KX3 and KPA500 ... full power out?

[Terry Domae]

Hi - I was wondering if there were any plans for what I'll call a "KX3
interface/adapter" to the KPA500 so that it could run at full rated output
power?

I know one can attach a number of "automatic" amps that get you up to 30w,
but not with the sort of complete integration as the K3 and KPA500.
Also, the connections seem a bit cumbersome to have the extra
/non-integrated amp. Or, is the filtering required too hard given the boost
level?

Ok, I'm sure someone will say the KXPA100 is the "adapter", but I still
believe there is some work to get the KPA500 working like it was part of
the KX3.
Maybe a 30w mod for the kx3 coming?

Thanks, Terry, NS6D
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[Brett Howard]

KPA500 is about 13dB so if you can get about 12 watts out of your KX3
you'll get about 250 out of your KPA500.

~Brett (N7MG)


On Mon, Jan 14, 2013 at 7:33 PM, Terry Domae <[hidden email]> wrote:

> Hi - I was wondering if there were any plans for what I'll call a "KX3
> interface/adapter" to the KPA500 so that it could run at full rated output
> power?
>
> I know one can attach a number of "automatic" amps that get you up to 30w,
> but not with the sort of complete integration as the K3 and KPA500.
> Also, the connections seem a bit cumbersome to have the extra
> /non-integrated amp. Or, is the filtering required too hard given the boost
> level?
>
> Ok, I'm sure someone will say the KXPA100 is the "adapter", but I still
> believe there is some work to get the KPA500 working like it was part of
> the KX3.
> Maybe a 30w mod for the kx3 coming?
>
> Thanks, Terry, NS6D
> ______________________________________________________________
> 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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[AD6XY]

Putting aside the curious logic of getting a portable QRP radio and then following it with a 600W amplifier.. this is a valid requirement for more than the KX3. The K3-10 for example is an excellent radio. It seems silly to have a 100W PA in one to drive a 600W amplifier that could easily be designed to be driven without the intermediate 100W amplifier. Logic and regulation don't always go together though.

In many HF amplifiers low the gain of only 13dB (limited by archaic FCC rules) is limited by an input attenuator that you can bypass. In the USA, the manufacturer can't remove the attanuator, that probably why there is not a low power input KPA500 option, but there is nothing to stop you doing it by yourself if the amplifier is designed that way. Amplifiers in this class are the THP HL-1.5KFX series. Bypassing the input attenuator means a 10W radio, e.g. K3-10 will easily drive them to our UK 400W legal limit.

Unfortunately there is no mod solution like this for the KPA500. There is an input attenuator - two actually, one switched to even the gain on 6m. They also do some matching and they can't readily be bypassed. Well maybe one could be, but the gain will only go up by a dB or so and you would need to modify the overdrive protection circuits to compensate. The reason the gain from the pair of VRF2933s is so low is negative feedback used to limit the nominal 22dB of gain at 30MHz that would otherwise be realised. This feedback equalised gain across HF, improves IMDs and helps prevent spurious oscillations. The feedback circuit could be changed but I wouldn't mess with it unless you know exactly what you are doing.

But ! All is not lost. It is going to be quite easy to take a 30W intermediate amplifier and insert it into the KPA3. It can be quite a simple amplifier, without switching and only limited filtering. It means modifying the amplifier, but that is what Hams do.  Not quite as elegant as a 10W input KPA500 but not bad.

Mike

[Paula Keezer]

Hi Mike,  I've been looking closely at the KPA 500 schematic and it looks like the PA actually has 4 pads that it uses.
1) Normally, there are two pads, the 3 resistor 3 watt pad just after the rf drive directional coupler, a 6 resistor 3 watt pad that is normally in by K1 (connected to NC)
2) if K1 is switched (6 meter enable) then the 6 resistor network is switched out and L1 inductance is switched in series.  
3) if K2 is switched in in either case 1 or 2 then a 50 ohm shunt is switched in (this if for overdrive protection)

It seems like one could pull the 3 resistor 3 watt pad with little impact beyond changing what the exciter sees for an impedance.  This can easily be handled by the KX3 tuner.

over drive and gain checking is another story.

[Ignacy]

Paula Keezer wrote

Unfortunately there is no mod solution like this for the KPA500. .... The feedback circuit could be changed but I wouldn't mess with it unless you know exactly what you are doing.

Actually there are mods from EU and according to their users they work very well. One forgoes the warranty, though.
Ignacy, NO9E