External add-on PA for HF, suggestions wanted.

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FYI Weighed mine at 646g (it has switched filters). A nice design as the step-up lets it run on 3x Li-Ion

But on the other hand, just weighed my my FT-891 at 1910g, so if I want QRO that’s the first choice now. It’s just more power hungry on both RX and QRP TX.

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What I wanted to say is, the MX-P50M ist small, lightweight, robust, simple, 10 dB, a clean signal, reliable and bang for bucks. Why think about another one apart from it? :wink:



I have a pair of HF Packer amps, and I have to change finals on one or the other every year due to forgetting to hit the LDG tuner. Whilst I have the replacement down to a science, I really want to install a simple SWR fuse. Diodes plus litz wire? Can’t find anything on line, and Virgil doesn’t respond. Help?

68 watts on 40m and 50 watts on 20m

Elliott, K6EL

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Look into self-resetting fuses.

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I had received a PA module from an FT-757 and set it up for the KX3. I created this compact SWR protection circuit, which works perfectly for me. The relay on the schematic is the transmit/receive switching relay from the PA. The SWR signal comes from an SWR measurement circuit. If the SWR is too high, the PA switches to receive. When the TRX goes to RX, the protection resets.

73, Peter - HB9PJT


There is also the FT857 in my drawer and a unfinished homebrew 50W linear next to it.

But ultimately I think: “Less power more fun”
73 Chris


There is also the DIY599 from DL4KA. It does up to 60 watts out with two to five watts in. It also has a built-in ATU and solar charger control. Yes, it’s very expensive. I love it.

As you narrow down your choices, consider the current draw on standby/ bypass.

The RO waves looks very good.

The hard Rock 50 now goes up to 100 amps with an upgrade kit and has a built-in ATU and QSK module if you’re interested in that.

The Neptune looks interesting.

I don’t have a 100 watt base station and have considered building a hard rock 50 plus with ATU and QSK.

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Thanks, Ken and Peter. The amps both have re-setting fuses already. If I catch on quickly enough, I can shut down for 30 seconds and re-start. Half of the time, the damage was done and you can hear the squeal. I’ll try the relay solution. There is enough room for a wrapped little board if the relay is small enough. Someone with time enough could sell the boards after-market, now that Virgil is done. The number of potential buyers is very large.

Elliott, K6EL

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Thanks for the suggestions. As I said the idea is to have something to boost the 10W I have (KX2, IC-705) when doing SOTA from more interesting locations. In the past 5W SSB/CW from my 817 was more than adequate from Maedira/Canary Islands etc. But I would like something a bit more ability to command my frequency.

Of the MX-P50 or Micro PA50+, I think the MX-P50 is the better amp.

The 1959 Ferrari Columbo 250 (3.0L) engine was a masterpiece, compact, reliable, 240bhp in user trim, 310bhp in competition trim. They continued look for improvements, 275 (3.3L), 275 4-cam, 330 (4.0L), 365 (4.3L) etc. They improved on perfection! For the same reason, the MX-P50 seems ideal but if you don’t check, something may have come out since that is better/cheaper/lighter/whatever.

And this is why the 250 engine is perfect… watch all 7minutes and have an eargasm.

Several people suggested CB amps. I have one here (£5 new in the box at Blackpool) but as of yet it has no filters and I’m not sure if it’s biased OK for SSB. Good for 40W CW on 10m. I see that there are filters available from eBay for not much money and that may be a way forward.

And that’s not a bad suggestion. However, I have far too many HF radios, I’ve sold one FT-817 and the other 817 and a 706 are going on eBay next 80%-off-fees day. But I’ve not really spent £2000 in the last year on a KX2 and IC-705 to buy another HF rig which will get very occasional use.

Like Peter HB9PJT I’ve thought about using a PA from a scrap HF radio (with real biasing and filters). When I bought my Codan 8525B it came with an unused spare PA board that’s good for 125W but I do not know if it’s ok on 10m as the 8525B is only for up to 24MHz. (I’ve just found VK3ZYZ’s 8525b synth updates… another project to make). The circuit will be useful should I go this way.

I should never have sold my HL-45B but the cash offer was useful and was spent on toys I would use as I had no use for a 50W amp for an 817 at the time. There was one on eBay last week but I was thinking ahead and didn’t bid. It went for a steal, less than an MX-P50 sells for.

So I may seek out an MX-P50 to borrow (I only need it for a week) on a "he who breaks it fixes it " principle. Or I may buy one of my own. And I’ll be keep a watch for a Packer, maybe one that needs some TLC and repair.

EDIT: and there’s an MX-P50 on eBay that some Scouse monkey has blown up. Worth watching to see how much it goes for.

I have the HRD 50 and at some point the KXPA 100 (we used it for Eric June memorial activation ages ago).

With all the cables and the “effort” to wire the amp to a kx2/kx3 on a peak, I ended up just dragging an 857d up a foreign mountain taking it as carry on with a monster battery. Figure it would all help me stay trim and invite excitement at second world airports.

Also the HRD50 isn’t exactly volumetrically small nor particularly light.

But if you do want an external amp how about Hans’s 50w mono band amp? Just pick a single band that you want to be “loud’ on or take two.

…maybe another interesting piece of information.

With the two 4Ah batteries from Eremit connected in parallel, I can operate the 80 watt PA for around 4 hours. (With the MX - P 50 it is twice as long.)

I can’t give any exact information about the RX/TX ratio… but there were always minute-long phases in which I only called CQ.

73 Armin

Why an external PA and not get something like a Yeasu FT-891 and run it at 50 Watts? No waterfall display so likely you can find something pre-loved at an attractive price. And the no-waterfall means also lower power consumption. No extra cables to carry and no ATU required after the PA.

Alternatively use two verticals, you can feed them both, in or out of phase, or even use one as a reflector or director by detuning. And working just below the top will give you gain in that direction too by lowering the take-off angle.

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It’s a very good idea. But why not an 891? Because most of the time it will sit here not being used and will cost 2.5 to 3x more than a PA with filters and I have an IC706 (with CW filter) I could man-handle already. (I’d rather sell that and put the money to something that will get used more often.) And as I said I didn’t spend £2000 in 2023/24 on rather excellent radios to not use them. Also I managed to activate 20 associations on 3 continents using 5W and no ATU. An ATU is nice but not essential.

However, I forgot to add to my thread that a very, very nice man has offered to lend me an external PA on the “if Andy breaks it Andy fixes it” principle which solves the problem. A very nice gesture.

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My small 2m linear is on permanent loan from a good friend who went off around the world in 1983 and it has been in use for SOTA since 2006. He is now settled in New Zealand and doesn’t have an interest in radio any more, so I guess it’s mine. Maybe this linear will become yours in 20 years time. :wink:

As for the FT-891 idea - the main pro is the convenience, the main con is that it’s a mobile with a thirsty receive current. That’s why I no longer have a FT-857D in my armoury. IMHO the FT-817 plus HF linear makes more sense, but as you say, for most of the time the rig on its own is sufficient :smiley:.

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@ZL1MJL John,

FT-891 just on RX takes something like 1.2A in current, while KX2/3 take quarter of that. You will not need big/heavy battery to run for couple of hours.

73 Marek