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KPA-1500 Power Output Variation

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Jim McCook W6YA

KPA-1500 Power Output Variation

I'm using my K-3 with the KPA-1500 and have a problem with extreme
changes in power output with a given drive level on 40m CW. This
happens to a lesser extent on other bands with different antennas. My
40m antenna is tuned to 7.025 where SWR is 1.1:1. On 7.000 it is 1.4:1
and at 7.050 it's 1.3:1. Within that range of frequencies the output
_without_ the tuner at a given drive level (30w) varies from 1300w to
well over 1700w. Using careful _tuner_ settings it's only slightly
better. I have to reduce drive to 24 or 25w to get below 1500w,
depending on the temperature of the amplifier (power output varies a lot
with temperature changes). Higher temp = lower output.

I have tried to compensate in the segments where output is excessive by
using manual settings via the Utility. This has helped slightly, but
still isn't adequate. I've resisted using ALC because of concern for
generating distortion. Having to ride the drive level from the K-3 is a
PIA, especially during contests.

It seems to me a good answer would be to have higher input attenuation
so drive requirement would be higher, maybe around 70+ watts. I'd
expect changing drive level by the same number of watts (5 or 6) would
not affect output as much. I'd also assume the power output would not
vary nearly so much with a given drive level.

Is this a possible solution? I've also understood that most
transmitters (even the K-3) have cleaner output near highest rated output.

Help!

Jim W6YA

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Bill K9YEQ

Re: KPA-1500 Power Output Variation

Seeing a 1.1:1.0 SWR at the amplifier doesn't necessarily mean your antenna is working as you expect. Without getting into details, I am sure others will chime in, your antenna setup is highly suspicious given what you have briefly reported. You didn't indicate antenna type, feedline, feedline distance, etc. You might find reading an antenna handbook very informative. You can also YouTube or do on line searches to learn about antenna matching. As a KPA1500 owner, my antenna setup doesn't give me anything close to what you are experiencing. Are you using the built in tuner? Have you used an antenna analyzer to determine the antenna's characteristics on the frequencies you included in your questioning?

73,
Bill
K9YEQ

https://wrj-tech.com/

-----Original Message-----
From: [hidden email] <[hidden email]> On Behalf Of Jim McCook
Sent: Saturday, August 24, 2019 1:45 PM
To: Elecraft Reflector <[hidden email]>
Subject: [Elecraft] KPA-1500 Power Output Variation

I'm using my K-3 with the KPA-1500 and have a problem with extreme changes in power output with a given drive level on 40m CW. This happens to a lesser extent on other bands with different antennas. My 40m antenna is tuned to 7.025 where SWR is 1.1:1. On 7.000 it is 1.4:1 and at 7.050 it's 1.3:1. Within that range of frequencies the output _without_ the tuner at a given drive level (30w) varies from 1300w to well over 1700w. Using careful _tuner_ settings it's only slightly better. I have to reduce drive to 24 or 25w to get below 1500w, depending on the temperature of the amplifier (power output varies a lot with temperature changes). Higher temp = lower output.

I have tried to compensate in the segments where output is excessive by using manual settings via the Utility. This has helped slightly, but still isn't adequate. I've resisted using ALC because of concern for generating distortion. Having to ride the drive level from the K-3 is a PIA, especially during contests.

It seems to me a good answer would be to have higher input attenuation so drive requirement would be higher, maybe around 70+ watts. I'd expect changing drive level by the same number of watts (5 or 6) would not affect output as much. I'd also assume the power output would not vary nearly so much with a given drive level.

Is this a possible solution? I've also understood that most transmitters (even the K-3) have cleaner output near highest rated output.

Help!

Jim W6YA

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Wes Stewart-2

Re: KPA-1500 Power Output Variation

I'm sorry but I don't think Jim deserves a condescending lesson on antennas.

Wes N7WS

On 8/24/2019 12:39 PM, Bill Johnson wrote:

> Seeing a 1.1:1.0 SWR at the amplifier doesn't necessarily mean your antenna is working as you expect. Without getting into details, I am sure others will chime in, your antenna setup is highly suspicious given what you have briefly reported. You didn't indicate antenna type, feedline, feedline distance, etc. You might find reading an antenna handbook very informative. You can also YouTube or do on line searches to learn about antenna matching. As a KPA1500 owner, my antenna setup doesn't give me anything close to what you are experiencing. Are you using the built in tuner? Have you used an antenna analyzer to determine the antenna's characteristics on the frequencies you included in your questioning?
>
> 73,
> Bill
> K9YEQ
>
> https://wrj-tech.com/
>
> -----Original Message-----
> From: [hidden email] <[hidden email]> On Behalf Of Jim McCook
> Sent: Saturday, August 24, 2019 1:45 PM
> To: Elecraft Reflector <[hidden email]>
> Subject: [Elecraft] KPA-1500 Power Output Variation
>
> I'm using my K-3 with the KPA-1500 and have a problem with extreme changes in power output with a given drive level on 40m CW. This happens to a lesser extent on other bands with different antennas. My 40m antenna is tuned to 7.025 where SWR is 1.1:1. On 7.000 it is 1.4:1 and at 7.050 it's 1.3:1. Within that range of frequencies the output _without_ the tuner at a given drive level (30w) varies from 1300w to well over 1700w. Using careful _tuner_ settings it's only slightly better. I have to reduce drive to 24 or 25w to get below 1500w, depending on the temperature of the amplifier (power output varies a lot with temperature changes). Higher temp = lower output.
>
> I have tried to compensate in the segments where output is excessive by using manual settings via the Utility. This has helped slightly, but still isn't adequate. I've resisted using ALC because of concern for generating distortion. Having to ride the drive level from the K-3 is a PIA, especially during contests.
>
> It seems to me a good answer would be to have higher input attenuation so drive requirement would be higher, maybe around 70+ watts. I'd expect changing drive level by the same number of watts (5 or 6) would not affect output as much. I'd also assume the power output would not vary nearly so much with a given drive level.
>
> Is this a possible solution? I've also understood that most transmitters (even the K-3) have cleaner output near highest rated output.
>
> Help!
>
> Jim W6YA
>

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Roger D Johnson

Re: KPA-1500 Power Output Variation

In reply to this post by Jim McCook W6YA

I just did some experiments with my K3/KPA1500 combo into a dummy load.
I used my LP-700 for power measurements. Drive from K3 was set to 30W
(28 on K3 indication).

I started at 35C with PO at 1362 watts. At 70C, the PO dropped to 1277
watts...about a 6% decrease. K3 output remained steady at 30W.

What we need is someone with a separate tuner that can set up loads
greater and lesser than 50 Ohms and with Xc and Xl components. We may
find greater output with loads less than 50 Ohms and/or loads with Xl
rather than Xc. This is one of the disadvantages of having a fixed output
network. With the old AL-1200, I could just tune for max smoke and set
the drive accordingly!

73, Roger

On 8/24/2019 2:45 PM, Jim McCook wrote:

> I'm using my K-3 with the KPA-1500 and have a problem with extreme changes in
> power output with a given drive level on 40m CW. This happens to a lesser
> extent on other bands with different antennas. My 40m antenna is tuned to 7.025
> where SWR is 1.1:1. On 7.000 it is 1.4:1 and at 7.050 it's 1.3:1. Within that
> range of frequencies the output _without_ the tuner at a given drive level (30w)
> varies from 1300w to well over 1700w. Using careful _tuner_ settings it's only
> slightly better. I have to reduce drive to 24 or 25w to get below 1500w,
> depending on the temperature of the amplifier (power output varies a lot with
> temperature changes). Higher temp = lower output.
>
> I have tried to compensate in the segments where output is excessive by using
> manual settings via the Utility. This has helped slightly, but still isn't
> adequate. I've resisted using ALC because of concern for generating
> distortion. Having to ride the drive level from the K-3 is a PIA, especially
> during contests.
>
> It seems to me a good answer would be to have higher input attenuation so drive
> requirement would be higher, maybe around 70+ watts. I'd expect changing drive
> level by the same number of watts (5 or 6) would not affect output as much. I'd
> also assume the power output would not vary nearly so much with a given drive
> level.
>
> Is this a possible solution? I've also understood that most transmitters (even
> the K-3) have cleaner output near highest rated output.
>
> Help!
>
> Jim W6YA
>
>
> ______________________________________________________________
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Wes Stewart-2

Re: KPA-1500 Power Output Variation

In reply to this post by Jim McCook W6YA

Jim I don't have a KPA1500, but do have a KPA500. I'm not particularly surprised
at your observations based on mine with the '500.

After a recent thread about amplifier efficiency I ran some tests on mine, using
a well characterized dummy load and an LP-100A wattmeter, at rated output.
Although I only tested at the bottom of each band, I observed a variation in
efficiency of a bit under 55% at 21 MHz to 75% at 3.5 MHz. The gain varied from
12.7 dB to 15.8 dB over the HF range (excluding 50 MHz) Additionally, I see
power jitter of several watts. In past two-tone IMD testing I've seen the
spectrum break up due to (according to headquarters) the K3 ALC loop hunting.
As you no doubt have noticed adjusting output power by tweaking the input power
isn't a smooth operation either. In short, this combination is anything but stable.

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.

Wes N7WS

On 8/24/2019 11:45 AM, Jim McCook wrote:

> I'm using my K-3 with the KPA-1500 and have a problem with extreme changes in
> power output with a given drive level on 40m CW. This happens to a lesser
> extent on other bands with different antennas. My 40m antenna is tuned to
> 7.025 where SWR is 1.1:1. On 7.000 it is 1.4:1 and at 7.050 it's 1.3:1.
> Within that range of frequencies the output _without_ the tuner at a given
> drive level (30w) varies from 1300w to well over 1700w. Using careful _tuner_
> settings it's only slightly better. I have to reduce drive to 24 or 25w to
> get below 1500w, depending on the temperature of the amplifier (power output
> varies a lot with temperature changes). Higher temp = lower output.
>
> I have tried to compensate in the segments where output is excessive by using
> manual settings via the Utility. This has helped slightly, but still isn't
> adequate. I've resisted using ALC because of concern for generating
> distortion. Having to ride the drive level from the K-3 is a PIA, especially
> during contests.
>
> It seems to me a good answer would be to have higher input attenuation so
> drive requirement would be higher, maybe around 70+ watts. I'd expect
> changing drive level by the same number of watts (5 or 6) would not affect
> output as much. I'd also assume the power output would not vary nearly so
> much with a given drive level.
>
> Is this a possible solution? I've also understood that most transmitters
> (even the K-3) have cleaner output near highest rated output.
>
> Help!
>
> Jim W6YA

Jim Brown-10

Re: KPA-1500 Power Output Variation

On 8/24/2019 2:40 PM, Wes wrote:
> Jim I don't have a KPA1500, but do have a KPA500. I'm not particularly
> surprised at your observations based on mine with the '500.
>
> After a recent thread about amplifier efficiency I ran some tests on
> mine, using a well characterized dummy load and an LP-100A wattmeter,
> at rated output.

I've had my KPA1500 for only a few months, and 99% of my operation has
been on 6M. I also have an LP100A inline, calibrated at their factory.
On 6M, with the tuner bypassed, The LP100A regularly reads about 100W
(0.3 dB) lower than KPA1500 on keydown transmissions, and the LP100A
reads about 1.8:1 SWR. The KPA1500 indicates 1.3:1 - 1.1:1 SWR
(depending on the weather). If I engage the tuner, output power as read
by the LP100A drops by 100W or so, indicating loss on the order of 0.3
dB in the tuner.

How could this be? Simple. There are a relay, two UHF connector
junctions, and 6-8 ft of RG400 (0.2 dB @ 50 MHz) between the LP100A and
the output of the KPA1500. The connectors each contribute a bit of
mismatch at 50 MHz, and the length of coax is transforming the load that
the amp sees, so the LP100A and the KPA1500 are measuring both power and
VSWR at different points.

Power accuracy of the LP100A is 5% (0.21 dB) worst case, 3% (0.13 dB)
typical, and the power reading compensates for mismatch. It can read
average, peak, and peak hold.

The point of all this is that, when you're trying to see small
differences, there are a lot of little things that can add errors or
make you think something is wrong when there isn't. :)

73, Jim K9YC

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n4cc

Re: KPA-1500 Power Output Variation

In reply to this post by Jim McCook W6YA

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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K9MA

Re: KPA-1500 Power Output Variation

In reply to this post by Jim McCook W6YA

Here’s a way to get a worst case estimate of the effect of SWR on power output. Assume the amplifier has constant VOLTAGE gain and an output impedance much smaller than 50 Ohms. A 1.3:1 SWR means the load impedance could be as low as 39 Ohms or as high as 65 Ohms. Let’s say the SWR is 1:1 at the frequency at which the drive is set for 1500 W output. Then you tune to where the SWR is 1.3:1. At that SWR, the output could be anything from 1150 to 1950 W. The range may be less, as the amplifier output impedance may not be all that low compared to 50 Ohms. In any case, this shows that the power output of a solid state amplifier may be very sensitive to load SWR.

This is why the KPA1500 ATU really needs to be able to keep the SWR low. The recently improved autotune firmware does this very well, if you “train” it across the band. It is also why it’s important to use the ATU, even if the antenna SWR is “only”, say, 1.5:1.

I keep hoping Elecraft will come up with a way to automatically adjust drive power to maintain constant output. That would also compensate for change in temperature.

73,

Scott K9MA

----------

Scott Ellington

--- via iPad

> On Aug 24, 2019, at 1:45 PM, Jim McCook <[hidden email]> wrote:
>
> I'm using my K-3 with the KPA-1500 and have a problem with extreme changes in power output with a given drive level on 40m CW. This happens to a lesser extent on other bands with different antennas. My 40m antenna is tuned to 7.025 where SWR is 1.1:1. On 7.000 it is 1.4:1 and at 7.050 it's 1.3:1. Within that range of frequencies the output _without_ the tuner at a given drive level (30w) varies from 1300w to well over 1700w. Using careful _tuner_ settings it's only slightly better. I have to reduce drive to 24 or 25w to get below 1500w, depending on the temperature of the amplifier (power output varies a lot with temperature changes). Higher temp = lower output.
>
> I have tried to compensate in the segments where output is excessive by using manual settings via the Utility. This has helped slightly, but still isn't adequate. I've resisted using ALC because of concern for generating distortion. Having to ride the drive level from the K-3 is a PIA, especially during contests.
>
> It seems to me a good answer would be to have higher input attenuation so drive requirement would be higher, maybe around 70+ watts. I'd expect changing drive level by the same number of watts (5 or 6) would not affect output as much. I'd also assume the power output would not vary nearly so much with a given drive level.
>
> Is this a possible solution? I've also understood that most transmitters (even the K-3) have cleaner output near highest rated output.
>
> Help!
>
> Jim W6YA
>
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
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> Post: mailto:[hidden email]
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Bob McGraw - K4TAX

KPA-1500 Power Output Variation

Has anyone actually used, observed, and measured the effect of the ALC
feature of the amp in combination with the K3/K3S with regard to output
power variation.

I do use the method and setup as suggested in the manual for both the
KPA500 and K3S with no known ill effects. The amp drives to rated
output with 20 watts on all bands. I find it never need more drive
that this for 160M - 6M. Output power variation with temperature is
less than 10% as measured over the range of 23°C to 60°C. Fan control
is automatic and steps up and down as needed.

I do view if one hasn't used it, observed it or measured it, then "old
opinions" of ALC implementation are not valid for comment.

73

Bob, K4TAX

On 8/24/2019 11:51 PM, K9MA wrote:

> Here’s a way to get a worst case estimate of the effect of SWR on power output. Assume the amplifier has constant VOLTAGE gain and an output impedance much smaller than 50 Ohms. A 1.3:1 SWR means the load impedance could be as low as 39 Ohms or as high as 65 Ohms. Let’s say the SWR is 1:1 at the frequency at which the drive is set for 1500 W output. Then you tune to where the SWR is 1.3:1. At that SWR, the output could be anything from 1150 to 1950 W. The range may be less, as the amplifier output impedance may not be all that low compared to 50 Ohms. In any case, this shows that the power output of a solid state amplifier may be very sensitive to load SWR.
>
> This is why the KPA1500 ATU really needs to be able to keep the SWR low. The recently improved autotune firmware does this very well, if you “train” it across the band. It is also why it’s important to use the ATU, even if the antenna SWR is “only”, say, 1.5:1.
>
> I keep hoping Elecraft will come up with a way to automatically adjust drive power to maintain constant output. That would also compensate for change in temperature.
>
> 73,
>
> Scott K9MA
>
> ----------
>

N4ZR

Re: KPA-1500 Power Output Variation

Not necessary to guess - Page 17 of the KPA-1500 manual discusses ALC.
"ALC is a useful safety mechanism, but do not use ALC as the principle
[sic] power control. ...Elecraft recommends using the K3 or K3s
PWR-SET-PER-BAND function to set proper drive levels, not ALC.

73, Pete N4ZR
Check out the Reverse Beacon Network
at <http://reversebeacon.net>, now
spotting RTTY activity worldwide.
For spots, please use your favorite
"retail" DX cluster.

On 8/25/2019 9:18 AM, Bob McGraw K4TAX wrote:

> Has anyone actually used, observed, and measured the effect of the ALC
> feature of the amp in combination with the K3/K3S with regard to
> output power variation.
>
> I do use the method and setup as suggested in the manual for both the
> KPA500 and K3S with no known ill effects. The amp drives to rated
> output with 20 watts on all bands. I find it never need more drive
> that this for 160M - 6M. Output power variation with temperature is
> less than 10% as measured over the range of 23°C to 60°C. Fan
> control is automatic and steps up and down as needed.
>
> I do view if one hasn't used it, observed it or measured it, then "old
> opinions" of ALC implementation are not valid for comment.
>
> 73
>
> Bob, K4TAX
>
>
> On 8/24/2019 11:51 PM, K9MA wrote:
>> Here’s a way to get a worst case estimate of the effect of SWR on
>> power output. Assume the amplifier has constant VOLTAGE gain and an
>> output impedance much smaller than 50 Ohms. A 1.3:1 SWR means the
>> load impedance could be as low as 39 Ohms or as high as 65 Ohms.
>> Let’s say the SWR is 1:1 at the frequency at which the drive is set
>> for 1500 W output. Then you tune to where the SWR is 1.3:1. At that
>> SWR, the output could be anything from 1150 to 1950 W. The range may
>> be less, as the amplifier output impedance may not be all that low
>> compared to 50 Ohms. In any case, this shows that the power output of
>> a solid state amplifier may be very sensitive to load SWR.
>>
>> This is why the KPA1500 ATU really needs to be able to keep the SWR
>> low. The recently improved autotune firmware does this very well, if
>> you “train” it across the band. It is also why it’s important to use
>> the ATU, even if the antenna SWR is “only”, say, 1.5:1.
>>
>> I keep hoping Elecraft will come up with a way to automatically
>> adjust drive power to maintain constant output. That would also
>> compensate for change in temperature.
>>
>> 73,
>>
>> Scott K9MA
>>
>> ----------
>>
>
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Bob Wilson, N6TV

Re: KPA-1500 Power Output Variation

In reply to this post by Jim McCook W6YA

One thought, have you run the Elecraft TX GAIN Calibration procedure
recently? It's built in to the K3 Utility.

Unfortunately, the Utility sets the TX gain at a midpoint frequency on
every band, frequencies I rarely use, and I was observing first dit "power
spikes" in the output of the KPA1500 when driven by a K3.

To eliminate these power spikes (thanks to Rene at Elecraft for this tip),
with the amp. in STBY and the tuner enabled for best match on your favorite
frequency (or if connected to a dummy load), set the drive power to exactly
50W and press and hold the XMIT button (TUNE), then once the display
stabilizes, tap it again (just takes a few seconds). This re-calibrates
the TX GAIN on that frequency, instead of the one used by the Utility. At
any other power setting other than 50W, the TUNE button doesn't change
anything, so change the power level back after you are done.

You could do this once on you favorite areas of the CW bands, then use the
K3 Utility to Save that configuration, then repeat the procedure on the
phone segments, and use the K3 Utility to Restore whatever configuration
you need that weekend (but this may not work so well for mixed mode
contests).

So try the simple 50W calibration test on 40m to see if it makes any
improvement.

As for KPA1500 output power variation with temperature, it is annoying, and
it seems to vary from KPA1500 to KPA1500, perhaps depending on how many
other devices or heat sources are located near the ventilation holes. The
best option when operating a contest seems to be to set the KPA1500 minimum
fan speed to 1, so that amplifier doesn't heat up too quickly, maintaining
a more stable (but not perfectly stable) power output level.

73,
Bob, N6TV

On Sat, Aug 24, 2019 at 11:45 AM Jim McCook <[hidden email]> wrote:

> I'm using my K-3 with the KPA-1500 and have a problem with extreme
> changes in power output with a given drive level on 40m CW. This
> happens to a lesser extent on other bands with different antennas. My
> 40m antenna is tuned to 7.025 where SWR is 1.1:1. On 7.000 it is 1.4:1
> and at 7.050 it's 1.3:1. Within that range of frequencies the output
> _without_ the tuner at a given drive level (30w) varies from 1300w to
> well over 1700w. Using careful _tuner_ settings it's only slightly
> better. I have to reduce drive to 24 or 25w to get below 1500w,
> depending on the temperature of the amplifier (power output varies a lot
> with temperature changes). Higher temp = lower output.
>
> I have tried to compensate in the segments where output is excessive by
> using manual settings via the Utility. This has helped slightly, but
> still isn't adequate. I've resisted using ALC because of concern for
> generating distortion. Having to ride the drive level from the K-3 is a
> PIA, especially during contests.
>
> It seems to me a good answer would be to have higher input attenuation
> so drive requirement would be higher, maybe around 70+ watts. I'd
> expect changing drive level by the same number of watts (5 or 6) would
> not affect output as much. I'd also assume the power output would not
> vary nearly so much with a given drive level.
>
> Is this a possible solution? I've also understood that most
> transmitters (even the K-3) have cleaner output near highest rated output.
>
> Help!
>
> Jim W6YA
>
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
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K9MA

Re: KPA-1500 Power Output Variation

Concerning the output variation, I’ve noticed that when I change bands while the amplifier is hot, the output on the new band starts out high, then gradually drops. This would seem to indicate that the LPF is heating, and that the loss in the filter is several hundred Watts. That’s no big deal if the filters are properly cooled, as they apparently are, but still requires manually riding the power control.

73,

Scott K9MA

----------

Scott Ellington

--- via iPad

> On Aug 26, 2019, at 7:34 PM, Bob Wilson, N6TV <[hidden email]> wrote:
>
> As for KPA1500 output power variation with temperature, it is annoying, and
> it seems to vary from KPA1500 to KPA1500, perhaps depending on how many
> other devices or heat sources are located near the ventilation holes.

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