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, [email protected] wrote:
essage: 15
Date: Mon, 16 Feb 2026 09:32:03 +0200
From: Victor Rosenthal<[email protected]>
To: Elecraft Reflector<[email protected]>
Subject: Re: [Elecraft] KPA1500 issues/blown finals
Message-ID:
<cajzhgajwd4_zpj8cnxlhwdprm31kesvvou7j-xlyrdr2ekb...@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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