[Elecraft] KPA-1500 on digital modes

[Edward R Cole]

Never realized that, but I checked dc input power vs output RF power 
on my LDMOS amps and that appears to be true.

I had intended to run my 2m 1500w amp at 600w for SSB (to be kinder 
and more linear), while running JT65 at 1300w  which is where 
saturation begins.

1385w RF: 37A x 47.2V = 1746w (disp = 361w)
600w RF: 25A x 48.5V = 1212w (disp = 612w)
http://www.kl7uw.com/test2.pdf

MRF-1K50N disp = 2941w at 25c (derate above 25c)
https://www.nxp.com/docs/en/data-sheet/MRF1K50N.pdf

But average current draw in SSB is less than JT65 (which is keydown 
carrier).  So I guess it depends on how long-winded your SSB talking is.
Typically voice duty cycle is 20% at max whereas JT65 is 50/120 seconds = 42%

Fortunately, I have a thermistor on the RF deck to monitor 
temperature with a digital voltmeter.  Control board illuminates 
"overtemp" lamp at 43c and 50v is shutdown at 55c.  Extended running 
of JT65 shows heatsink is a shade under 43c (lots of fans cooling things).
http://www.kl7uw.com/144_1500W_PA.htm

73, Ed - KL7UW

Date: Sat, 31 Aug 2019 17:04:38 -0500
From: K9MA <k9ma at sdellington.us>
To: elecraft at mailman.qth.net
Subject: Re: [Elecraft] KPA-1500 on digital modes

This is nothing unique to the KPA1500: ALL class AB amplifiers behave
this way. Even an "ideal" class AB amplifier has a maximum dissipation
at around half of full output.

One thing running at lower output probably will buy you is lower IMD.

73,
Scott K9MA


73, Ed - KL7UW
   http://www.kl7uw.com
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