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KPA500 Power Output

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N4CW

KPA500 Power Output

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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Jack Brindle

Re: 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
>
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html

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Bill Gerth

Re: KPA500 Power Output

In reply to this post by N4CW

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
[hidden email]

On Jan 27, 2012, at 10:57 AM, [hidden email] wrote:

Vic Rosenthal

Re: KPA500 Power Output

Of course! It's rated in MFJ WATTS. One regular watt = about 10 MFJ watts. Elecraft is one
of the few companies making products rated in real watts.

On 1/27/2012 10:21 AM, Bill Gerth wrote:

> 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
> [hidden email]
>
>
>
>
> On Jan 27, 2012, at 10:57 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
>>
>>

--
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/

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Bob Wolbert, K6XX

Re: KPA500 Power Output

In reply to this post by Jack Brindle

I believe the most significant contributor to key-down power sag is the
increase in MOSFET channel resistance with temperature. As the MOSFETs heat,
their ON resistance increases slightly, which drops output power slightly.

Fortunately this is a small effect, especially when measured in dB.

73 de Bob, K6XX

> -----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
> >
> >
> > ______________________________________________________________
> > Elecraft mailing list
> > Home: http://mailman.qth.net/mailman/listinfo/elecraft
> > Help: http://mailman.qth.net/mmfaq.htm
> > Post: mailto:[hidden email]
> >
> > 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:[hidden email]
>
> 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:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html