[Elecraft] KPA500 HV concerns

[Brett Howard]

My KPA500 sits at about 69-70V at idle.  On TX it drops to about 49-50V.
In my home that circuit sags a good bit under load.  When I go to the green
tap I get about 83V at idle and sit around 59-60V under TX.  Based on this
I believe that you'll be just a tad over if not right at 85V when you go to
the green tap.  I notice that that latest software has moved the HV max
fault point to 90V rather than 85V yet the manual still states that you
should not go over 85V.  The manual was VERY recently republished to the
site and still states the 85V max so I'd believe it best to honor that.
However even though I am able to go to the green tap in my installation I
choose to remain on the yellow tap.  I mainly do this so that when I move
the amp to other locations its in a safer position.  However there are
efficiency/thermal results that aren't without benefit as well.

I get almost no transformer hum what-so-ever when using the yellow tap and
get quite a lot of hum on the green tap.

Furthermore when doing TX thermal tests there is another interesting
difference.  On the yellow tap I can transmit at 500 watts essentially
brick on the key with no issues and the temp hovers right around 60C and
doesn't really move much at all once it hits that point.  However on the
green tap when doing the same test the KPA500 PA temp continues to rise.
It slows down greatly once it hits about 65 but it does finally reach 70C.
At which point the fan hits full speed and the temp starts to fall.  The
temp will fall till about 67C at which point the fan speed slows down
again.  The temp then rises to 70C again and the fan speeds up.  It
continually cycles between 67C and 70C with the fan speed change until I
got bored watching the test and I was at double the rated duty cycle (a
little over 20 minutes).

Any way hope this information is useful to you.

~Brett (N7MG)


On Sat, Jan 5, 2013 at 8:22 AM, Ron W3ZV <w3zv at verizon.net> wrote:

> I am running my KPA 500 on 120v. My actual line voltage is about 122v. I
> am using the yellow tap. HV with the amp in standby is about 73 volts. With
> power set to deliver 400W rtty, the voltage drops to about 58v. My reading
> of the manual indicates that the amp might start to be less than a good
> neighbor at that voltage.
>
> Without checking, I would guess that using another tap would set the
> standby HV higher than recommended.
>
> Are my concerns justified, and is the only answer decreasing power until
> the voltage sag reaches an acceptable level?
>
> Ron W3ZV
>
>
>
>
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