KPA3 for QRP

[Julian, G4ILO]

I run QRP out of practical necessity. I'm forced to use attic antennas and I don't wish to induce more RF into the domestic electrical equipment (or me, for that matter) than I absolutely have to.

Nevertheless, I'm thinking about getting a KPA3, because it probably will never be cheaper than it is now, and the occasion might arise in future when I might be able to use it.

I'm just wondering if there are any advantages to using the PA when running QRP. As I understand it, the PA is switched out of circuit until you dial the power above 10W, just as in the K2. There is no option for class A operation at a lower power level to produce a cleaner signal. If I run 10W of FM, the QRP PA will do all the work, and warm up very quickly, instead of letting the KPA3 do most of the work and barely getting warm. If I run 5W of PSK31, the IMD generated by the low power PA running at half its rated power will be higher than the KPA3 loafing along at the same power level.

Unless I'm missing something, there would be no benefit to me at the moment from having the KPA3 unless I want to wind the power control above 10W. Am I right?

[Brett Howard]

>From my understanding of it the radio has the KPA3 switched out of the
loop until you cross the 12W mark.  Then it starts actually putting it
into the signal path.  Until you crank the power up beyond 12Watts the
KPA3 is nothing more than an added current drain.  But if you turn
things up to 12 or 13 watts you're probably still quite fine with those
levels for RFIing yourself and the rest of the house / neighborhood and
your KPA3 will have TONS of headroom for you in that configuration.  Yet
as far as the low power amp its still probably still rated at the same
100% duty cycle at full power for 10 minutes.  I say probably because
the only technical spec listed is that it will handle 100 watts for 10
minutes easily on the web site and in the manual.  I'd imagine that
similar margins are used on both PA's being that both are designed to
have a 20% overhead which is more than you get from Yaecomwood out of
the gate!  

One big advantage is that getting the KPA3 now is a smart investment as
it is probably only going to go up in price and you have a great
advantage with the current exchange rate as it is.  Not to mention that
if you were ever to want to sell your K3 the fact that a KPA3 is already
installed makes it much more attractive to many more people.  My motto
has always been its better to have than to want. ;)


On Mon, 2008-03-17 at 02:55 -0700, G4ILO wrote:

> I run QRP out of practical necessity. I'm forced to use attic antennas and I
> don't wish to induce more RF into the domestic electrical equipment (or me,
> for that matter) than I absolutely have to.
>
> Nevertheless, I'm thinking about getting a KPA3, because it probably will
> never be cheaper than it is now, and the occasion might arise in future when
> I might be able to use it.
>
> I'm just wondering if there are any advantages to using the PA when running
> QRP. As I understand it, the PA is switched out of circuit until you dial
> the power above 10W, just as in the K2. There is no option for class A
> operation at a lower power level to produce a cleaner signal. If I run 10W
> of FM, the QRP PA will do all the work, and warm up very quickly, instead of
> letting the KPA3 do most of the work and barely getting warm. If I run 5W of
> PSK31, the IMD generated by the low power PA running at half its rated power
> will be higher than the KPA3 loafing along at the same power level.
>
> Unless I'm missing something, there would be no benefit to me at the moment
> from having the KPA3 unless I want to wind the power control above 10W. Am I
> right?
>
> -----
> Julian, G4ILO  K3 s/n: 222 K2 s/n: 392
> G4ILO's Shack: www.g4ilo.com
> Zerobeat Ham Forums: www.zerobeat.net/smf

_______________________________________________
Elecraft mailing list
Post to: [hidden email]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
 http://mailman.qth.net/mailman/listinfo/elecraft   

Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com

[Bill W4ZV]

G4ILO wrote

I'm just wondering if there are any advantages to using the PA when running QRP. As I understand it, the PA is switched out of circuit until you dial the power above 10W, just as in the K2.

        The switch point in the KPA3 is actually 13W.  For 12 and less it'is switched out.

Unless I'm missing something, there would be no benefit to me at the moment from having the KPA3 unless I want to wind the power control above 10W. Am I right?

        Nothing I'm aware of.

[AD6XY]

Yes Julian - you are right. There is no advantage in having a KPA3 for QRP working. In fact, there is a bit of a disadvantage as it takes up space in the case which you might be able to use for something else, like a battery or a VHF transverter.

Regarding the price, at $450 + VAT it is not particularly cheap compared to any other broadband 100W HF amplifier you might buy. Couple it with the KAT3 at $300 you are into the $750 territory and I am sure there are cheaper and more flexible alternatives. A good example being the K2 100W PA and ATU in an external case.

If you really want a very clean signal, an external amplifier fed with a few watts and in class A would be a better bet. You could then use a device like an MRF151G with a 45V supply and if you wish to home brew check out the CCI website for amplifier kits. A 200W amplifier running off 28V or 48V will be very clean at 50W or so in class A.



Mike

[n6wg]

Hi Julian
There is one drawback, however.
Increased current drain, even though the KPA3
is not in use at QRP levels.  If this isn't a
problem for you, then go right ahead.
However, if you want to take your K3 out to
the field for a Field Day operation, that
extra drain can create battery problems.

Good luck and 73
Bob N6WG

----- Original Message -----
From: "G4ILO" <[hidden email]>
To: <[hidden email]>
Sent: Monday, March 17, 2008 1:55 AM
Subject: [Elecraft] KPA3 for QRP


>
> I run QRP out of practical necessity. I'm forced to use attic
antennas and I
> don't wish to induce more RF into the domestic electrical equipment
(or me,
> for that matter) than I absolutely have to.
>
> Nevertheless, I'm thinking about getting a KPA3, because it probably
will
> never be cheaper than it is now, and the occasion might arise in
future when
> I might be able to use it.
>
> I'm just wondering if there are any advantages to using the PA when
running
> QRP. As I understand it, the PA is switched out of circuit until you
dial
> the power above 10W, just as in the K2. There is no option for class
A
> operation at a lower power level to produce a cleaner signal. If I
run 10W
> of FM, the QRP PA will do all the work, and warm up very quickly,
instead of
> letting the KPA3 do most of the work and barely getting warm. If I
run 5W of
> PSK31, the IMD generated by the low power PA running at half its
rated power
> will be higher than the KPA3 loafing along at the same power level.
>
> Unless I'm missing something, there would be no benefit to me at the
moment
> from having the KPA3 unless I want to wind the power control above
10W. Am I
> right?
>
> -----
> Julian, G4ILO  K3 s/n: 222 K2 s/n: 392
> G4ILO's Shack: www.g4ilo.com
> Zerobeat Ham Forums: www.zerobeat.net/smf
> --
> View this message in context:
http://www.nabble.com/KPA3-for-QRP-tp16091280p16091280.html

> Sent from the Elecraft mailing list archive at Nabble.com.
>
> _______________________________________________
> Elecraft mailing list
> Post to: [hidden email]
> You must be a subscriber to post to the list.
> Subscriber Info (Addr. Change, sub, unsub etc.):
>  http://mailman.qth.net/mailman/listinfo/elecraft
>
> Help: http://mailman.qth.net/subscribers.htm
> Elecraft web page: http://www.elecraft.com

_______________________________________________
Elecraft mailing list
Post to: [hidden email]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
 http://mailman.qth.net/mailman/listinfo/elecraft   

Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com

[WILLIS COOKE]

People talk about the extra current drain.  How much
is it?  I don't have my K3 yet to make any tests and I
seldom run QRP (only when the other station wants 2
way QRP), but it would be of interest to us all if
anyone had measured the current draw of the K3 at
receive with and without the KPA3, transmit at 5 watts
without the KPA3 installed and transmit at 5 watts
with the KPA3 installed. I am sure that Elecraft has
the information, but I don' recall seeing it
published.  My guess is that the KPA3 does not require
much if any current when it is switched out.  Anybody
do any tests to know for sure?
 
--- Robert Tellefsen <[hidden email]> wrote:

> Hi Julian
> There is one drawback, however.
> Increased current drain, even though the KPA3
> is not in use at QRP levels.  If this isn't a
> problem for you, then go right ahead.
> However, if you want to take your K3 out to
> the field for a Field Day operation, that
> extra drain can create battery problems.
>
> Good luck and 73
> Bob N6WG
>
> ----- Original Message -----
> From: "G4ILO" <[hidden email]>
> To: <[hidden email]>
> Sent: Monday, March 17, 2008 1:55 AM
> Subject: [Elecraft] KPA3 for QRP
>
>
> >
> > I run QRP out of practical necessity. I'm forced
> to use attic
> antennas and I
> > don't wish to induce more RF into the domestic
> electrical equipment
> (or me,
> > for that matter) than I absolutely have to.
> >
> > Nevertheless, I'm thinking about getting a KPA3,
> because it probably
> will
> > never be cheaper than it is now, and the occasion
> might arise in
> future when
> > I might be able to use it.
> >
> > I'm just wondering if there are any advantages to
> using the PA when
> running
> > QRP. As I understand it, the PA is switched out of
> circuit until you
> dial
> > the power above 10W, just as in the K2. There is
> no option for class
> A
> > operation at a lower power level to produce a
> cleaner signal. If I
> run 10W
> > of FM, the QRP PA will do all the work, and warm
> up very quickly,
> instead of
> > letting the KPA3 do most of the work and barely
> getting warm. If I
> run 5W of
> > PSK31, the IMD generated by the low power PA
> running at half its
> rated power
> > will be higher than the KPA3 loafing along at the
> same power level.
> >
> > Unless I'm missing something, there would be no
> benefit to me at the
> moment
> > from having the KPA3 unless I want to wind the
> power control above
> 10W. Am I
> > right?
> >
> > -----
> > Julian, G4ILO  K3 s/n: 222 K2 s/n: 392
> > G4ILO's Shack: www.g4ilo.com
> > Zerobeat Ham Forums: www.zerobeat.net/smf
> > --
> > View this message in context:
>

http://www.nabble.com/KPA3-for-QRP-tp16091280p16091280.html

> > Sent from the Elecraft mailing list archive at
> Nabble.com.
> >
> > _______________________________________________
> > Elecraft mailing list
> > Post to: [hidden email]
> > You must be a subscriber to post to the list.
> > Subscriber Info (Addr. Change, sub, unsub etc.):
> >  http://mailman.qth.net/mailman/listinfo/elecraft
> >
> > Help: http://mailman.qth.net/subscribers.htm
> > Elecraft web page: http://www.elecraft.com
>
> _______________________________________________
> Elecraft mailing list
> Post to: [hidden email]
> You must be a subscriber to post to the list.
> Subscriber Info (Addr. Change, sub, unsub etc.):
>  http://mailman.qth.net/mailman/listinfo/elecraft   
>
>
> Help: http://mailman.qth.net/subscribers.htm
> Elecraft web page: http://www.elecraft.com
>

Willis 'Cookie' Cooke
K5EWJ
_______________________________________________
Elecraft mailing list
Post to: [hidden email]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
 http://mailman.qth.net/mailman/listinfo/elecraft   

Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com

[Brett Howard]

>>> People talk about the extra current drain.  How much is it?

If you go by what W0CZ measured you get this:

The current tests before I put in the KPA3.
Receive 0.94 to 1.1 A
TX (NO POWER) 1.56 A
TX 5W 2.56 A
TX 10W 3.07 A
TX 12W 3.23 A

The same test after the KPA3 was installed, Set up and working fine.
Receive 0.94 to 1.1 A
TX (NO POWER 2.16 A
TX 5W 3.10 A
TX 10W 3.62 A
TX 12W 3.78 A


_______________________________________________
Elecraft mailing list
Post to: [hidden email]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
 http://mailman.qth.net/mailman/listinfo/elecraft   

Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com

[Ralph Tyrrell]

post from the past  ref K3 current draw.
--------------------
Fri, 22 Feb 2008 21:53:25 -0600
From: "Kenneth A. Christiansen" <[hidden email]>  Add
Mobile Alert
To: [hidden email]
Subject: [Elecraft] K3 NR 457 now working in Fargo ND


I got a pleasant surprise yesterday when K3 457
arrived. It was ordered
 
June 13 and I got my Lisa Gram last week end.
It has KAT3, KBPF3, KXV3, 500 hz filter, 2700 hz
filter, 6000 hz filter
 
and the FM filter on back order.

I made a special test on it because I wanted to know
how the KPA3
changes the current draw.  I know others have asked
the same question here and never did get an answer so
here it is. .

The current tests before I put in the KPA3.
Receive 0.94 to 1.1 A
TX (NO POWER) 1.56 A
TX 5W 2.56 A
TX 10W 3.07 A
TX 12W 3.23 A

The same test after the KPA3 was installed, Set up and
working fine.
Receive 0.94 to 1.1 A
TX (NO POWER 2.16 A
TX 5W 3.10 A
TX 10W 3.62 A
TX 12W 3.78 A

All the above testes were done with a digital meter on
the 10 amp
 scale.
I would say the KPA3 adds a constant 0.6 Amp for any
given power level
below 12.1 Watts.

I started the inventory last night about 6:00 PM and
finished it
 tonight
about 8 PM. I had a fire run, slept, went to a meeting
etc.
so my build time must have been about the magic 10
hours.
I sure do like the receiver but have just scratched
the surface of what
 
it can do.
It was worth the wait.
73
Ken W0CZ
-----------------

Hope this helps.

73, Ty, W1TF, K1 #1423 waiting for a K3

__________________________________________________
Do You Yahoo!?
Tired of spam?  Yahoo! Mail has the best spam protection around
http://mail.yahoo.com 
_______________________________________________
Elecraft mailing list
Post to: [hidden email]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
 http://mailman.qth.net/mailman/listinfo/elecraft   

Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com

[Darwin, Keith]

But I think we missed the point of the original post.

I believe the question was not "how much dead weight is the KPA3 when I
run QRP".  That has been answered.  The question was "wouldn't the rig
be a better QRP rig if the KPA3 was used even at QRP levels?".

With the K2, current drain was an issue and shutting down the after
burner for QRP made sense.  With the less portable K3, current drain is
largely a non-issue so shutting down the big finals below 10 watts isn't
an obvious good thing in my book.

Of course, if I were smart enough I could just answer the question.  As
it is, I'm barely able to simple rephrase it :-)

- Keith N1AS -
- K2 5411.ssb.100 -
- K3 Wave 3 -

-----Original Message-----
>>> People talk about the extra current drain.  How much is it?

If you go by what W0CZ measured you get this:

*snip*
_______________________________________________
Elecraft mailing list
Post to: [hidden email]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
 http://mailman.qth.net/mailman/listinfo/elecraft

Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com