I know the MXP-50 is a popular amp for the 817/8. I’ve seen a some have used the amp with the KX2 successfully, but it requires making a keying cable. I did notice that a buffer relay may be suggested to prevent voltage from damaging the radio. Anyone used this combo?
Thanks!
Yes - although I now usually don’t bother with the extra weight and just run 10w. I think the issue isn’t the relay, but the back emf generated by the relay in the amplifier which I believe may damage the KX2 by presenting the keying line with a large reverse voltage. Some amplifiers / relays have diode across the relay coil so this is harmlessly dissipated, AFAIK the MX50 doesn’t The solution I used was a diode across this keying line so any back EMF generated by the relay would not get to the KX2. I have used it and my KX2 is still working. Hopefully this may point you in the right direction, but given the cost of the KX2 I would research more fully and not rely on my memory… 73. Paul
This problem has been solved by KB9LRW. He has built a protection circuit. I have built it as well and it works flawlessly.
Origin: https://www.youtube.com/watch?v=k8dwJMOdEMI
Kind Regards,
Chris
Hi AJ,
I don’t have a KX2 however when I have run amps with QRP radios in the past, I made up the switching lead using a reed-relay. The switched PTT current is minimal and using the reed relay isolates both units from each other. Check what voltage the KX2 has available on its accessory socket (and choose a reed relay coil voltage to suit). Also, check whether the PTT pin earths when on transmit or goes to +V. Some radios have a two-way PTT pin, so be sure to find which way is being switched otherwise plugging in the cable might put the KX2 onto transmit.
If the MXP-50 also has an ALC line, that would be good to wire back to the KX2 to stop the amplifier from being overdriven - again check the required voltages if this feature is on the KX2.
73 Ed.
I rarely use my amp now but for years I did, with no mod and issues with my KX2.
Malen
I also successfully use a similar circuit. But to protect the KX2 in the event of an accident, I would connect a resistor (1-10k) between the KX2 and the base of the transistor.
73, Peter - HB9PJT
I also use this power amplifier with the KX2 and have no protection circuit… and haven’t had any problems yet.
I only used it at events (e.g. S2S QSO Party, NA-EU events,…)
But maybe I’ll install the recommended circuit… safety first
73 Armin
Snap! … but for KX2 substitute KX3. However, I’m also thinking of doing the modification so Murphy doesn’t come on an activation with me. 
I use the MX-P50M with my KX2. I highly recommend making the modification to prevent a power spike from being sent back into your radio (I made the modification to my unit). Be sure to remember to change the band setting dial on the amp when you change bands on your KX2. I use it for SOTA activations. Within the first three outings, all four rubber “feet” came unglued, and had to be repaired/replaced. Otherwise, it seems to work fine.
I am driving my MX-P50M directly with an IC-705 and have not experienced any problems. I suspect there are possibly differences in the keying circuit between different versions of these units. Before spending a lot of effort building a buffer circuit, I would do a little investigation first. First, I would actually measure the current in the keying circuit with a DVM. On my unit, which is version 1.6S the keying current is 0.56 MA. Not enough current to directly drive the relay. The relay coil is spec’d at 450 mw at 12 volts which is 37.5 ma. This indicates to me there is a buffer driving the relay and not the radio. Maybe people could measure their amps and report the current and the revision here to allows us to understand which versions might need a buffer circuit to drive the relay.
Thx
Burke
KF7NP
On my MX-P50M, which is version V1.5 the keying current is also only 0.54 mA.
Therefore when I prepared it to be used with my KX2 I decided that an additional switching transistor was NOT needed.
However, I added a diode between the keying line and ground to avoid any (negative) spikes which might come from the MX-P50M.
