http://elecraft.85.s1.nabble.com/KPA500-Power-Output-tp7230465p7230955.html
increase in MOSFET channel resistance with temperature. As the MOSFETs heat,
> -----Original Message-----
> From:
[hidden email]
> [mailto:
[hidden email]] On Behalf Of Ron
> D'Eau Claire
> Sent: Friday, January 27, 2012 10:18 AM
> To:
[hidden email]
> Cc:
[hidden email]
> Subject: Re: [Elecraft] KPA500 Power Output
>
> A quick way to check Jack's suggestion that the mains source
> voltage may be
> drooping is to monitor the HV on the KPA500 while
> transmitting to see if it
> decreases over time.
>
> Ron AC7AC
>
> -----Original Message-----
> From:
[hidden email]
> [mailto:
[hidden email]] On Behalf Of Jack Brindle
> Sent: Friday, January 27, 2012 9:55 AM
> To:
[hidden email]
> Cc:
[hidden email]
> Subject: Re: [Elecraft] KPA500 Power Output
>
> Are you sure it is the KPA500 that is causing the drop off?
> There are
> a lot of variables here. The exciter could be scaling back,
> the power
> source (110/220) could be limiting you or something else may be
> happening. As power cords heat, they tend to restrict the electron
> flow. Especially if you are powered from 110V, that could be the
> source of several problems. Be sure not to replace the
> supplied power
> cord with a smaller one - this is a recipe for reduced output
> in itself.
>
> We have also seen transmitters decrease their output power
> when keyed
> down for a long time. This might be due to heating in the TX, ALC
> action, or other things specific to the radio.
>
> I would suggest adding a wattmeter between the exciter and
> the KPA500
> so you can monitor power between the devices. Also check the power
> source wiring to make sure there are no limitations there. In
> general,
> look to see if there are other factors external to the KPA causing
> power reduction.
>
> Jack B, W6FB
>
>
> On Jan 27, 2012, at 8:57 AM,
[hidden email] 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
> >
> >
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