Bert, In addition to the other suggestions you received, I'd suggest that you make sure your dummy load is capable of this kind of key-down power for that amount of time. I have an MFJ Dummy Load that is called a 1.5KW Dummy Load but the chart on the back panel shows it is only capable of around 120 watts for 60 seconds. It can only handle 1.5 KW for less than 10 SECONDS. Some fan-cooled or oil-cooled dummy loads might be OK for 500 watts of key-down for 1 minute, but my air- cooled 1.5 KW Dummy Load is NOT. The non-inductive resistor gets very hot and begins to smell even after relatively short key-down periods. Your power change might be due to the dummy load changing its effective resistance due to heating.
Just a thought. 73, W4RK - Bill Gerth Jefferson City, MO [email protected] On Jan 27, 2012, at 10:57 AM, [email protected] wrote: > Sorry if this has been covered before, but I can't recall that it > has. Is > there any reason why power output > falls off over time? Here's what I did: using MMTTY, I "diddled" > into a > 50-Ohm dummy load for one minute. > I initially had a power output of 525 Watts which went to 500 W > shortly and > went down to about 465 W > when MMTTY "diddling" timed out. I repeated that process later, > monitoring > current and noted that it also > went down over time...makes sense! > But my question is WHY? I was under the impression that I needed to > run the > amp at 500 W on RTTY for > optimum efficiency because in an earlier posting I noted that at 150W > output (in BPSK31), the amp was very inefficient and the fan was > pretty loud. If > I can't derate the output and I can't run full output, what do I do > with > the KPA500 in the digital modes? > > Bert, N4CW > > > ______________________________________________________________ > 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 ______________________________________________________________ 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
