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
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[Bob McGraw - K4TAX]

I run my amp on 240 VAC with the yellow tap as supplied from the
factory.   No hum or buzz when operating 550 watts output.  Drive on 6M
is 30 watts.  SWR is 1.2:1 or better.  Other bands the same.   The
KAT500 memory is "trained" every 50 kHz on all band below 10M.    In
fact with my ear against the amp, nothing is heard. FAN CTL is on 1.  
Other than the LED's flashing, I have no clue the amp is even active.  
Very satisfied with the results in all regards.

73

Bob, K4TAX

KPA500F s/n 3519


On 7/29/2018 6:28 PM, ANDY DURBIN wrote:

> 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
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
> Message delivered to [hidden email]
>

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