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After building my KPA500 and doing initial testing I encountered a fairly loud hum during CW transmissions. During idle periods or SSB transmitting, the amp was very quiet.
After reading the posts here I moved the transformers wiring around to avoid contact with the top cover and this helped somewhat. Further investigation showed that under full load the voltage dipped below 60 and the current was up to 11.2A. The output was still 500+ Watts and clean as observed on a scope and monitored. I decided to experiment with different transformer taps, all previous testing was done with the yellow tap connected to the rectifiers.
I changed to the red tap and now the amp runs very quietly in full output power. The quiescent voltage is 76.8 and under load it is 66V @ 10A.
I hope this helps, remember to not exceed the 85V maximum as stated in the manual.
K2MTG
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