"If anybody has any lessons learned I would appreciate hearing them."
I built a KPA500 kit and I too was bothered by the transformer hum. I learned that Elecraft has used several different transformer suppliers and that transformer sizes vary. This means the clearances between the transformer and the case may be very different for the supplied transformer than as shown in the manual. I carefully shimmed the transformer before tightening the retaining nut so there was no case contact.
I also learned that hum level is directly related to PA current. If the PA load is mismatched the efficiency goes down, PA current is higher for same power output, and hum level is higher. I ignore the 1.5:1 match is ok recommendations and tune for better than 1.2:1.
Perhaps the most important thing I learned is that one can get used to almost anything. The hum and clicks that drove me crazy in the first few months of ownership are now ignored. I found something else to worry about.
Andy, k3wyc
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