Friday 21 June 2024



Most hams who love to operate outdoors, prefer to work at QRP levels. Being in the great outdoors usually gives you a fabulous take-off and provides a super-low noise-floor.  10W, 5W, even a couple of Watts can get you across the Atlantic on SSB and that’s something that just thrills me and keeps me interested in the hobby.

Back home however, all the stars need to be aligned for long distance QSO’s to take place. It’s doable of course, but it can be hard work for people at the other end of your transmission, which means that I avoid joining in on any overseas Nets. It’s the main reason that I don’t do much transmitting from the shack. No big deal, cos during Spring, Summer and Autumn it’s easy to setup outdoors. A pleasure, in fact!

I actually have a Xiegu XPA125B amp (boxed and on sale (now sold)), which produces 100W from a 5W input, but it’s useless to me (it came with a radio that I bought (X6100)). The problem with it is that you can’t turn down the power - it’s 100W or nothing!  I’d keep it if I could feed it with 1W and get 25W out. Why?? Because at the end of February OFCOM will be allowing Foundation License holders to use 25W.

What I wanted was an amp which would provide me with 25W comfortably without putting strain on it. I looked at the Neptune  50W but, for the amount of use it would get, it was more than I wanted to pay (and I couldn’t find any used models anywhere).

And then I read some reviews of the MX-P50 made by Sun Xiao in Shandong, China.  Being a very happy owner of a fabulous radio from China (FX-4CR), I'm not one of those "Let's hate China" brigade, so the MX-P50 seemed to fit the bill. It was just a case of finding a used one for a decent price. 

With the 80-10M MX-P50 being able to produce 45W from a 5W input, I knew that it would be positively coasting along at the 25W limit I’d be setting, so longevity shouldn’t be an issue. 

When it arrived, I could see that it had never really been used. It had been a backup amp to a HardRock/Hermes setup and had spent most of its short life in the box. The amp is really very compact and light - great for those who want to take it with them on a Field Day.

Apparently, it works well with a range of radios including the FT-817, KX3, IC-705, etc, but people using more obscure transceivers may experience issues. 

The amp’s power cable was fitted with Anderson Power Poles and the interface cable was terminated with a 3.5mm stereo jack plug. This was perfect for my Icom IC-705 and worked straight from the get-go.

I know I'm drifting a bit here, but I think it's important to realise that amps can send current back into your radio if a spike is generated by the relays during switching. This current might be small, but it can cause damage to your radio's keying circuitry. This isn't a problem unique to the MX-P50M - it can happen with any generic amp.

I'm guessing that most people ignore this potential problem (or just aren't aware of it) and probably get away with it, but I don't want to put my radios at risk, so I use a buffer device from a guy called RADIODAN in the USA. He calls it an RBI-1 (Relay Buffer Interface) and it can be adapted to work with virtually any radio (although he sells specific interface kits anyway). It will work with any radio that has a keying output which goes to GROUND when transmitting.


Using this relay buffer provides peace of mind and is (IMHO) a very worthwhile accessory for all your QRP radios - all you need to do is make up a different lead for each keying connection type - for example on many radios it will be a simple 3.5mm jack, while on others it will be phono (RCA) or even something like a multi-pin DIN.

There is also a 'mod' which can be carried out to prevent spikes into your radio involving you opening up the amp, cutting a couple of wires and soldering few small components. It's detailed HERE in this video where the construction method is better explained. The mod was designed by Kevin Loughin and his video can be found HERE. When I opened up my amp, I discovered that the previous owner had already carried out the mod.

So, back to the amp itself. When connected to power the MX-P50M circuit is live all the way through to the relays even though the Power Toggle is in the OFF position, so don't switch the amp off thinking that it's not draining any power from your battery - it is - only a tiny amount, but it's there.

With all the connections made between the MX-P50M, the RBI-1 and the IC-705, I switched the amp on, set the radio's power to 1W, switched to CW and briefly stabbed the key - the amp worked! I sent a few more dits and dahs before switching over to RTTY mode and keying the mic to send a more sustained output. Everything went well.

Now it was time to insert a Power Meter so that I could find out how much power into the amp would result in 25W out (to the dummy load). With the meter inline, I observed the following results...

The power-out reading on the radio pretty much mirrored the power-out on the meter (x10), so that was great. It altered slightly on different bands, but not a mile off. So 2.5W in, 25W out 👍

Obviously, for those who are licensed to use it, there's 45W available. So there you have it, a nice little amp which is just perfect for UK Foundation License holders and their QRP radios.

73, Tom, M7MCQ.



Frequency Band : 80m 40m 30m-17m 15m-10m
Operating Modes : SSB CW AM,  RTTY and FM-Reduced duty cycle
RF input : 5W
Output power : 45W+
Band mode : Manual
Power requirement : 13.8V 8A ,  RED “+” BLACK “-”
Cooling method:passive air cooling
Antenna connector : SO-239 50Ω
Size : 155*100*35 ( mm )D*W*H
Weight : 0.55Kg

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