817/8 and MXP-50 amplifier?

Wondering if anyone here uses the Yaesu and Chinese amp combo?

How has your experience been? What type of battery do you use for rig/amp? Are they separate?

I’ve heard you need a special keying cable on top of the din they give you.

And to avoid overdriving.

Thanks for any input


The cable shown in the eBay adverts seems to be everything you need… it uses the TX GND pin on the 817 to key the amp. What else do you think you need?

True of any and every amp. Taking your 5W to about 25/30W is going to make a difference to your signal but going from 25W to 45W is somewhat more marginal. Drive it to about 25W, have a cleaner output and have the battery last longer.

You probably need to be able to check your antenna match is OK if you use antennas that are not docile and independent of their environment before you key up. I’d always use a separate battery for the amp.


Thanks for the info, Also, is a buffer relay needed to prevent voltage from coming back down into the radio?

That’s a good question… I just had quick look at the circuit and I didn’t see any back emf diodes shown for the change over relays. They may be on the board and not on the diagram. If there was a problem then the “internets” would be filled with people complaining their MXP50 blew up their 817 keying circuit. I’d be tempted to check my amp and fit some if there are none.

1 Like

I’ve got one, but I bought it second hand and all the cables were ready assembled and inlcuded. It came with an Anderson Powerpole connector so I run it off a 12V LifePO4 battery.
I usually run the 817 off the internal battery and the amp off the external one, but I have a Y cable so I can run both off the external battery if needed.
It doesn’t seem to have blown up my radio yet.

On the eBay listings it looks like the keying cable comes without a connector attached, so for the FT817 you’ll have to solder on the mini DIN yourself, but that’s so you can use other plugs with other radios.

It gets warm but not hot when being used a lot, sometimes that’s a good thing on a cold mountain :slight_smile:


I have used that combination and it worked well. FT817 set to 2.5 watts and it made a huge difference.

Review here MX-P50M HF AMPLIFIER REVIEW. | VK2QR Adventures in Ham Radio

There was some debate about more recent quality - a different unmarked device in the PA but I’ve had no bother when driven with 2.5 watts. I usually switch it out to check the SWR before going QRO…

(Now switched to a KX2 but the effect of the amp seems much smaller (10 watts with a speech processor seems much more effective than the 5 from the 817 … or it could just be conditions improving). 73. Paul


I am using FT817 + MX-P50M time by time, most with 2.5 W from the TRX. Battery is a 6.6 Ah 12.8 V LIFEPO. Until now without problems.

Output @ 50 Ohm is
between 7 W and 8.5 W @ 0.5 W in on 80…12 m and 6 W on 10 m
between 15 W and 17.5 W @ 1 W in on 80…12 m and 12.6 W on 10 m
between 27 W and 29 W @ 2.5 W in on 80…12 m and 24 W on 10 m

73, Ludwig


I use my MX-P50M amp with my FT-817, KX3 and IC-703, all set at 2.5 watts output to produce around 25 - 30 watts output. The lead for the Icom rig has an opto-coupler in line, but the other rigs key the amp directly. It is certainly worth having the advantage of more power with the 817, less so with the other rigs which run more power, but the amp does provide the option of delivering around 45 watts if required from 5 watts drive. It’s a balancing act… the weight of the amp plus battery versus the effort required to get up the hill.

I run the rigs off a 2200mAH LiPO and the amp off a 4200mAH LiFePO4 which gives me a good couple of hours intensive operation (CW and SSB).

73, Gerald


I think that’s a good point to highlight - I understand the amp doesn’t have any built in SWR protection.


I second that caveat. My pair of HF Packer amps have both blown finals when I forgot to check SWR… Same batteries as Gerald.

Elliott, K6EL


I use one with my 817 and love it, has made such a difference. Like Peter I run it and the 817 on an external battery with a Y connector that I made. I only run the 817 to a max of 2.5 watts.


I have one, I have used it with the 817 and it can make a big difference even though 30w is only six times the output of the radio. That’s 7.7 db which is theoretically only just over one S unit, but in practice it can be making a weak signal move from “in the noise” to an S point above the noise.

There is some concern about the lack of a diode to absorb the back emf generated by the relays when they are released. To be honest I had not thought about that so I didn’t install a diode. There is a post on youtube showing how one owner did install a diode and a keying transistor. There has been some hysteria on the IC705 forum (groups.io) on this topic. I think the diode is a good precaution. The 817 can clearly key the relays in the amp so the current amp is probably not necessary.

73 Andrew VK1DA/VK2DA


I run my 817 with this amp using a 9amp LIFEPO4 battery to power both.

  1. Have run it for over 3 hours, at 5W input and no problems with overheating and have never depleted the battery. I have also run it with 2.5W drive and have never put a dent in the battery. A friend uses a 6amp battery with the HF Packer which is 30 -35 watts I think, also has never run his battery down.
  2. DO make sure your antenna SWR is below 1.5 and as others mentioned - verify before using the amp.
  3. I also have the SOTA Beams speech compressor: DO use without amp and DO NOT use with amp as it will overdrive the amp and provide a ‘dirty’ signal.
  4. My best example of this combination was running it mobile with a 20M hamstick and making a contact from my location in Los Angeles to Japan
  5. This is perhaps the most important thing to keep in mind: ENJOY USING IT!

Howard KE6MAK


I should have added that my tests with the amplifir showed that the 2.5w level on the 817 was sufficient to produce 40w output. Therefore the 5w level will definitely overdrive the amp as it is not capable of 80w output, as doubling the input should double the output, right? That’s the definition of Linear, right? So I suggest never use more than the 2.5w drive level. Your amplifier may have different gain and sensitivity, but unless you measure its output at each 817 power level, you won’t know what level it stays or stops being linear. Overdriving an amplifier causes a wide signal and users of adjacent frequencies will rightly complain about your splatter.
Andrew VK1DA/VK2DA


Have used the MX-P50M with my KX2/KX3. Ran the amp with about 2.5W or even sometimes 5.0W if I get enthusiastic.
Needs a separate keying line as others have discussed.
During hot weather the amp will get hot so attached a cooling fan which can be switched in as needed.
Watch out for high VSWR which will pop the transistor. I always check the VSWR before switching the amp on.

Run it off either a 18AH SLA or a LiFE 7.0A lithium battery (nominal 12.8V). Have never run either battery down for the usual SOTA activation.

73’s Wal VK2WP


As discussed elsewhere - the MX-P50M I have produces ~20w out from 5w in. 2.5w in gives ~10w. That’s using an uncalibrated power meter - but using the same power meter to measure input and output power - so I’m confident that I am getting a 4x power gain from the amp, not 8x to 10x as others suggest.

Now, the MX-P50M I bought was cabled up for another radio model (KX2, I believe), not the FT818. I had to chop off the connector and fit the mini-din. But either I have a non-typical amp, or these come configured for different input powers - possibly based on the radio it’s set up for.

Hi Matt,

Yours is certainly not producing the power gain that most of them do. I wonder if a previous owner installed an attenuator of some kind on the input?

There are a few of them in the vk1 group and I think all show about the same performance, with 40-45w output on the lower HF bands and less above 20 mhz.

Anyway I hear you (usually) so it does help!

73 Andrew VK1DA/VK2DA

1 Like

Alternatively, perhaps half of the output device has failed. From memory it is a MRF286 dual device in push pull config. If one half fails, you have a distorted half power output. Any reports of poor signal quality?

The dual device that is the output of the FT817 failed in my 817 taking out one half of the active device. Replacing the complete final amp board with the final stage of the 817ND brought the power and quality back to rated levels.

1 Like

Never had any reports of distorted signal. Output spectrum looks clean on SDR. Harmonics - can’t recall the figures but were acceptable. Will check again when I get home - but signal certainly looked good last time I checked it.

1 Like

It looks like Yaesu decided to add their own protection. Here is the TXGND circuit and you can see D1057 on the line to protect againts back emf. Unless I’ve misread its function. If so, full marks to Yaesu for adding $0.001 worth of electronics to protect the keying transistor.