Elecraft KX3 - Increased Output Power with Latest Firmware Update

I just noticed something that might be of interest to those of you with an Elecraft KX3. The latest firmware update increases the output power to 15 watts on the 80m-20m bands if the supply voltage is above 12.8v.


73, James M0JCQ

That is interesting.

I haven’t got mine to hand at the moment. I wonder how long it would key down at 15W with the standard heat-sink ?

I wish Lyle would change the s-meter code, I don’t consider putting the pre amp into the SP to get a correct reading particularly satisfactory.

I also wonder if he has corrected the tendency for the power to jitter as its been adjusted as well in the latest release.


As mentioned in the firmware release notes, not every KX3 will reach 15W. Mine delivers around 14W, measured on a R&S communications monitor.
As activator I will not change my usual 5W CW output setting. More W need more Ah need more kg…

73, Markus HB9BRJ


It works at 12.8V, but Elecraft recommends at least 13.8V.

Also, that is a beta firmware release.


I hope the heatsinks, final transistors, acceptable reflected power ratings are all OK at the increased power. There are settings in the FT-817’s hidden engineering settings to be able to boost output up to at least 10W (up to 14W on some bands) but those changes have been known to take out the final transistors because of bad VSWR (where running at 5W there was enough tolerance). :cold_sweat:

I think Elecraft would have checked this however. Of course running a KX3 at 15W SSB output means you can no longer claim to be running QRP, which is 10W PEP SSB or 5W CW. :innocent:

[quote=“G2HFR, post:2, topic:12450”]I wonder how long it would key down at 15W with the standard heat-sink ?[/quote]The newer KX3s ship with a slightly beefier heatsink than the early models, but I expect that will depend a bit on how brass-monkeys the summit you’re operating from is… In a nice warm shack, possibly not so long. :wink:

Good spot! I had initially missed this. I think I’ll hold fire until it makes it to a production release.

I had also wondered about this.

I have an older version but have only had issues when the KX3 was in direct sunlight in Spain :sunny:

I figure I’ll be putting a beefier heatsink on mine some point, as I noticed it getting rather warm while I was using it in Kenya over Xmas (before I put the new beta on it). I’ve had mine cut the power a bit on summits, but I figure the battery voltage is usually mostly responsible…

Its terrible on 6m. Can barely run it at full output for more then 5 mins calling CQ.

SN is somewhere in the 6xxx region. Its just a flat piece of aluminium on mine :confused:


Never had any issues with mine on 6m (or any other band).
It’s never gone over temp or rolled back power.


You’re clearly not long winded enough.

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The KX3 is rated for 10 W up to 15m and 8 W at 12m and above.

A bigger heatsink ships with SN 7292 for kits, SN 7255 for factory assembled. Elecraft says, "The additional mass and area of this heat sink provides as much as twice the operating time at full power before the KX3’s protective circuitry automatically reduces the power to 5 watts. "

Order page for bigger heatsink: http://www.elecraft.com/order_form_parts.htm#kx3

More details: http://www.elecraft.com/manual/E740254%20KX3HSMDKT%20Heat%20Sink%20Installation%20rev%20A.pdf


Saturday, I was chasing K0MOS (California to Colorado) on 17m and not getting his attention with 5W SSB. I cranked it up to 15W and got him on the next call. Might have been the power, might have been that the other chasers were done. He reported 48, so something was working.


I’ve also not had any issues on 6m and have run it during contests at 10w without an issue. My heatsink is also the flat piece of aluminium. So not sure what’s going on with yours Jonathan?

73 James M0JCQ

Don’t know ! I will give it ago when I get home on a dummy load. Maybe its a bad section of the LPF.


What battery do you use? The KX3 needs voltage at the top end of the range to make full power.


Some people may have overlooked that Wayne explicitly speaks of key-down voltage:
“Supply voltage must be over 12.8 V on key-down as indicated by the KX3’s voltage display”.

[quote=“HB9BCB, post:18, topic:12450”]as indicated by the KX3’s voltage display[/quote]…and that voltage is measured by the KX3 from the rig side of the diode-protected supply, so the battery voltage needs to be (IIRC) 0.3V higher than that (so 13.1V at the battery on key-down)…

Correct, that goes without saying…long/thin/fused power cords may cause an additional voltage drop…

Perhaps your PA on 6m is doing its impression of an Owl? :wink:

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