Excellent video! This is a perfect primer for the MX-P50M and rig connections. I bought one of these when they came out and set up like Kevin but with a tethered T/R switch on the logbook. It worked perfect for ease of my radios (FT-817ND, HW-9, uSDX+). Now I am just entering FT-8 and needed the amp to auto-PTT, and there was so much confusion on the FT-817 ACC pin-outs. One extra thing I did on the ACC connector plug (they are very tiny and close). After soldering the wires, I stripped 1/4" insulation off a scrap wire and slid over the +13.8V pin next to the TX GND pin to insulate. This is in case if something accidentally yanks the cable, it won't touch pins and smoke the precious rig. Thanks again Kevin!
Thanks for this video. 2 years later I bought this amp. I saw your video before using this amp first time, so, I modified it using your schematic and info. I am using it with a new Xiegu X6100 and it works perfectly. I am getting around 45 Watts which is OK for this little amp. I am happy my new Radio is protected and safe. Again, thank you for posting this and sharing your knowledge with us. 73 Rob YS1RS
Great video Kevin. Thanks for going into detail about this little amp. It really does help the amateur radio community as a whole and also the addition of the keying circuit to protect the radio. Good DX.. KG5IF
Thank you, Kevin. I just implemented the circuit in my MX-P50M. It works like a charms with my FT-817, the original version obtained about a year after the model came out. It has never failed me and now, it is further protected. 73, Chris VE2MW
I had a design to replace mercury whetted relays (which for obvious reasons became unreasonably expensive), and it was switching inductive loads. I was amazed at the inverse voltage spikes I found that were causing severe damage to the solid state relays I was using to replace the mercury whetted relays. A MOV was added to to reduce the spike. It worked, but the MOV would incinerate on occasion. Great video!!
Thanks for the tip on how to connect a FT817, most of the documentation talks about PTT which is wrong. The transistor modification will also be useful.
There is a diode, look up on top part of the schematic. The ft-817 has a 2sd2211 switching transistor rated to 160 volts and 1.5A and a protection diode built in. Thanks for the video, nice mod!
I was going to chime in with the same comment. But it is not brilliant design to "expect" a pretty much unknown input (in the case of the radio keying the PA) to have flywheel protection. It SHOULD, but in these days with so much junk on the market, you never know....
Very clever idea to use an extra transistor this way. Although @16:50 the way you place the diode to protect the transistor is not correct. The current in the relay wants to keep flowing in the same direction as it did when the transistor was still on, so still towards the emitter. It will raise the emitter and thus also the base voltage and the spike ends up in the rig. The diode doesn't protect this way because it is still reverse biased after the transistor opens. You have to place the diode across the relay coils towards the supply in order to work as intended. Actually - it will work a little. As the emitter voltage rises and the base voltage increases to a few volts above the 12V supply, there will be a small current flow from the base into the pull up R towards the supply, keeping the transistor in a linear conduction mode. It will then dissipate the collapsing current into the transistor without too much over voltage to the rig. But that's a side effect. Better have the diode do the job.
Don Reid A zener would help but the zener voltage then needs to be higher than the maximum expected supply voltage to stay off when the relay is not powered. Maybe 15 or 18V to be safe? But at 12V this still allows enough voltage across the base pull up resistor to bring the transistor into a linear conduction and dissipating the tail of the relay current. Which is fine but less defined. This way of clamping would work best if the pull up resistor to the base is not too big, say max. a few kOhm. Then the voltage spike of the relay would be clamped by the transistor to just above the supply voltage.
When the supply is removed from the relay the collapsing magnetic field induces a voltage spike in the coil which is in REVERSE polarity to the supply. Therefore the protective diode must be cathode to the positive supply side of the relay coil and anode to the ground side. If it were the other way around the diode would simply conduct and effectively short out the supply.
At 14:30 make that diode a 15V zener and it will properly clip the spike which tries to go more positive than the power supply. A .047uF capacitor will also do the trick by absorbing most of the spike. --N2GX
Kevin, I guess there must be multiple revisions of the Amp. In my case, when the power switch is off the keyline is floating, not at input voltage. When i switch on the power, the keyline goes to 13.8V dc which is my input voltage from the PS. I agree about adding in the snubber diode which i will add. 73 de k1jbd
Hi Kevin, nice job. But apparently 40ma is not a problem for original mchf. It has optocoupler as keyer. The question is how RS implemented it, probably they simplified it. Also mchf is known to have problems with pin diode rx/tx switch. Mine had terrible distortions at 14Mhz 5w. So I removed diodes and replaced it with relay. The results are excellent. A little bit more output power and improved sensitivity. Probably you would like to check yours.
Hello dear Kevin! Thanks for the very interesting information, but it is a pity that your Blog page with the spectral purity testing of the amplifier does not open. Is it possible to download this information somewhere else? Thank you and good health. Regards Dima UT4NJ 73!!!
Hello Dima, thanks for watching. That was not my blog page, that was another person's blog page. And apparently his blog has been deleted. I have no control over that, I'm sorry. I've edited the description to indicate that that page is no longer available.
It's pretty typical to have 12V hot on the power transistors all the time, most 100 watt radios are wired this way. Even small voltage drops are undesirable and running the high current wiring around the cabinet, through switches, etc. will cause voltage drops and affect power and linearity negatively. So anytime power is connected the RF power section usually is hot but biased off. 73 de w6akb.
Is that the stock output transformer? It looks like the ferrites have been taken out of snap-on ferrites and taped together. Not necessarily bad, just interesting.
Most modern radios the PA is always powered when connected to a supply because that little switch on the front cannot handle the current draw of the pa. You know what happens when you let the magic smoke excape.
I don't know about that. The most intelligent thing to do is have a dpdt switch and have one pole as show and the other one switches off the bias supply to the Mosfets. That way the Mosfets don't get a Vcc belt every time the amp goes to Tx mode yet the Mosfets are not cooking any amps in standby mode. Tim Fidler/ Nz
It goes across the collector and emitter on the transistor. As shown in the schematic I put on screen. You can pause the video and do a screenshot to get the schematic.
Kevin love your videos. I have different problem but similar. I have this amp and trying to use it with a Xiegu G1M. On that rig the ACC PPT out has a 8.2V bias voltage on it when in RX. On TX the G1M puts the voltage on the PPT out to ~0v. (not 100% a ground but it seems to work for this amp). I wired the amp up as the instructions say but that Bias voltage causes the TX LED to light dim. Kind of like you had. It is not actually transmitting. When I do press TX on the rig the Amp kicks in and it amplifies fine. The problem is sometimes when I release TX on the rig the amp does not completely switch back. The TX light goes back to dim and the power out the amp drops but it the rig is not receiving so I assume at least one of the relays is staying closed. Likely because of the 8.2 bias voltage. Looks like you studied this little think pretty good so any ideas would be appreciated. Seems like a little circuit like this that filters out the bias or keys of the ground would work.
I think my circuit would work as is. The base of the transistor is already biased up to the 12v rail, so the voltage coming from the radio would affect nothing. Since it's keying the amp now, we know it grounds the control line. So, I think this little transistor driver I did should probably solve your problem as well.
@@loughkb I tried it be it did not work. It was really strange. It seemed like it was working but the once it kicked back into receive mod the rig was not receiving anything. Like it was causing one of the relays to hang or something in the amp. In the end I used a opt relay module set to trigger on high but connected the amp to the normally closed side of the relay. I drive the relay off the main 12v input to the amp. This works well with the only downside being if you turn the radio off it keys the amp..... Ok for my use.
@@jlacy8234 It doesn't make sense. The high side of the amps relays is connected to the 12V rail. The low side goes out to the rig. The only way it can trigger the relays is if it's grounded. Although it is very odd that the control line from the radio would go only up to 8V when not in TX. That's a 4V difference though, so if it can carry some current when in that mode, it could be enough to hold the relays in. Super weird.
Hi Kevin, Nice when a plan comes together and works. Great job. Have you done any work on the RV's A/C? Took my Extra exam last night and passed, so I can append /AE to my call until the FCC upgrades my license. Stay safe. 73 WB3BJU /AE
Great video Kevin. This sort of information will help me lessen the chance of letting the magic smoke out of my equipment... ;) Take care and looking forward to your future videos. de VA3NIE
Hello Kevin. I've modified my MX-P50M for my IC-705 and it works perfectly due to your explanation on how to mod the amp. My next question is, could I used this modified amp on my FT-818? Any thoughts? Thx.
@@frankwc0o As I recall, and old PS/2 style mouse or keyboard uses the same type of DIN connector on the back of the 817 and 818. You could get a 3.5mm jack and wire it up that way as an adapter cable. That way you could keep the 3.5mm plug on the amps cable for use with the other radio.
Have you tried the FT-817 + amp combination on CW? That's the mode I will probably operate with the amp. Do you think the amp's relays are capable of handling the FT-817's version of QSK? And, thanks for the keying circuit mod.
I'm pretty sure the amp's relays would be a problem operating qsk, especially at higher speeds. I think I recall that amps have to be designed for and rated for qsk, probably using pin diodes or other solid state switching. I've used it in CW, but not QSK. It's fine if you have a half second or more delay before switching back to RX. I've never liked QSK myself though. The RX noise popping up between elements glitches my brain a bit and makes it harder to send. But that's just me.
@@loughkb Thanks for the suggestions. So, it will be more like working with my old SB-220. It doesn't do QSK either. And, I want to try your little switching circuit. Simple enough. I just need to locate the parts. One more question. Do you use the 817's CAT interface along side the TX switching? 73 de Peter, AE1T
Yes. I have a serial port connection on the acc port as well. I used an old PS/2 mouse cable and it had all the pins brought down the cable, so I just found the right wire and the cable is doing double duty.
Thank you so much Kevin for the great video. I have a query: I have a tr(uSDX) and I want to connect this new amplifier to it. Is there any possibility that the amplifier is activated only when it receives the RF signal from the tr(uSDX)? Not with the ACC wire? Thank you very much for your answer.
Thanks for the great video! I'm making a little PCB for this. Quick question, did you happen to measure the pin spacing on the connector on the amplifier's PCB?
Well I am late to the party, but I want to run one of these with my FT-818 and use a Digirig so I can do digitalmodes. What sort of cable setup will key the amp zgiven that both the amp and the digirig want to use the same Din?
Hi Kevin. I've noticed there are buffered interfaces that are sold on eBay, is that something different than you mod? Some say, its important to have a buffer to protect the switch transistors.
While it's probably a worthwhile modification, for the peace of mind of anyone already using an unmodified amplifier, the (I believe) 2SD2211 transistor used for PTT switching in the FT-817/818 is quite a meaty beast rated at 1.5 Amps collector current and 3 Amps pulse ... so the 40mA drawn by the relays in the amplifier is very unlikely to be a problem. There is also a protective diode already fitted across the switching transistor in the radio to suppress any back EMF spike from the relays. Edit or delete this
@@loughkb - Even knowing that the switching transistor in the 817 is sufficiently rated, I'm still going to modify my MX-P50M. Like you, switching those relays directly just doesn't sit well with me and it's all too easy to take out the PTT switching in a rig which can't handle that sort of current.
Thank you for the videos and the schematic. I bought one of the amps and, since I found the heat sink getting warm with 2.5 watts of drive, I added a 12V cooling fan that fits neatly over the heat sink. I've also integrated the CAT interface to my computer. So now, the FT-817 and little amp together act more like my K3 + SB-220. I plan to add your keying circuit later. Would you consider a mod to the amp to actually switch off the 12V to the circuitry? de Peter, AE1T
Switching the supply would require a relay or something that can handle the current. Even bigger panel switches are usually only rated for a max of 2 amps or so. No surprise you need the cooling. Amplifiers are never intended to run near full output at 100% duty cycle modes. It doesn't get very warm for me running SSB voice. I'd bet you're running digital modes?
Hi Kevin. Excellent video. I like your mod to it? I have had one of these amps for many years, using it with FT817ND. Now, I would like to use it with the Xiegu X6100? Have you any tios, please? 73 de Pete GI0FZT.
I just subscribed to your channel. watched the videos of you reviewing, testing and modifying this interesting inexpensive little RF amplifier. I wanted to ask if you could also review the MiniPA50 HF amplifier. It is very similar to this. I was curious if it may have a better circuit design Or perhaps it may need your modifications. It is about the same price as this. I do believe one of its features is that it has auto band decoder switching for the 817 on the QRZ forum there are schematics for modifying it to work on 6 meters. Just for kicks there is a very high quality 50 watt amplifier called the Hardrock-50 that you build from a kit and you can also bild as an option into it an antenna tuner, Thank you.
Looking at the schematic for the MiniPA50, it's a better design. They have diodes across the relays and proper drivers for the relays on board. I can't review it, I don't have one.
The MiniPa50 as shown here does not have auto RF freq detection. www.joom.com/en/products/5c2f190e8b2c3701017d41a5 It is a poorer design thermally. There appear to be a number of "mafrs" in the PRC who take a common PCB (populated) and put it in a case and call it a different beast. The Hardrock 50 amp is a magnificent design but at a high price too. It uses six off RDHH16 Mosfets, which are easy to obtain and it does not overstress them so it has a potential to last forever depending on the build quality.
Hey Kevin, I ordered this amplifier, to use on my xiegu g106. I know I have to hook the relay of the amplifier to the ptt and ground of the ACC. do you think i'll have any trouble with the radio tripping the relay ? Any information you might have would be greatly appreciated. Thank JR.
Hello jr, sorry I don't know anything about that particular radio. But if it cannot handle the current and key the amplifier, you can do my modification.
Hello Kevin, Thanks for the informative video. A friend just gave me a MX-P50M that he never used. I'd like to use the PA with my QCX+ from QRP Labs. The QCX+ signal is 0V when the QCX+ is in Receive mode, and +5V when the QCX+ is in Transmit mode. Do you see any issue using this fix with the QCX?
The signaling you describe is inverted. Most key lines from radios ground when in transmit. Also, being a CW rig, you'd want something that stays switched rather than switching with each key down. You probably wouldn't want the relays in the amp chattering with each key down. It's not an amp that's designed for that kind of operation. So, to answer your question, no, this mod wouldn't work in your case. You'd need something extra. maybe a 555 timer circuit that is triggered by the signal and has a bit of a delay before releasing the key line.
You didn't fix the power switch, what are your thoughts on that? Thx anyways - very use full, got it running on my IC705 :) Also, another question, you added a pull-up 10K resistor, but I don't think you need that, there is already a LED and R11 (6.8K) doing that - don't you agree? Ahh - cancel my question, I see, you'll need it because you cut the signal wire instead of modifying the board.
To switch the power to the finals, you'd need a relay or switch that can handle 8 amps draw. It's easier to just switch power external to the amp if you want to power it down completely.
@@loughkb super, thank you. Now I have all the parts, waiting for the MX-P50 to come and I can do the mod. It will be great when it works on the IC705!
That's the radio I use it with most of the time. It compliments it well. Don't push your drive level above 3 watts though. The amp becomes a bit non-linear and you'll start splattering.
@@loughkb Thank you for the tip. I'm very excited to use this amp, for many people tell me that I'm at the noise level and I hope that I can squeeze out 1S unit with the amp. Although, conditions are getting better everyday. I'm here at LAX and I was able to talk to someone at Chile, South America with 10 watts and my report was a 57.... incredible!
Kevin I suggest you use a voltage divider to keep the base current down otherwise you can damage the 2N3904 PNP Bare Minimum Added to your schematic You need A Resistor in Series with the Base ( 300ohm to 2Kohm ) to limit the Base Current You could also add to Another Resistor if you want to make sure to keep the 2N3904 is biased off when the PTT is not keyed across the 2N3904 ! The High Side (with No PTT), depending on the radio can be 5V to 12V which may or may not be current limited and depending on the Radio you use with the Amp, you can exceed the Base Current of the 2N3904 and cause it to either die over time or make it leaky 73 Mike Circuit Designer
Base current to or from where? Emitter to base or base to collector? Most radios that I've looked at the schematic for will use a transistor switch to ground the keying line with a resistor limited voltage in only a few cases. I haven't seen one that could feed any real current over the line.
@@loughkb A PN junction, such as a diode or a base-emitter junction has a nominal voltage difference of 0.5V though in practice it tends to be about 0.7V when we measure it due to some resistance within the diode or transistor. At 16:42 in the video, you show a PNP transistor with the base connected to the same point (when keyed) as the collector, and the emitter connected to(wards) the other side of the power supply. It looks like the base-emitter junction has the full 12V across it and that would burn up the transistor. What I missed, at first glance, is the emitter connects to supply THROUGH the relay - not directly. When the base goes low: current flows through the transistor (emitter to collector), that pulls current through the relays, the voltage at the emitter goes down from 12V to about 0.7V, and all is well for the base-emitter junction. There's negative feedback because the relay is wired between the emitter and supply rail. Much like an NPN transistor with a resistor between emitter and ground. Maybe Mike's concern is an initial surge through the base-emitter junction as it starts to turn on. There is the full 12V at the emitter (no current flowing through the relay yet) and we are taking the base to 0V. That is 12V across the junction, for a very short time. That said, it's probably not a problem because as soon as ANY current starts to flow through B-E, current will flow through C-E and drop the voltage at the collector. All should be well. I predict a long and happy life for your mod and copies.
My hands are not steady enough to trust them on other people's gear now. You might check with members of your closest ham radio club. You might find someone there that will help you out.
If I sent you 300$ would you send me a modified Px 50 to work for my Ic705? I don’t have the skills to make an amp and I need small amp. Can’t wait for the pa500
I don't get QRP people who buy a 5 watt radio and then have to buy and incorporate a 100 amp. Buy a 100 watt radio with a power adjustment capability. Easier?????
It's a 35 watt amp. The combination of the FT-817 and amp when transmitting a solid 30 watt CW carrier was drawing just under 70% of the current of my 7300 transmitting the same 30 watts. It's more power efficient for an off-grid solar power setup. In my use case, this is an advantage.
A QRO Radio powered down to QRP level works, yes. But is not power efficient at all. A QRP Radio with a small 30-40W amp is way more power efficient than a big radio powered down. If you work off grid then a QRP radio + this little amp is a winner power efficient combo!
Excellent video! This is a perfect primer for the MX-P50M and rig connections. I bought one of these when they came out and set up like Kevin but with a tethered T/R switch on the logbook. It worked perfect for ease of my radios (FT-817ND, HW-9, uSDX+). Now I am just entering FT-8 and needed the amp to auto-PTT, and there was so much confusion on the FT-817 ACC pin-outs. One extra thing I did on the ACC connector plug (they are very tiny and close). After soldering the wires, I stripped 1/4" insulation off a scrap wire and slid over the +13.8V pin next to the TX GND pin to insulate. This is in case if something accidentally yanks the cable, it won't touch pins and smoke the precious rig. Thanks again Kevin!
Thanks for this video.
2 years later I bought this amp. I saw your video before using this amp first time, so, I modified it using your schematic and info.
I am using it with a new Xiegu X6100 and it works perfectly. I am getting around 45 Watts which is OK for this little amp. I am happy my new Radio is protected and safe.
Again, thank you for posting this and sharing your knowledge with us.
73
Rob
YS1RS
Great video Kevin. Thanks for going into detail about this little amp. It really does help the amateur radio community as a whole and also the addition of the keying circuit to protect the radio. Good DX.. KG5IF
Good to see the old KB9RLW back. Ive missed your electronic vids.
Good mod Kevin and glad it worked 😀.
When I saw the TX flash on the amp, my first thought too was that the 918 wasn't sinking enough current.
Thank you, Kevin. I just implemented the circuit in my MX-P50M. It works like a charms with my FT-817, the original version obtained about a year after the model came out. It has never failed me and now, it is further protected. 73, Chris VE2MW
Spot on! Tight space, but small mod. Very interesting video.
Nice job! It has been so long since I have looked at a schematic. Thanks for the refresher.
I had a design to replace mercury whetted relays (which for obvious reasons became unreasonably expensive), and it was switching inductive loads. I was amazed at the inverse voltage spikes I found that were causing severe damage to the solid state relays I was using to replace the mercury whetted relays. A MOV was added to to reduce the spike. It worked, but the MOV would incinerate on occasion. Great video!!
Congratulations! Thank you for sharing
Thanks for the tip on how to connect a FT817, most of the documentation talks about PTT which is wrong. The transistor modification will also be useful.
Dude your mod works great. I can key with the 705 now. Its a lottle bit of a tight space in the amp but works fine.
Nice mod. Thanks for taking the time to make this video and others.
There is a diode, look up on top part of the schematic. The ft-817 has a 2sd2211 switching transistor rated to 160 volts and 1.5A and a protection diode built in. Thanks for the video, nice mod!
Great. Thanks for info ;)
I was going to chime in with the same comment. But it is not brilliant design to "expect" a pretty much unknown input (in the case of the radio keying the PA) to have flywheel protection. It SHOULD, but in these days with so much junk on the market, you never know....
Very clever idea to use an extra transistor this way. Although @16:50 the way you place the diode to protect the transistor is not correct. The current in the relay wants to keep flowing in the same direction as it did when the transistor was still on, so still towards the emitter. It will raise the emitter and thus also the base voltage and the spike ends up in the rig. The diode doesn't protect this way because it is still reverse biased after the transistor opens. You have to place the diode across the relay coils towards the supply in order to work as intended.
Actually - it will work a little. As the emitter voltage rises and the base voltage increases to a few volts above the 12V supply, there will be a small current flow from the base into the pull up R towards the supply, keeping the transistor in a linear conduction mode. It will then dissipate the collapsing current into the transistor without too much over voltage to the rig. But that's a side effect. Better have the diode do the job.
It could work if it was a zener diode but OP is right normally you want a diode to the 12V rail.
Don Reid A zener would help but the zener voltage then needs to be higher than the maximum expected supply voltage to stay off when the relay is not powered. Maybe 15 or 18V to be safe? But at 12V this still allows enough voltage across the base pull up resistor to bring the transistor into a linear conduction and dissipating the tail of the relay current. Which is fine but less defined. This way of clamping would work best if the pull up resistor to the base is not too big, say max. a few kOhm. Then the voltage spike of the relay would be clamped by the transistor to just above the supply voltage.
When the supply is removed from the relay the collapsing magnetic field induces a voltage spike in the coil which is in REVERSE polarity to the supply. Therefore the protective diode must be cathode to the positive supply side of the relay coil and anode to the ground side. If it were the other way around the diode would simply conduct and effectively short out the supply.
I got the Radiodan RBI-1 to interface the MX-P50M with my IC-705. 73 AJ6RE
I just ordered this amp for my TX-500 - I will need this mod. You should sell this to folks like me with very bad eyes.
At 14:30 make that diode a 15V zener and it will properly clip the spike which tries to go more positive than the power supply. A .047uF capacitor will also do the trick by absorbing most of the spike. --N2GX
Sharp troubleshooting, slick solution!
Thanks for sharing the mod.
Kevin, I guess there must be multiple revisions of the Amp. In my case, when the power switch is off the keyline is floating, not at input voltage. When i switch on the power, the keyline goes to 13.8V dc which is my input voltage from the PS. I agree about adding in the snubber diode which i will add. 73 de k1jbd
Hi Kevin, nice job.
But apparently 40ma is not a problem for original mchf. It has optocoupler as keyer.
The question is how RS implemented it, probably they simplified it.
Also mchf is known to have problems with pin diode rx/tx switch. Mine had terrible distortions at 14Mhz 5w. So I removed diodes and replaced it with relay. The results are excellent. A little bit more output power and improved sensitivity.
Probably you would like to check yours.
That's really good. This will help a lot of folk. Thanks very much.
Thanks Kevin I think I actually learned something watching this , you do a good job on the teaching side! 73 Chuck
great vid, lots of gold in this one!
Great mod and good spotting on the relays!
Of course that begs the question: Low ball and modify, or save your pennies and get a "better" amp?
Hello dear Kevin! Thanks for the very interesting information, but it is a pity that your Blog page with the spectral purity testing of the amplifier does not open. Is it possible to download this information somewhere else? Thank you and good health. Regards Dima UT4NJ 73!!!
Hello Dima, thanks for watching. That was not my blog page, that was another person's blog page. And apparently his blog has been deleted. I have no control over that, I'm sorry. I've edited the description to indicate that that page is no longer available.
Got it, thanks!@@loughkb
It's pretty typical to have 12V hot on the power transistors all the time, most 100 watt radios are wired this way. Even small voltage drops are undesirable and running the high current wiring around the cabinet, through switches, etc. will cause voltage drops and affect power and linearity negatively. So anytime power is connected the RF power section usually is hot but biased off. 73 de w6akb.
Is that the stock output transformer? It looks like the ferrites have been taken out of snap-on ferrites and taped together. Not necessarily bad, just interesting.
Most modern radios the PA is always powered when connected to a supply because that little switch on the front cannot handle the current draw of the pa. You know what happens when you let the magic smoke excape.
I don't know about that. The most intelligent thing to do is have a dpdt switch and have one pole as show and the other one switches off the bias supply to the Mosfets. That way the Mosfets don't get a Vcc belt every time the amp goes to Tx mode yet the Mosfets are not cooking any amps in standby mode. Tim Fidler/ Nz
Any chance of video or pics on how to wire the diode to prevent spikes?
It goes across the collector and emitter on the transistor. As shown in the schematic I put on screen. You can pause the video and do a screenshot to get the schematic.
Kevin love your videos. I have different problem but similar. I have this amp and trying to use it with a Xiegu G1M. On that rig the ACC PPT out has a 8.2V bias voltage on it when in RX. On TX the G1M puts the voltage on the PPT out to ~0v. (not 100% a ground but it seems to work for this amp). I wired the amp up as the instructions say but that Bias voltage causes the TX LED to light dim. Kind of like you had. It is not actually transmitting. When I do press TX on the rig the Amp kicks in and it amplifies fine. The problem is sometimes when I release TX on the rig the amp does not completely switch back. The TX light goes back to dim and the power out the amp drops but it the rig is not receiving so I assume at least one of the relays is staying closed. Likely because of the 8.2 bias voltage. Looks like you studied this little think pretty good so any ideas would be appreciated. Seems like a little circuit like this that filters out the bias or keys of the ground would work.
I think my circuit would work as is. The base of the transistor is already biased up to the 12v rail, so the voltage coming from the radio would affect nothing.
Since it's keying the amp now, we know it grounds the control line.
So, I think this little transistor driver I did should probably solve your problem as well.
@@loughkb I tried it be it did not work. It was really strange. It seemed like it was working but the once it kicked back into receive mod the rig was not receiving anything. Like it was causing one of the relays to hang or something in the amp. In the end I used a opt relay module set to trigger on high but connected the amp to the normally closed side of the relay. I drive the relay off the main 12v input to the amp. This works well with the only downside being if you turn the radio off it keys the amp..... Ok for my use.
@@jlacy8234 It doesn't make sense. The high side of the amps relays is connected to the 12V rail. The low side goes out to the rig. The only way it can trigger the relays is if it's grounded.
Although it is very odd that the control line from the radio would go only up to 8V when not in TX. That's a 4V difference though, so if it can carry some current when in that mode, it could be enough to hold the relays in.
Super weird.
I ordered the amp.so hopefully it will come someday. Could I just use a foot switch like I do on my big boat anchor base station amp.
hello Kevin whether the conversion is also worthwhile for the xiegu x5105 I also bought this power amplifier. the conversion is simply 73 TomDG1XT
Hi Kevin,
Nice when a plan comes together and works. Great job. Have you done any work on the RV's A/C? Took my Extra exam last night and passed, so I can append /AE to my call until the FCC upgrades my license. Stay safe. 73 WB3BJU /AE
Congrats on getting your extra Don!
No, I haven't worked on the A/C yet. I'll be doing that when I finally get to my summer spot next week.
Great video Kevin. This sort of information will help me lessen the chance of letting the magic smoke out of my equipment... ;)
Take care and looking forward to your future videos. de VA3NIE
Great post, thank you very much, I plan to use it with IC-705. One quick question, would 1N4148 diode work fine?
Yes, should be fine.
Hello Kevin. I've modified my MX-P50M for my IC-705 and it works perfectly due to your explanation on how to mod the amp. My next question is, could I used this modified amp on my FT-818? Any thoughts? Thx.
Yes, of course. The mod simply reduces the amount of current the radio has to deal with to key the amp. It will still work with any radio.
@@loughkb I thought so, had to check with the expert. Now I need to figure out how to connect to the 818 with the 3.5” plug. Any suggestions? -73
@@frankwc0o As I recall, and old PS/2 style mouse or keyboard uses the same type of DIN connector on the back of the 817 and 818. You could get a 3.5mm jack and wire it up that way as an adapter cable. That way you could keep the 3.5mm plug on the amps cable for use with the other radio.
@@loughkb smart. That for the motivation. I might have an old mouse in the house.
Bottom line: It works. Nuff said by me. 73 de N5XS
Have you tried the FT-817 + amp combination on CW? That's the mode I will probably operate with the amp. Do you think the amp's relays are capable of handling the FT-817's version of QSK? And, thanks for the keying circuit mod.
I'm pretty sure the amp's relays would be a problem operating qsk, especially at higher speeds. I think I recall that amps have to be designed for and rated for qsk, probably using pin diodes or other solid state switching.
I've used it in CW, but not QSK. It's fine if you have a half second or more delay before switching back to RX.
I've never liked QSK myself though. The RX noise popping up between elements glitches my brain a bit and makes it harder to send. But that's just me.
@@loughkb Thanks for the suggestions. So, it will be more like working with my old SB-220. It doesn't do QSK either. And, I want to try your little switching circuit. Simple enough. I just need to locate the parts. One more question. Do you use the 817's CAT interface along side the TX switching? 73 de Peter, AE1T
Yes. I have a serial port connection on the acc port as well. I used an old PS/2 mouse cable and it had all the pins brought down the cable, so I just found the right wire and the cable is doing double duty.
Thank you so much Kevin for the great video. I have a query: I have a tr(uSDX) and I want to connect this new amplifier to it. Is there any possibility that the amplifier is activated only when it receives the RF signal from the tr(uSDX)? Not with the ACC wire?
Thank you very much for your answer.
No. That amplifier doesn't sense RF to switch. It needs to be triggered by a ground on the keying line.
@@loughkb Thank you Kevin for the answer.
Thanks for the great video! I'm making a little PCB for this. Quick question, did you happen to measure the pin spacing on the connector on the amplifier's PCB?
Kevin, thanks so much for doing this video. Very instructive. Always enjoy your insights. 73 Kevin KI0EB
+1 for the coffee mug.
Kevin, you inform the manufacturer of this mod the cost does not cost them too much this mod it is better a damaged radio?
Hey Kevin, The problem with the Ic-705 is that it's low voltage triggered....
Pretty much all radios keying lines ground or pull down close to ground when keyed. I now have a 705 and this mod allows it to key the amp just fine.
Well I am late to the party, but I want to run one of these with my FT-818 and use a Digirig so I can do digitalmodes. What sort of cable setup will key the amp zgiven that both the amp and the digirig want to use the same Din?
Same din, but different pins I believe. Use the documented wiring for the FT-818 and just run a second set of wires off the same DIN connector.
Hi Kevin. I've noticed there are buffered interfaces that are sold on eBay, is that something different than you mod? Some say, its important to have a buffer to protect the switch transistors.
If the idea of the 'buffer' is to protect the radio's switching transistor from high current draw then yes, my mod is the same thing.
While it's probably a worthwhile modification, for the peace of mind of anyone already using an unmodified amplifier, the (I believe) 2SD2211 transistor used for PTT switching in the FT-817/818 is quite a meaty beast rated at 1.5 Amps collector current and 3 Amps pulse ... so the 40mA drawn by the relays in the amplifier is very unlikely to be a problem. There is also a protective diode already fitted across the switching transistor in the radio to suppress any back EMF spike from the relays.
Edit or delete this
I was going to have a look at the 817's schematic, thanks for saving me the time.
@@loughkb - Even knowing that the switching transistor in the 817 is sufficiently rated, I'm still going to modify my MX-P50M. Like you, switching those relays directly just doesn't sit well with me and it's all too easy to take out the PTT switching in a rig which can't handle that sort of current.
Thank you. Very informative and well explained. I had done mine . 73
Thank you for the videos and the schematic. I bought one of the amps and, since I found the heat sink getting warm with 2.5 watts of drive, I added a 12V cooling fan that fits neatly over the heat sink. I've also integrated the CAT interface to my computer. So now, the FT-817 and little amp together act more like my K3 + SB-220. I plan to add your keying circuit later.
Would you consider a mod to the amp to actually switch off the 12V to the circuitry?
de Peter, AE1T
Switching the supply would require a relay or something that can handle the current. Even bigger panel switches are usually only rated for a max of 2 amps or so.
No surprise you need the cooling. Amplifiers are never intended to run near full output at 100% duty cycle modes. It doesn't get very warm for me running SSB voice. I'd bet you're running digital modes?
Hi Kevin.
Excellent video. I like your mod to it?
I have had one of these amps for many years, using it with FT817ND.
Now, I would like to use it with the Xiegu X6100? Have you any tios, please? 73 de Pete GI0FZT.
tios? And I have no experience with the Xiegu radios yet.
I just subscribed to your channel. watched the videos of you reviewing, testing and modifying this interesting inexpensive little RF amplifier. I wanted to ask if you could also review the MiniPA50 HF amplifier. It is very similar to this. I was curious if it may have a better circuit design Or perhaps it may need your modifications. It is about the same price as this. I do believe one of its features is that it has auto band decoder switching for the 817 on the QRZ forum there are schematics for modifying it to work on 6 meters. Just for kicks there is a very high quality 50 watt amplifier called the Hardrock-50 that you build from a kit and you can also bild as an option into it an antenna tuner, Thank you.
Looking at the schematic for the MiniPA50, it's a better design. They have diodes across the relays and proper drivers for the relays on board.
I can't review it, I don't have one.
The MiniPa50 as shown here does not have auto RF freq detection. www.joom.com/en/products/5c2f190e8b2c3701017d41a5 It is a poorer design thermally. There appear to be a number of "mafrs" in the PRC who take a common PCB (populated) and put it in a case and call it a different beast. The Hardrock 50 amp is a magnificent design but at a high price too. It uses six off RDHH16 Mosfets, which are easy to obtain and it does not overstress them so it has a potential to last forever depending on the build quality.
Hey Kevin, I ordered this amplifier, to use on my xiegu g106. I know I have to hook the relay of the amplifier to the ptt and ground of the ACC. do you think i'll have any trouble with the radio tripping the relay ? Any information you might have would be greatly appreciated. Thank JR.
Hello jr, sorry I don't know anything about that particular radio. But if it cannot handle the current and key the amplifier, you can do my modification.
Sounds good, thanks for your time. Ya I'll test it and if it doesn't work, I'll try your modification. Thank again.
Can this modification be done at the CAT-cable itself outside the amp? E.g. a P-MOSFET ?
Possibly, you'd need to bring power to it.
You have cup from StarTrek Discavery on Netflix :)
Hi Kevin, I really enjoy your videos. I have a dumb question, can I use a 1/4 watt resistor for this modification?
If you're talking about the resistor that brings the bias voltage to the transistors base then yes. It's only carrying 1 to 2 ma of current.
@@loughkb Thanks for your reply. This mod is so helpful thank you so much!
Hello Kevin, Thanks for the informative video. A friend just gave me a MX-P50M that he never used. I'd like to use the PA with my QCX+ from QRP Labs. The QCX+ signal is 0V when the QCX+ is in Receive mode, and +5V when the QCX+ is in Transmit mode. Do you see any issue using this fix with the QCX?
The signaling you describe is inverted. Most key lines from radios ground when in transmit. Also, being a CW rig, you'd want something that stays switched rather than switching with each key down. You probably wouldn't want the relays in the amp chattering with each key down. It's not an amp that's designed for that kind of operation.
So, to answer your question, no, this mod wouldn't work in your case. You'd need something extra. maybe a 555 timer circuit that is triggered by the signal and has a bit of a delay before releasing the key line.
@@loughkb Thanks Kevin. Just getting started in the hobby. I'm sure I'll find a good use for the amplifier later.
You didn't fix the power switch, what are your thoughts on that? Thx anyways - very use full, got it running on my IC705 :)
Also, another question, you added a pull-up 10K resistor, but I don't think you need that, there is already a LED and R11 (6.8K) doing that - don't you agree?
Ahh - cancel my question, I see, you'll need it because you cut the signal wire instead of modifying the board.
To switch the power to the finals, you'd need a relay or switch that can handle 8 amps draw. It's easier to just switch power external to the amp if you want to power it down completely.
Huh. Looks like it's just a standby switch. Also, some relays have diodes built in.
Yes ... the switch would probably be more accurately labelled "operate" and "standby" as with many larger HF linear amplifiers.
Nice little mod, job well done. 73 KE8JWE Vic.
Congrats!
Hello Kevin. Which reverse bias diode would you recommend?
Any silicon signal diode. Like a 1N914 . It's not too critical.
@@loughkb super, thank you. Now I have all the parts, waiting for the MX-P50 to come and I can do the mod. It will be great when it works on the IC705!
That's the radio I use it with most of the time. It compliments it well. Don't push your drive level above 3 watts though. The amp becomes a bit non-linear and you'll start splattering.
@@loughkb Thank you for the tip. I'm very excited to use this amp, for many people tell me that I'm at the noise level and I hope that I can squeeze out 1S unit with the amp. Although, conditions are getting better everyday. I'm here at LAX and I was able to talk to someone at Chile, South America with 10 watts and my report was a 57.... incredible!
Just a quick question, is that a Zener diode you’re using?
No, just a common signal diode.
Are you some kinda frick'in genius or what ???? .... lol !!!!
If only the unit was for UHF then you could use RF Circulators for RF switching.
👍👍👍 good Job 👍👍🤘
Are you selling these for the less technologically advanced?
I would love to purchase a completed board that I could solder in...
Kevin
I suggest you use a voltage divider to keep the base current down
otherwise you can damage the 2N3904 PNP
Bare Minimum Added to your schematic
You need A Resistor in Series with the Base
( 300ohm to 2Kohm ) to limit the Base Current
You could also add to Another Resistor if you want to make sure to keep the
2N3904 is biased off when the PTT is not keyed across the 2N3904 !
The High Side (with No PTT), depending on the radio can be 5V to 12V which may or may not be current limited and depending on the Radio you use with the Amp, you can exceed the Base Current of the 2N3904 and cause it to either die over time or make it leaky
73 Mike
Circuit Designer
Base current to or from where? Emitter to base or base to collector? Most radios that I've looked at the schematic for will use a transistor switch to ground the keying line with a resistor limited voltage in only a few cases. I haven't seen one that could feed any real current over the line.
@@loughkb
A PN junction, such as a diode or a base-emitter junction has a nominal voltage difference of 0.5V though in practice it tends to be about 0.7V when we measure it due to some resistance within the diode or transistor.
At 16:42 in the video, you show a PNP transistor with the base connected to the same point (when keyed) as the collector, and the emitter connected to(wards) the other side of the power supply. It looks like the base-emitter junction has the full 12V across it and that would burn up the transistor.
What I missed, at first glance, is the emitter connects to supply THROUGH the relay - not directly. When the base goes low: current flows through the transistor (emitter to collector), that pulls current through the relays, the voltage at the emitter goes down from 12V to about 0.7V, and all is well for the base-emitter junction. There's negative feedback because the relay is wired between the emitter and supply rail. Much like an NPN transistor with a resistor between emitter and ground.
Maybe Mike's concern is an initial surge through the base-emitter junction as it starts to turn on. There is the full 12V at the emitter (no current flowing through the relay yet) and we are taking the base to 0V. That is 12V across the junction, for a very short time. That said, it's probably not a problem because as soon as ANY current starts to flow through B-E, current will flow through C-E and drop the voltage at the collector. All should be well. I predict a long and happy life for your mod and copies.
@@loughkb
"and drop the voltage at the collector" should be "and drop the voltage at the emitter". I'm not used to thinking in PNP mode.
Thanks for restating the obvious.
Not sure what you mean here. However, what is obvious to some will be news to others who are learning.
@@loughkb Vaughn 101 sound like a bored Troll with an empty mind and a keyboard in front of him.. TIm Fidler
Hey could you make me one of these? I also bought a MX-P50M for my RS-918 but I cant solder worth a crap. Ill pay you whatever you want.
My hands are not steady enough to trust them on other people's gear now. You might check with members of your closest ham radio club. You might find someone there that will help you out.
Elegant! N9QIL
Do you have a Instagram account as where I can message you? I do not have Facebook anymore.
No, but my public email is good on QRZ.com.
Thanks? Very good/ Subscribe
Optoisolator!
If I sent you 300$ would you send me a modified Px 50 to work for my Ic705? I don’t have the skills to make an amp and I need small amp.
Can’t wait for the pa500
It's a very simple circuit and mod. Check with the members at your local radio club. You'll probably find someone that can help make the mod.
🇺🇸
😁
I don't get QRP people who buy a 5 watt radio and then have to buy and incorporate a 100 amp. Buy a 100 watt radio with a power adjustment capability. Easier?????
It's a 35 watt amp. The combination of the FT-817 and amp when transmitting a solid 30 watt CW carrier was drawing just under 70% of the current of my 7300 transmitting the same 30 watts.
It's more power efficient for an off-grid solar power setup. In my use case, this is an advantage.
A QRO Radio powered down to QRP level works, yes. But is not power efficient at all.
A QRP Radio with a small 30-40W amp is way more power efficient than a big radio powered down. If you work off grid then a QRP radio + this little amp is a winner power efficient combo!