Hi James/ Allan & all,
Well done on going out in such rotten WX. You are obviously very enthusiastic which is brilliant.
On the subject of amps. I am currently using the MX-P50M and have given it a fair testing both in the UK and abroad during 2015. The amp weighs 513 grams and the leads another 70gm. (My Sainsonic MX-P50A weight is 1.12kg plus 0.1kg for leads etc).
Bench testing on all bands concluded that the MX-P50M more or less does what it should (as did the MX-P50A which is now on the shelf). For SOTA I normally use an 11.1V, 6Ah Li-Po to power both the amp and the FT817 in parallel. However I have added separate circuit breakering/ rev. pol. protection to both DC input lines - the amplifier has a 10 amp and the 817 a 2 Amp resettable circuit breaker. That’s maybe overdoing it. One fuse would do both but the 817 already had the 2 Amp breaker and it sometimes has to work on its own. The system is also OK with a 2.2Ah battery which I sometimes take as backup.
I have no absorption Wattmeter but I transmitted into a dummy load via my cheap (through) wattmeter, setting the meter to 50 Watts (FSD) with 13.8 Volts applied and an FT817 drive level of 5 Watts. Reducing the drive level to 2.5W; 1W and 0.5W produces readings of 36W; 15W and 5W respectively from the amplifier. These are the results for the 14MHz band and I set the 817 to FM for convenience. Other bands give similar results - maybe a bit less on 28MHz?
A 50W output drops to 33W when the input voltage is set down from 13.8V to 11V. The 817/ amp combination still produced a useable output at 10.2 Volts but it was zero at 9V. Where Li-Po’s are concerned that is a safety feature. Typical input current for the combination is 7.6 Amps at 11.3V. The amplifier was briefly transmitted (on 80m) with 5W input at 13.8V into an open circuit three times without damage. It was pleasantly warm (not hot) after completion of the test schedule which had many (albeit brief) input voltage/ RF drive/ frequency band combinations.
Unlike the MX-P50A, which has automatic band switching when used with the FT817, you need to manually select the required LPF on the MX-P50M front panel.
Selecting the wrong filter has no detrimental effect. If a lower filter than the band in use is selected, output is greatly curtailed as expected. If the filter is higher than the frequency of RF coming from the 817, the power output reading increases somewhat. Either that is due to harmonics getting through to the power meter and dummy load and adding to the reading or the filters for each band start to cut off too close to the band in use when the amp is properly set.
Though not specified for the band, the amplifier does function apparently at full power on the 160m band. However on 160 the 2nd harmonic is not filtered as the lowest LPF which can be set is the 80m band one. A separate (external) LPF would be needed for Top Band.
If you switch the linear off, the FT817 output still reaches the antenna OK via the receive path (HF). That would need checking if VHF / UHF was being used with the linear still in circuit and switched off. The path through the amp, though presumably direct (apart from relay contacts), might attenuate higher frequencies? To get around this I use the rear SO239 connection for HF and the front BNC connection for VHF on the FT817.
I agree about the flimsy PTT connection system between the amp and FT817.
The MX-P50M cost 139.95 GBP on Ebay in early 2015 but there was no instruction sheet or manual. It is smaller and lighter than the Sainsonic MX-P50A which in my opinion has a heavier heatsink than it really needs. (I did a lightening exercise on that one which got me close to 1kg all told.) After that, the MX-P50M (at not much more than half a kilo) was a ‘must have.’ The MX-P50A cost 154.59 GBP from Amazon in 2013 but I can’t find them for sale anywhere now.
Though the interconnection system is tailor made to fit the FT817, the suppliers of both these linears seem to imply (rather vaguely) that they will work with (5 Watt) rigs other than the FT817. I’m sure they would if an interconnection system could be devised for each different rig. You can short two pins on the rear connector to put the amplifier into the transmit state but research and work would be required to make that automatic when using other rigs.
I like both these amplifiers, especially the smaller and lighter MX-P50M. Weight saved when compared with my lightened IC706-2G is over 0.5kg and I don’t need the Icom’s extra half ‘S’ Point very often. I often employ an FT817 drive level of 2.5 Watts. That saves battery power with full power in reserve for more difficult contacts. I tell people I’m running about 30 Watts - near enough. That said, I have used it with a 5 Watt input for an entire activation in summer without problem.
It did ‘appear’ to fail near the end of an activation of CT3/MI-004 so I bought a second MX-P50M on return to the UK. However, when bench tested, the first one worked perfectly and continues to do, so I now have a spare. I must have dreamt the failure??? Perhaps it was just the battery voltage after all.
The above information is based on simple tests and observations at G4YSS and on SOTA’s and is for reference only. I do not have the kit (or the inclination these days) to do more exhaustive testing.
Above: FT817ND and MX-P50M active on CT3/MI-001 2015 (G4YSS)
Above: FT817ND and MX-P50A active on EA8/LP-002 2014 (G4YSS)
Thanks to G4SSH in bringing my attention to this thread.