[Elecraft] KPA500 hum/buzz

[ANDY DURBIN]

When I first started using my KPA500 I found the hum/buzz during high power CW operation to be intolerable.  Elecraft support sent me a paper on hum reduction, a longer transformer bolt, and additional rubber washers.  I did not use any of this new hardware but now have acceptable hum/buzz during TX.


I learned that hum/buzz is strongly related to PA current.  I first made a significant reduction in hum/buzz by changing the transformer tap from yellow to red.  This gave a high voltage below warning level at my highest line voltage.  A second significant decrease in hum/buzz was achieved by matching all antenna loads to better than 1.2:1 SWR.  Although the KPA500 is specified to accept loads of 1.5:1 SWR, the PA efficiency  is degraded and PA currents increase as the match become worse.   For bands where I use my KAT500 tuner I have manually selected L and C to gives a match better than 1.05:1.


Bottom line - if you don't like the KPA500 hum/buzz level consider changing the transformer tap and improving load matching.  Neither of these factors is mentioned in "KPA500 Application Note - Hum Mitigation.pdf".  I went from a hum/buzz level greater than fan speed 4 to less than fan speed 1.


73,

Andy k3wyc