[Elecraft] KPA1500 issues/blown finals

[Bob McGraw]

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