[Elecraft] KPA1500 reduces power as it gets warmer?

[Bob McGraw]

Kevin:

The duty cycle on FT-8 mode is ~50%.  The KPA1500 and KPA500 are both 
very adequately rated for this mode and for rated outputs, 1500 watts 
and 500 watts respectively.

Your K2 is rated at 10 watts output!   At 15 watts, just because it will 
do it, is no reason to run that power level.  I can understand why the 
PA failed.   Also SWR will contribute to heating of the PA devices.  
Poor antenna match will eat PA devices.

Hams seem to think running at reduced power will "save something".  This 
is bad ham advice and basically "old ham lore" which is not correct.

Conversely, my measurements show my KPA500 is 38% efficient at 250 watts 
and 57% efficient at 500 watts on 20M.  This relates to 404 watts of 
dissipation at 250 watts and 368 watts of dissipation at 500 watts.   
Thus 500 watts is less strenuous, producing less heat on the components, 
than 250 watts.

Yes, FT-8 is a weak signal mode.  I run near legal limit on 2M to a nice 
array just to have a weak signal off of the moon.   Low power just won't 
get it.

73

Bob, K4TAX

On 6/6/2022 11:29 AM, elecraft-request at mailman.qth.net wrote:
> From:elecraft-bounces at mailman.qth.net  <elecraft-bounces at mailman.qth.net>  on behalf of Kevin Robertson via Elecraft<elecraft at mailman.qth.net>
> Sent: Saturday, June 4, 2022 6:52 AM
> To: Rick NK7I<rick.nk7i at gmail.com>
> Cc:elecraft at mailman.qth.net  <elecraft at mailman.qth.net>
> Subject: Re: [Elecraft] KPA1500 reduces power as it gets warmer?
>
> FT8 is a weak signal mode, designed for low power. Given its high duty cycle, you are asking for trouble if you operate your power amplifier in FT8 mode using your KPA1500 at anywhere near high power. I burned out my final on my K2 at 15 watts on FT8.
>
> 73,
> Kevin Robertson KD2WCY

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