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KPA-1500 Power Output Variation

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Jim McCook W6YA

KPA-1500 Power Output Variation

Antenna is 2 el. delta. Uses 1/4 wave 70 ohm matching section.
Described in ON4UN's Low Band DXing. Used for 38 years with various
tube amplifiers and no tuners. All coax has been replaced as recently
as 10 years ago. SWR measurements always similar. Power output with
tube amps does not vary visibly on a Bird WM. Same situation currently
with a tube amp substituted. Feedline from matching section about 50
ft. I know how to match antennas.:-) I have seen the usual reactance
measurement on each side of design frequency of 7.025. The built in
tuner is the one I was referencing. The KPA-1500 comes with one.

Jim
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Don Wilhelm

Re: KPA-1500 Power Output Variation

Jim,

It would be informative to know where you are observing the power variation.
In the feedline after the KAT1500 with your Bird WM or on the KPA1500
panel display (or the KPA1500 Utility)?

73,
Don W3FPR

On 8/24/2019 3:58 PM, Jim McCook wrote:

> Antenna is 2 el. delta. Uses 1/4 wave 70 ohm matching section.
> Described in ON4UN's Low Band DXing. Used for 38 years with various
> tube amplifiers and no tuners. All coax has been replaced as recently
> as 10 years ago. SWR measurements always similar. Power output with
> tube amps does not vary visibly on a Bird WM. Same situation currently
> with a tube amp substituted. Feedline from matching section about 50
> ft. I know how to match antennas.:-) I have seen the usual reactance
> measurement on each side of design frequency of 7.025. The built in
> tuner is the one I was referencing. The KPA-1500 comes with one.
>
> Jim
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Bill K9YEQ

Re: KPA-1500 Power Output Variation

In reply to this post by Jim McCook W6YA

Thank you, Jim. That is more helpful. I use a remote tuner rather than using the built in for matching. Thus my situation would not match yours. I use a 350 loop on 40 meters, thus I would not be helpful in your situation. Using a tube amp is much different in matching versus the solid state amplifier as I remember when I last used a tube amp. I believe your specifics will now lead to a better answer to your situation.

73,
Bill
K9YEQ

https://wrj-tech.com/

-----Original Message-----
From: [hidden email] <[hidden email]> On Behalf Of Jim McCook
Sent: Saturday, August 24, 2019 2:58 PM
To: Elecraft Reflector <[hidden email]>
Subject: [Elecraft] KPA-1500 Power Output Variation

Antenna is 2 el. delta. Uses 1/4 wave 70 ohm matching section.
Described in ON4UN's Low Band DXing. Used for 38 years with various tube amplifiers and no tuners. All coax has been replaced as recently as 10 years ago. SWR measurements always similar. Power output with tube amps does not vary visibly on a Bird WM. Same situation currently with a tube amp substituted. Feedline from matching section about 50 ft. I know how to match antennas.:-) I have seen the usual reactance measurement on each side of design frequency of 7.025. The built in tuner is the one I was referencing. The KPA-1500 comes with one.

Jim
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ANDY DURBIN

KPA-1500 Power Output Variation

In reply to this post by Jim McCook W6YA

"I believe that additional attenuation might be helpful if you can implement it without upsetting the RX path, although on most bands 2 or 3 dB extra attenuation wouldn't matter."

Both the KPA500 and KPA1500 have built in drive attenuators (3 dB for KAT500, KPA1500 unknown) but there seems to be no serial command to activate them (only control is internal for over-drive protection). It would seem to be a fairly simple firmware change to allow the user to activate the attenuator but that would remove a layer of protection from accidental over-drive.

Anyone willing to make a (minor?) hardware mod could activate the built in drive attenuator without impacting the RX signal path.

Like Wes, I'm not surprised he KPA1500 is sensitive to load. I am surprised that sensitivity is still seen when the load is matched by the internal tuner.

73,
Andy, k3wyc

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Jim McCook W6YA

Re: KPA-1500 Power Output Variation

In reply to this post by Jim McCook W6YA

Don,

I'm seeing the power changes on the front panel of the amp, on the
Utility, and on the Bird external to the amplifier. The Bird 2500w slug
is about 40 years old, so it probably needs recalibrating. Still, I can
see the relative differences at all 3 spots. There is no fault in the
amp when it hits the high power level around 1700w.

My attenuator idea was to put it at the input of the transmit section of
the amp, out of the receive line..... in the same place as the built in
attenuator. It still seems to me it would help for reasons I mentioned
before. With low drive levels the output is most sensitive to
relatively small changes in drive. I think the idea of running the K-3
closer to rated output is still desirable.

Thanks, guys, for all the input.

Jim W6YA
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Elecraft mailing list

Re: KPA-1500 Power Output Variation

Jim;

It would be interesting to see the source of the variance. You might insert the Bird between the transceiver and the KPA to see if there is any variance there.

The amplifier has fixed gain - it has no ability to vary its gain during transmission other than a protective pad that is inserted when problems are detected. The power is not
changing enough for that to be indicated, which makes me wonder just where the actual variance comes from.

73!
Jack, W6FB

> On Aug 24, 2019, at 3:58 PM, Jim McCook <[hidden email]> wrote:
>
> Don,
>
> I'm seeing the power changes on the front panel of the amp, on the Utility, and on the Bird external to the amplifier. The Bird 2500w slug is about 40 years old, so it probably needs recalibrating. Still, I can see the relative differences at all 3 spots. There is no fault in the amp when it hits the high power level around 1700w.
>
> My attenuator idea was to put it at the input of the transmit section of the amp, out of the receive line..... in the same place as the built in attenuator. It still seems to me it would help for reasons I mentioned before. With low drive levels the output is most sensitive to relatively small changes in drive. I think the idea of running the K-3 closer to rated output is still desirable.
>
> Thanks, guys, for all the input.
>
> Jim W6YA
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Jim McCook W6YA

Re: KPA-1500 Power Output Variation

In reply to this post by Jim McCook W6YA

Jack,

Thanks for the idea. I've not checked power in the drive line from the
K-3. However, when running the K-3 barefoot with the amp on or off at
any power level I see none of these power changes in the K-3 output.

Other than my previous comment about drive levels, I have to wonder if
it has to do with tuner firmware in the KPA-1500. If nobody else is
seeing a similar problem, there must be something before or after the
amp that creates the problem.

73, Jim
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Eric Swartz - WA6HHQ

Re: KPA-1500 Power Output Variation

Administrator

In reply to this post by ANDY DURBIN

The internal attenuator is only rated for short term usage in cases of over drive or high reflected power. Any hw modification forcing it on will risk damage to the input attenuator components.

73,
Eric
elecraft.com
_..._

> On Aug 24, 2019, at 3:21 PM, Andy Durbin <[hidden email]> wrote:
>
> "I believe that additional attenuation might be helpful if you can implement it without upsetting the RX path, although on most bands 2 or 3 dB extra attenuation wouldn't matter."
>
> Both the KPA500 and KPA1500 have built in drive attenuators (3 dB for KAT500, KPA1500 unknown) but there seems to be no serial command to activate them (only control is internal for over-drive protection). It would seem to be a fairly simple firmware change to allow the user to activate the attenuator but that would remove a layer of protection from accidental over-drive.
>
> Anyone willing to make a (minor?) hardware mod could activate the built in drive attenuator without impacting the RX signal path.
>
> Like Wes, I'm not surprised he KPA1500 is sensitive to load. I am surprised that sensitivity is still seen when the load is matched by the internal tuner.
>
> 73,
> Andy, k3wyc
>
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Jim McCook W6YA

Re: KPA-1500 Power Output Variation

In reply to this post by Jim McCook W6YA

Eric,

If the attenuator had more attenuation that caused the drive to be
closer to the K-3 output I would think this would reduce the sensitivity
of the drive level with respect to change in power output. 6w change in
drive (from a larger input attenuator) from 80w would result in less
output change (with the smaller attenuator) than with 6w change from
30w. Is this right?

Jack,

When I change to the tube amp I use all the same cables and connectors
for RF. None of the problem occurs. I have tried substituting cables.
I have not substituted the feedline of the 40m antenna, though.

Jim

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Nr4c

Re: KPA-1500 Power Output Variation

What about a dummy load?

Sent from my iPhone
...nr4c. bill

> On Aug 24, 2019, at 7:56 PM, Jim McCook <[hidden email]> wrote:
>
> Eric,
>
> If the attenuator had more attenuation that caused the drive to be closer to the K-3 output I would think this would reduce the sensitivity of the drive level with respect to change in power output. 6w change in drive (from a larger input attenuator) from 80w would result in less output change (with the smaller attenuator) than with 6w change from 30w. Is this right?
>
> Jack,
>
> When I change to the tube amp I use all the same cables and connectors for RF. None of the problem occurs. I have tried substituting cables. I have not substituted the feedline of the 40m antenna, though.
>
> Jim
>
>
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Jim McCook W6YA

Re: KPA-1500 Power Output Variation

Dummy load is no problem and never has been.
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Jim McCook W6YA

Re: KPA-1500 Power Output Variation

In reply to this post by Nr4c

That's not the point at all. It's about having to ride the drive level
control on the K-3 constantly, especially in a contest, to avoid sudden
output of 1700w.

> Why worry about it? No one on the other end would be able to tell one bit
> of difference. I see power drop -- maybe 5 - 10 percent -- as the KPA1500
> heats up...likely normal. The small bit of reflected power may also have
> some minor effect on the wattmeter readings you see as well. 73, Greg-N4CC

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ANDY DURBIN

KPA500 attenuator rating

In reply to this post by Eric Swartz - WA6HHQ

Eric,

I have taken, perhaps obsessive, precautions to ensure that my KPA500 is not damaged by operator error. I don't expect to ever trip the attenuator in my station. However, I don't see anything in the documentation that imposes a limit on the amount of time that the KPA500 could be driven with power that trips the attenuator.

As an example - assume 30 W gives 500 W out and the antenna load is well matched. Now I increase drive to 60 W. The attenuator trips and the KPA500 is back to 30 W drive and a well matched load. What causes the firmware to progress to a hard fault to protect the attenuator resistor?

Thanks and 73,
Andy, k3wyc
________________________________
From: Eric Swartz - WA6HHQ, Elecraft <[hidden email]>
Sent: Saturday, August 24, 2019 4:37 PM
To: Andy Durbin <[hidden email]>
Cc: [hidden email] <[hidden email]>
Subject: Re: [Elecraft] KPA-1500 Power Output Variation

The internal attenuator is only rated for short term usage in cases of over drive or high reflected power. Any hw modification forcing it on will risk damage to the input attenuator components.

73,
Eric
elecraft.com
_..._

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