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Using Elecraft KPA-1500 for 2 radios

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Charlie Wirth

Using Elecraft KPA-1500 for 2 radios

I own a Flex 6400 and an Icom 7700. I currently have the amplifier connected to the Flex. There may be times I want to use the Icom 7700. Other than disconnecting the send and control cables from the 1500 that go to the Flex and plugging in the same cables from the 7700 is there any other way to have both sets of cables connected to the amplifier safely? I would only be using one radio at a time and the other one would be turned off. I know that I would have to go into the menu settings of the amplifier to change the baud radio and radio when I use the other radio.

Thanks,

Charlie W3DDF
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Jim Brown-10

Re: Using Elecraft KPA-1500 for 2 radios

On 3/17/2019 1:26 PM, Charlie Wirth wrote:
> Other than disconnecting the send and control cables from the 1500 that go to the Flex and plugging in the same cables from the 7700 is there any other way to have both sets of cables connected to the amplifier safely?

Several thoughts. First, both of Elecraft's power amps nearly
instantaneously detect the frequency of the drive signal, no there is no
need for the rig to send band information to the amp. Second, ALC should
never be used to set TX power -- that should always be done by setting
the output power of the rig.

The only necessary connections are the "amp key" line that puts the amp
in transmit mode and the coax between the rig and the amp input.
Switching the coax is easy with a SPDT coax switch or relay.

Most of the rigs I've owned use a short on the key line to key the amp.
If both of yours do that, you can probably connect them in parallel to
the Key input of the power amp using an RCA Y-cable. I've done that for
years to switch my K3 between my KPA500 and a 1500w tube amp (for many
years a Ten Tec Titan, now an 87A).

73, Jim K9YC

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Mike / W5JR

Re: Using Elecraft KPA-1500 for 2 radios

I have a KPA/KAT500 combo connected to my Flex 6600 and K3. The K3 has all of the cables connected, so when it’s on, it controls the package with full integration. I use the “key interupter” to break out the key line. It’s “Y’d” with the key line from the Flex as Jim mentions into the KAT500. I switch input coaxes via a two port manual selector. That could be automated using one of several options if desired.

Works great.

tnx
Mike / W5JR
Alpharetta GA

> On Mar 17, 2019, at 4:53 PM, Jim Brown <[hidden email]> wrote:
>
>> On 3/17/2019 1:26 PM, Charlie Wirth wrote:
>> Other than disconnecting the send and control cables from the 1500 that go to the Flex and plugging in the same cables from the 7700 is there any other way to have both sets of cables connected to the amplifier safely?
>
> Several thoughts. First, both of Elecraft's power amps nearly instantaneously detect the frequency of the drive signal, no there is no need for the rig to send band information to the amp. Second, ALC should never be used to set TX power -- that should always be done by setting the output power of the rig.
>
> The only necessary connections are the "amp key" line that puts the amp in transmit mode and the coax between the rig and the amp input. Switching the coax is easy with a SPDT coax switch or relay.
>
> Most of the rigs I've owned use a short on the key line to key the amp. If both of yours do that, you can probably connect them in parallel to the Key input of the power amp using an RCA Y-cable. I've done that for years to switch my K3 between my KPA500 and a 1500w tube amp (for many years a Ten Tec Titan, now an 87A).
>
> 73, Jim K9YC
>
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Bill K9YEQ

Re: Using Elecraft KPA-1500 for 2 radios

In reply to this post by Charlie Wirth

I use a KR5L switch, but can no longer find on the internet. Possibly there is a circuit available that you could duplicate. I use mine to select rigs and a choice of Two amps, I only have one.

73,
Bill
K9YEQ

https://wrj-tech.com/

-----Original Message-----
From: [hidden email] <[hidden email]> On Behalf Of Charlie Wirth
Sent: Sunday, March 17, 2019 3:27 PM
To: [hidden email]
Subject: [Elecraft] Using Elecraft KPA-1500 for 2 radios

I own a Flex 6400 and an Icom 7700. I currently have the amplifier connected to the Flex. There may be times I want to use the Icom 7700. Other than disconnecting the send and control cables from the 1500 that go to the Flex and plugging in the same cables from the 7700 is there any other way to have both sets of cables connected to the amplifier safely? I would only be using one radio at a time and the other one would be turned off. I know that I would have to go into the menu settings of the amplifier to change the baud radio and radio when I use the other radio.

Thanks,

Charlie W3DDF
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john@kk9a.com

Re: Using Elecraft KPA-1500 for 2 radios

In reply to this post by Charlie Wirth

So the different voltage sources from each amp do not bother each other when
connected together with a Y or was only one amp turned on at a time?

John KK9A

Jim Brown K9YC wrote:

<snip>

Most of the rigs I've owned use a short on the key line to key the amp.
If both of yours do that, you can probably connect them in parallel to
the Key input of the power amp using an RCA Y-cable. I've done that for
years to switch my K3 between my KPA500 and a 1500w tube amp (for many
years a Ten Tec Titan, now an 87A).

73, Jim K9YC

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Bill Steffey NY9H

Re: Using Elecraft KPA-1500 for 2 radios

i use topten a/b coax switches,,,, to switch between the kpa500 &
acom 2000 ... one in front of the amps , one in back .... also have
them switching between K3 & an Icom....

https://toptendevices.com/products/ab-station-selector/

On 3/17/2019 8:56 PM, [hidden email] wrote:

> So the different voltage sources from each amp do not bother each other when
> connected together with a Y or was only one amp turned on at a time?
>
> John KK9A
>
>
> Jim Brown K9YC wrote:
>
> <snip>
>
> Most of the rigs I've owned use a short on the key line to key the amp.
> If both of yours do that, you can probably connect them in parallel to
> the Key input of the power amp using an RCA Y-cable. I've done that for
> years to switch my K3 between my KPA500 and a 1500w tube amp (for many
> years a Ten Tec Titan, now an 87A).
>
> 73, Jim K9YC
>
> ______________________________________________________________
> 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
>

Jim Brown-10

Re: Using Elecraft KPA-1500 for 2 radios

Not a great choice. SWR gets increasing nasty above 15M because the
construction uses chassis as signal return rather than carrying it on
the circuit board. It's a two-sided board, but the "ground" layer is
broken under the traces that carry the RF, so return current can't flow
on that layer. I didn't have the heart to try to measure crosstalk.

5-6 years ago, I bought several of them for antenna switching, and
learned that the hard way. I modified them by paralleling copper braid
from the connectors to the board and following the RF paths through the
relays across the board as best I could. That reduced the SWR by about
half.

I've since replaced all of them, and they're for sale. :)

73, Jim K9YC

On 3/17/2019 6:15 PM, Bill Steffey wrote:
> i use topten a/b coax switches,,,, to switch between the kpa500 &
> acom 2000 ... one in front of the amps , one in back .... also
> have them switching between K3 & an Icom....
>
> https://toptendevices.com/products/ab-station-selector/

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Jim Brown-10

Re: Using Elecraft KPA-1500 for 2 radios

In reply to this post by john@kk9a.com

Nope. The key inputs of both amps are buffered. Partial schematic of
Titan is here.

http://k9yc.com/TitanKeyingLogic.pdf

K4XU once told me that it was his initials on the published schematics,
and I think he later worked for Alpha. For operation with most rigs,
the PTT position of the PTT/KEY switch is used for all modes. The QSK
position is for a keying method used by early Ten Tec rigs where the
operator keys the amp and the amp keys the rig.

73, Jim

On 3/17/2019 5:56 PM, [hidden email] wrote:

Tom Georgens

Re: Using Elecraft KPA-1500 for 2 radios

In reply to this post by Jim Brown-10

Hi Jim

I bought a bunch of these for my station in 8P. When you reported your findings with these devices, I was very concerned. I had one in 8P with me at the time and I measured it in my AIM 4170. I could not reproduce your observations and used it on 10M with no obvious issues.

Since I have been home, I took another set of measurements with my DG8SAQ VNA. They were essentially identical to what I measured in 8P.

Just so we are clear on definitions, I measured them in in both directions using both ports and a precision 50 ohm load. In-A had a steadily rising SWR with frequency, maxing out at 1.1:1 on 28Mhz. In-B was slightly worse with the same shape and a max SWR of 1.15:1 at 28 Mhz. It was the same in both directions and did not change if the unused port was terminated or open. The isolation was essentially as advertised, over 80 dB up to 28 Mhz.

Admittedly 1.15:1 is not world class, and could be better, but I don't consider it unacceptable and I plan on using the switches I bought. Are we drawing different conclusions from the same data?

If you had different measurements, I would be curious if there was some design change along the way. I did not open them up. The units I have were bought from another ham, but they were unused.

They also make a product called an Op Swapper that has some other functionality. What I am testing is the A/B station selector.

73, Tom W2SC

-----Original Message-----
From: [hidden email] <[hidden email]> On Behalf Of Jim Brown
Sent: Sunday, March 17, 2019 8:12 PM
To: [hidden email]
Subject: Re: [Elecraft] Using Elecraft KPA-1500 for 2 radios

Not a great choice. SWR gets increasing nasty above 15M because the construction uses chassis as signal return rather than carrying it on the circuit board. It's a two-sided board, but the "ground" layer is broken under the traces that carry the RF, so return current can't flow on that layer. I didn't have the heart to try to measure crosstalk.

5-6 years ago, I bought several of them for antenna switching, and learned that the hard way. I modified them by paralleling copper braid from the connectors to the board and following the RF paths through the relays across the board as best I could. That reduced the SWR by about half.

I've since replaced all of them, and they're for sale. :)

73, Jim K9YC

On 3/17/2019 6:15 PM, Bill Steffey wrote:
> i use topten a/b coax switches,,,, to switch between the kpa500 &
> acom 2000 ... one in front of the amps , one in back .... also
> have them switching between K3 & an Icom....
>
> https://toptendevices.com/products/ab-station-selector/

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Jim Brown-10

Re: Using Elecraft KPA-1500 for 2 radios

On 3/18/2019 2:38 PM, Tom Georgens wrote:
> Admittedly 1.15:1 is not world class, and could be better, but I don't consider it unacceptable and I plan on using the switches I bought. Are we drawing different conclusions from the same data?

Hi Tom,

Yes, we are. I was using a pair of them to switch inputs and outputs of
a KPA500 an Ten Tec Titan. I use the KPA mostly on 6M, and what you saw
on 10M was significantly worse there.

Another important point. It is totally unrealistic to terminate un-used
ports for isolation measurements, because un-used ports are often not
matched at the frequency where isolation is of concern. I measure
isolation between ports with a VNA (the DG8SAQ VNWA3e). In the case of
this unit, I would measure between the two outputs.

73, Jim

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Tom Georgens

Re: Using Elecraft KPA-1500 for 2 radios

Thanks Jim

I did not go all the way to 50Mhz. You comment indicated a knee at 21 Mhz,
I did not see that. I saw a linear rise in SWR with frequency. At 28Mhx,
the resistive component was still close to 50 ohms. It looks like a fixed
inductance or capacitance that likely could be compensated away if I were so
motivated.

FWIW - their website only provides specifications up to 30 Mhz. It is
possible that the design scope did not contemplate 50Mhz use.

With 80db of relay isolation, termination should be moot. I was just
verifying that it was. I saw no difference in a 50 ohm load or an open. In
my use case, the unused port will be grounded (I think) so I wanted to be
sure.

They should be fine for my use case, but I can see why they did not work for
you.

I feel better knowing I was not missing anything in my measurements.

Tom

-----Original Message-----
From: [hidden email] <[hidden email]> On
Behalf Of Jim Brown
Sent: Monday, March 18, 2019 4:54 PM
To: [hidden email]
Subject: Re: [Elecraft] Using Elecraft KPA-1500 for 2 radios

On 3/18/2019 2:38 PM, Tom Georgens wrote:
> Admittedly 1.15:1 is not world class, and could be better, but I don't
consider it unacceptable and I plan on using the switches I bought. Are we
drawing different conclusions from the same data?

Hi Tom,

Yes, we are. I was using a pair of them to switch inputs and outputs of a
KPA500 an Ten Tec Titan. I use the KPA mostly on 6M, and what you saw on 10M
was significantly worse there.

Another important point. It is totally unrealistic to terminate un-used
ports for isolation measurements, because un-used ports are often not
matched at the frequency where isolation is of concern. I measure isolation
between ports with a VNA (the DG8SAQ VNWA3e). In the case of this unit, I
would measure between the two outputs.

73, Jim

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k6mrmagnet

Re: Using Elecraft KPA-1500 for 2 radios

I have been using 4 of these switches to share one amplifier between two K3s in an SO2R setup for the past 4 years. I was running two KPA500s in parallel, but recently switched to a KPA1500. No problems.

I don’t operate 6 meters, and 10 meters hasn’t been open in so long I can honestly say I don’t operate there either!

Ken K6MR

________________________________
From: [hidden email] <[hidden email]> on behalf of Tom Georgens <[hidden email]>
Sent: Monday, March 18, 2019 6:01:04 PM
To: [hidden email]; [hidden email]
Subject: Re: [Elecraft] Using Elecraft KPA-1500 for 2 radios

Thanks Jim

I did not go all the way to 50Mhz. You comment indicated a knee at 21 Mhz,
I did not see that. I saw a linear rise in SWR with frequency. At 28Mhx,
the resistive component was still close to 50 ohms. It looks like a fixed
inductance or capacitance that likely could be compensated away if I were so
motivated.

FWIW - their website only provides specifications up to 30 Mhz. It is
possible that the design scope did not contemplate 50Mhz use.

With 80db of relay isolation, termination should be moot. I was just
verifying that it was. I saw no difference in a 50 ohm load or an open. In
my use case, the unused port will be grounded (I think) so I wanted to be
sure.

They should be fine for my use case, but I can see why they did not work for
you.

I feel better knowing I was not missing anything in my measurements.

Tom

-----Original Message-----
From: [hidden email] <[hidden email]> On
Behalf Of Jim Brown
Sent: Monday, March 18, 2019 4:54 PM
To: [hidden email]
Subject: Re: [Elecraft] Using Elecraft KPA-1500 for 2 radios

On 3/18/2019 2:38 PM, Tom Georgens wrote:
> Admittedly 1.15:1 is not world class, and could be better, but I don't
consider it unacceptable and I plan on using the switches I bought. Are we
drawing different conclusions from the same data?

Hi Tom,

Yes, we are. I was using a pair of them to switch inputs and outputs of a
KPA500 an Ten Tec Titan. I use the KPA mostly on 6M, and what you saw on 10M
was significantly worse there.

Another important point. It is totally unrealistic to terminate un-used
ports for isolation measurements, because un-used ports are often not
matched at the frequency where isolation is of concern. I measure isolation
between ports with a VNA (the DG8SAQ VNWA3e). In the case of this unit, I
would measure between the two outputs.

73, Jim

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john@kk9a.com

Re: Using Elecraft KPA-1500 for 2 radios

In reply to this post by Charlie Wirth

N3RD (a fellow contester) is very responsive and knowledgeable on Top Ten
products. You should contact him if you have any questions or concerns. I
am bringing a Top Ten Decoder and 6 way switch to Bonaire for WPX SSB later
this month.

John KK9A - PJ4R

Tom Georgens W2SC wrote:

Hi Jim

I bought a bunch of these for my station in 8P. When you reported your
findings with these devices, I was very concerned. I had one in 8P with me
at the time and I measured it in my AIM 4170. I could not reproduce your
observations and used it on 10M with no obvious issues.

Since I have been home, I took another set of measurements with my DG8SAQ
VNA. They were essentially identical to what I measured in 8P.

Just so we are clear on definitions, I measured them in in both directions
using both ports and a precision 50 ohm load. In-A had a steadily rising
SWR with frequency, maxing out at 1.1:1 on 28Mhz. In-B was slightly worse
with the same shape and a max SWR of 1.15:1 at 28 Mhz. It was the same in
both directions and did not change if the unused port was terminated or
open. The isolation was essentially as advertised, over 80 dB up to 28 Mhz.

Admittedly 1.15:1 is not world class, and could be better, but I don't
consider it unacceptable and I plan on using the switches I bought. Are we
drawing different conclusions from the same data?

If you had different measurements, I would be curious if there was some
design change along the way. I did not open them up. The units I have were
bought from another ham, but they were unused.

They also make a product called an Op Swapper that has some other
functionality. What I am testing is the A/B station selector.

73, Tom W2SC

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