency}
>  1.8MHz DC (11.5) [63.1] {70}
>  3.5MHz   (11.2) [63.5] {71}
>  7MHz    (12.2) [62.5] {67}
>  10.1MHz  (not measured)
>  14MHz  (13.4)[61.8] {60}
>  18.1MHz  (12.9)[62.8] (63}
>  21MHz   (15.8)[61.2] {52}
>  24.9MHz   (14.6) [60.1] {56}
>  29.7MHz   (14.7)[60.4] {56}
>
>These are in reasonable agreement with David's except for perhaps 10M.
>
>The problem with these measurements are the wattmeter accuracy (10%?) 
>and the current measurement inaccuracy. Default wattmeter calibration 
>factors were used above.  Elecraft is using a Hall effect device to 
>measure PA current.  It would be nice to know just what accuracy to 
>expect from it.
>
>The efficiencies appear to be in the right ballpark.  They tend to fall 
>on a straight line log plot vs frequency, decreasing at higher freq's.
>
>If nothing else, the above numbers serve as a benchmark for future 
>comparisons down the road for my amp.
>
>Note: The current measurement can be significantly affected by RF on the 
>outside shield of the coax.  Until a choke was employed on the feedline, 
>the 24.9 MHz current with a real antenna measured 3 amps too low.  With 
>the choke it agreed with the above dummy load value.  I suspect lots of 
>KPA-500 users do have this condition-- given the types of 
>antennas/tuners et al described by uses on this reflector.
>
>73 de Brian/K3KO
>
>On 2/4/2012 20:27, david park wrote:
>> 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
>>
>
>
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