[Elecraft] KPA1500 issues/blown finals

[dyno lab]

You bet!
Don't hesitate to contact me if you need more info.
73,
Hal W7YNC

> On 02/17/2026 12:38 AM PST Doug Turnbull via Elecraft <elecraft at mailman.qth.net> wrote:
> 
>  
> Dear OMs,
> Mny TU for this information.    It will inform my ongoing use of the KPA
> 1500.    Wanted to express my thanks.
> 73 Doug EI2CN
> 
> On Mon 16 Feb 2026, 19:29 Bob McGraw via Elecraft, <elecraft at mailman.qth.net>
> wrote:
> 
> > One should understand and adhere to the duty cycle at 1500 watts.  From
> > the manual:   "100% ICAS for 100% duty cycle modulation modes (FM, RTTY,
> > FT8, JT65, etc.) 1.8-30 MHz: Maximum 5 minutes key down / Minimum 5
> > minutes receive (after 5 minute TX) 50 MHz: Maximum 50 seconds key down
> > / Minimum 50 seconds receive."
> >
> > These values must be considered using a 50 ohm resistive load.  Hence, a
> > simple 1.1:1 SWR does not necessarily indicate a 50 ohm non-reactive
> > load.  Also I would comment, person opinion and experience, a transmit
> > time of 5 minutes and a receive time of 5 minutes equals a duty cycle of
> > 50% with one cycle being 10 minutes.   Yes, if one considered only the
> > transmit time for referenced modes, the duty cycle is 100%.  (But that
> > isn't the way I was taught for pulse modulation.)
> >
> > As a reference, FT-8 duty cycle is slightly less then 50%  based on
> > RX/TX timing.   MS-144, a meteor scatter mode,  runs about the same but
> > with longer transmit time and longer receive time.
> >
> > Yes I view from my measurements on the KPA500, the efficiency varies
> > from a high of 60% to a low of 25% based on power level and band.  For
> > this amp, the dissipation high was 444 watts on 15M at 400 watts output
> > to a low of 150 watts on 80M at 500 watts output.    The gain of the
> > amplifier was 13.7 dB to 14.8 dB, again depending on band and power
> > value.  It was clear that signal compression started in the 400 to 500
> > watt power range.   My measurements were based on using two Bird 43
> > meters and suitable elements, one for input power and one for output
> > power.  A 1kW Bird 50 ohm load was part of the test equipment lineup.
> > These are not considered to be lab grade equipment, but equipment of
> > reasonable accuracy along with measurement methods employed.  (FYI - all
> > purchased new by me, thus not hamfest or e-bay, etc. products)
> >
> > My conclusion is the same measurements should be performed on the
> > KPA1500, thus allowing one to see the variations, band to band, and
> > power value to power value.  The use of and Excel spreadsheet and simple
> > formulas are an aid in producing reliable results and accuracy.
> >
> > 73
> >
> > Bob, K4TAX
> >
> > On 2/16/2026 11:00 AM, elecraft-request at mailman.qth.net wrote:
> > > essage: 15
> > > Date: Mon, 16 Feb 2026 09:32:03 +0200
> > > From: Victor Rosenthal<k2vco.vic at gmail.com>
> > > To: Elecraft Reflector<elecraft at mailman.qth.net>
> > > Subject: Re: [Elecraft] KPA1500 issues/blown finals
> > > Message-ID:
> > >       <
> > CAJZhGajwd4_ZPj8cNXLHwdPrm31KEsvvOU7j-xLYrdR2eKb9uw at mail.gmail.com>
> > > Content-Type: text/plain; charset="UTF-8"
> > >
> > > I recall Elecraft saying that the KPA1500 runs hotter at lower power
> > levels
> > > because it is designed for maximum efficiency at full output.
> > > So while it's possible there will be lower voltage peaks when output is
> > > reduced, heat might become an issue.
> > >
> > > Victor 4X6GP
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