[Elecraft] KPA500 finals blown

[mcduffie at ag0n.net]

On Thu, 21 Jan 2016 11:26:27 -0500, Irwin Darack wrote:

> The fans on mine do come on high and I keep an eye on the temp reading. I
> do need to stop occasionally to allow the amp to cool down. I am not sure
> if there is a way to add an addition external fan to increase the air flow?

I don't have any comparison between mine and others.  However, I can offer a few
points about how mine runs.

I use the amp at pretty much full power for meteor scatter modes (digital) on
six meters daily.  I have found two things.  First, temperature does depend on
the antenna.  Even though the VSWR on the KPA indicates 1.0 for all directions
that I point the antenna, the heat is higher in some directions than others. You
can bet that higher SWR will cause the fan to run more than a perfect match.

Second, if I keep the transmit/receive sequences down to 15 seconds each (JT9g
or h, JTMSK, ISCAT), I rarely hit fan speed 5.  It cycles as high as 4 every
cycle after initial warm up.  These on and off cycles (50% duty cycle) can
continue for from a few minutes to an hour or more for one completed QSO with
only an occasional excursion to fan speed 5.

When I point to the southeast, the fan steps on up to speed 5 almost every other
transmission period.

On some modes that have longer transmit cycles, like FSK 441 with ~45 seconds on
and one minute off, the fan regularly steps up to 6 after the initial few
cycles.  Step 6 always comes into play for me during those longer transmissions
on six meters.

I've found that I can reduce the heat cycle a little (noticeably) by using a
small fan 2-3 feet away on the right side, blowing at the hot spot on the right
rear side of the cover (this is the side panel, not the rear panel).  It doesn't
take much air, but it does reduce the high speed cycles.

All of this is on six meters.  Six meters generates more heat than HF.  I never
come above speed 4 (usually 3 at the most) on ten meters and below.

Just because I like to be conservative and strongly dislike heat in solid state
devices, I typically manually put the fan on speed 4 when I start a digital
exchange and back to NORmal when done.  That alone, keeps it from hitting speed
5 most of the time.

Drive power for all of this is 27 watts from my xcvr (at least that's what it is
set at and is likely slightly less).  Power fades to lower than 450 watts on
some modes when it is hot, but that could be the xcvr.  Most solid state devices
fade power when they get hot.  Some modes run slightly less power, but that's
more a function of the audio frequencies being used for the mode and response of
the radio.

Anyway, that's my story, and I'm stickin' to it.  :)

Love my KPA!

Gary - AG0N