Hi Matt
This may help.
Loads of info on the operating voltage vs output of the MX50 amps.
73
Tim G4YTD
The one Andy is referring to (available from SOTABeams as a kit or pre-built - Speech Compressor for FT-817, 818 - SOTABEAMS) is a dynamic speech compressor which goes in-line with the FT-817/818 microphone. Plug its cable into the microphone socket and the microphone lead into the other end of what is about a lipstick sized box. Power comes from the rig. you will only need to open the box if need to adjust the units settings for compression.
73 Ed.
The 3S LIPO 2500 maH 14 GBP battery from Hobby king ( https://hobbyking.com/en_us/turnigy-nano-tech-2500mah-3s1p-5-10c-transmitter-lipo-pack-futaba-6ex-and-3pks.html ) fits into the battery bay of the FT-817/818 and will run the rig at 5 watts out for a couple of activations without problems. You may want to take a look at that approach rather than having to carry an external battery?
Take a look at FT-817 Low Cost Lipo Battery | VK3YY
73 Ed.
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4 cells in parallel to up the ampage and there will be 4 banks of parallel joined in series to up the voltage Andy . 16 cells I have got
Matt
Adjust compression is that in the speech compressor itself or in the radio ?
Am I guessing it comes with instructions?
Matt 2E0FGX
In the little black box itself - you must not have compression turned on in the FT818 if you have an external speech processor. Does the FT-818 have built-in compression? If it does it isn’t very good - most compression options except in the more expensive radios, tend to be bodges in the audio stages and create more “splatter” than “punch”.
Any setting in the box is a “set and forget” though. You need to set it to suit your voice and then leave it.
Here’s an article (in German and English) on it http://www.sotabeams.co.uk/content/817COMPRESSOR.pdf
73 Ed.
I got an Ft-891 to avoid the 817 plus amplifier issue and gain a newer receiver and 32bit DSP
So I can roll out 5 to 100 watts without any cables to worry about
Yes the current drain is more on receive but its worth it
Just pack a bigger battery
John ve3ips
Yes there cracking radios and the dsp work well . But I wanted a lighter radio with vhf as well as hf so was only feasible choice. Not done summits like snowdon yet but imagine that be a vhf activation
Matt 2E0FGX
That’s known as a 4S4P setup Matt. Yes, then it will have a capacity around 4x the single cell capacity.
…and lose 2m and 70cm! A pity, its such a tempting little radio.
I believe not - but it is worth remembering that the FT857D does have a built in compressor which I find very effective. Its one of those things that you tend to set and forget about, so beware of adding an external compressor too!
My wife and I are both participants in SOTA. She is strictly phone while I am primarily CW. We have had summits where she barely made the four contacts needed due to conditions.
I purchased one of the MX-P50M amplifiers and did a few bench tests. The device used for amplification is robust and capable of 100 watts at 900 MHz. The output is clean as long and you have the band switch set to the correct band. It functioned from 10 to 15 volts, the PA could take more voltage but I wasn’t sure about the rest of the circuit. The output power using 5 watts drive, varied from 35 to 45 watts, depending on voltage and bands.
We take it up on the hills and she now regularly makes 10 or better contacts. We use a 6 Ahr LiFePO4 12V battery, which seems to be all we need, so far we have activated three summits on one charge and the battery had plenty more to offer.
So far 70+ activations and no issues.
By the way, there is a SOTA Reflector thread with how to connect the FT817. FT817 + 45w Amp cable info needed
Lee N7LP
That’s the view from north of the border. 
My opinion is that they are a worthwhile buy. I use mine with both my FT-817ND and IC-703. The main problem with building your own amp is sourcing the components for the filters. Quite frankly I would rather spend my spare time on the hills, not trying to build an equivalent of this amp in so small a package with eyesight which is not what it used to be.
Well I found a UK source on eBay. I did note this is another thread some months back.
As for the power source, I run mine off a 4.2AH 4S LiFePO4 and at 2.5 watts drive I get around 30 watts out depending upon the frequency. That gives me at least 3 hours operating time. I agree with Malen that 30 watts out is the sweet spot for battery life / QSO’s. In practice it does actually make a difference when running SSB relative to an 817 barefoot and as Andrew says a speech processor is a worthwhile addition. The downside of running an external amp is the additional cabling. I also have the FT-857D which is a much neater solution, but it is less economical in terms of battery power.
If i need to do the vhf/uhf bands I pack the 817 or a handheld to use along with the 891. However, for car to summit then the weight is nothing to worry about and I can also bring a ladder and a picnic table.
If I was being a ultralight then I would use the 817 or the MTR3B and a handheld
Depends on the summit but why pack another amplifier and cable set
John ve3ips
Not very nice Brian, I might refer that remark to the Moderator.
CW was but one of seven approaches I suggested that I consider to be very much preferable to a linear amp. I think that’s a perfectly valid point. You even agree with one of my other suggestions but trust you won’t attract a similarly sneering remark!
A linear amp to me is extra weight, extra volume and extra expense, as well as denying oneself all the fun and satisfaction associated with each of my suggestions above.
For what it is worth I have had one of the amps for two years, and it does seem to make a difference. Having only been active in a period of low solar activity I struggled with SSB with 5w on HF. I tend to activate on 80/60/40 and using an FT817 feeding the amp at 2.5w I get good reports, and by putting in a speech processor too I seem to be able to make contacts despite the poor solar conditions. Had one poor report, think I was overdriving the amp by putting 5w in.
I am old enough to have the CW ticket - but never found it easy and despite occasional attempts am still reluctant to try it on an activation.
I use a couple of 4200mAH LiPo batteries with a voltage regulator to bring it down to about 13v and find that I am cold, or the dog is bored before the battery is flat!
I think SOTA is an individual challenge and quite how you make the contacts is up to you. Personally I don’t get much of a buzz out of a pair of running shorts and a 2m HH and I know I am nearer the “lucky anvil” end of lightweight operating but I enjoy it and that is probably quite important
PS Got my amp second hand off e-bay and never had a problem with it.
PPS Now the new challenges and the prospect of operating top band from the summit might be a bit harder as the amp does not go down to 160m - unless someone knows how to do it? ( Dont fancy operating without the right LPF to get rid of harmonics )
73 Paul
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That’s more than “Quite” important Paul, (IMHO) the enjoyment has to be the primary reasion for activating (or chasing for that matter)!
Good on 'Yer!
73 Ed.
You forgot to add the smiley, , which should have told you that it was light hearted in intent. Furthermore, it was more aimed as a gentle poke at Andy, who poked a “learn morse” comment at Matt when there had been no mention of mode in his post. Just to head off further discussion of an aside, I am perfectly happy to acknowledge the superiority of CW as a weak signal mode, furthermore I have learned morse code myself, can currently read at a speed in the middle teens and think that if we still had the 12 wpm code test I would be able to pass it. I just think that code and computer modes lack the charge that I get from the flavour of the human voice and choose not to use them at present, though I make no committments for the future - and anyway, I don’t have a key!
Now back, I hope, to the much more interesting discussion of linear amps.
Cheapest PA you can buy.
Is it? How much per hour is your time worth, and how many hours does it take to become proficient at Morse code? I can’t argue that it isn’t a worthwhile investment but it costs! FT8 is cheaper!