I would like to use the above amp with a 14,8 V (16,8 V) 4S2P 4.4 Ah Li-Ion battery in order to gain some more power for short SOTA activities.
Does anybody use this amp at the higher voltage successfully? I know that there is a 18V zener diode (D5). But with maximum 16.8V (fresh battery) this should be no issue. Any doubts?
My amp survived this when I accidentally omitted the voltage reduction diodes on an activation - once - but I didn’t repeat the experience. 73 Paul
I regularly connect my mx-p50m directly to my 4s LiPo and have had no issues.
MX-P50M HF AMPLIFIER REVIEW. | VK2QR Adventures in Ham Radio
Quote from the blog:
I conducted tests at a higher input voltage (14.5v) and found the amp can produce more power output if required. I did find some specifications for the encapsulated RF device inside the amplifier stating it can accept input voltages up to 24v to produce 100w output. I am not going to try it, the results could be catastrophic!
keep in mind that the mx-p50m does not have a high SWR protection. Due to the higher output, using a perfectly matched antenna may be crucial when driving the amp at higher voltages.
From my reading of datasheets most RF MOSFET power transistors can easily handle 20VDC and often more. I read somewhere this amp was run at 24vdc quite happily. Go for it.
I have done it a few times with a 4 cell LiPo without problems.
Plan to do it next week for 9 activations over 3 days.
Things to consider when over volting a PA and other transmitting equipment.
Dissipation of heatsink. The output chip may get too hot if the heatsink isn’t able to get rid of the extra power. Manufacturers don’t like to waste aluminium.
Rating of electrolytic capacitors. 15 V is a common rating. They won’t go bang at 15.1 V but 24 V is probably suicidal. Check what’s on the board first.
Matching to the PA chips is designed for 13 V not 18 V or 24 V. This results in efficiency dropping at some point in the over voltage. See first comment.
Resistive losses in pa transformers will go up 60% for just under 30 % increase in PA current. Expect the current to increase with increased voltage.
Output filters will get hotter but should be OK for modest overvoltage on the PA.
Could anyone encouraging others to apply lots of volts please do it themselves first and then report back.
Hi Ron, thanks for your insights.
73 de Geoff vk3sq
Thanks all for the information and insights. I hope that the slightly higher voltage (14,8V) will do no harm to the amp. At the moment I am driving the amp with 13,2V and get about 35-39 watts power output when driven with 4 watts (more input does not further increase the outpout power). If I get 50-60 watts out of a 14,8V source, I would be happy.
You should be okay up to just under 18 volts because that is the rating of the over voltage and reverse polarity zener diode in the Amp.
I was hoping to hear that. Indeed, the zener diode D5 in the circuit is rated for 18V. I have read though that some people replaced it with a 22V one. But I wouldn’t try more than 18V with this amp…
I have found 30 watts to be the “sweet spot” where the battery power requirement against the on air performance in terms of being heard is best. Therefore I run my MX-P50M with 2.5 watts input and a 4S LiFePO4 as the power source to produce the 30 watts. It really all depends on what you are trying to achieve.
I was interested enough to do some more investigating.
It seems the ‘Power’ switch is really an ‘Enable’ switch, the radio is always powered up if the power lead is connected. This includes the PA bias circuit so the current draw is significant!
I also found a video describing some problems with the amps keying circuit design, https://www.youtube.com/watch?v=k8dwJMOdEMI
It seems the radio has to sink the total current from the Amp Tx/Rx changeover relays! The video includes a fix
You will need 16 V to have a chance of 50 W based on 35 W with your voltage… By using decent power wiring that is only just long enough I get 45 - 47 W from mine with a LiFe battery and an FT817.
Now ask yourself how many dB is that. It’s an ego thing not a technical requirement. Going to 60 W is 2.3 dB up on 35 W.
Going from 4 W to 40 W is 10 dB and worthwhile.
I run my PA with 4S2P LiPo and my findings are very similar to Gerald’s @G4OIG.
I have included this fix in my amp. Works great. It is a simple circuit.
Info on the active device(s) (MRF186) here:
Looks plenty robust enough for higher voltages but as Ron VK3AFW points out you need to check the peripherals…
Never had a problem with 4s2p LiIon pack. For contests, I put a small 12 volt fan on the heat sink.
73, Peter - HB9PJT
Today the Li-Ion 4S2P 4,4 Ah battery pack arrived. Fully charged, the voltage is 16.5 V. The amp works perfectly at this voltage and the power output is approximately 45 W at 4 W input. On the higher bands, 50 W is the maximum.
Interesting enough, if the LPF filter is switched to the 30-17m position, power reaches 70 W on the 40m band. But then the current draw is nearly 9A. See pics. But I don’t think that the amp is supposed to be clean at this LPF position.