Hi Bob, With regard to reduced power you mentioned in your previous reply, it is true that the power dissipation does increase with reduced power but not by a large amount. Typically, when reducing from 1500 watts, the maximum dissipation increase is less than 5 percent and occurs at about 1000 watts output. I don't think Elecraft was suggesting that the amp should be run at full power but more that reducing power would not reduce the heat load. Reducing power does however greatly reduce the chance of FET failure due to "Hot Carrier Injection".
I am in complete agreement with your reply of today. 73, Hal-W7YNC > On 02/16/2026 11:28 AM PST Bob McGraw via Elecraft <[email protected]> > 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, [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 > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[email protected] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > Message delivered to [email protected] ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [email protected]
