The Fine Art Of Making Power

Make no mistakes, I think real amp designers are witch doctors and that amp design is more voodoo than art. But this has never stopped me from having a go and seeing what I can come up with and what I can learn.

Today I hit a new milestone, I raised peak power to 50W. 50W might not sound like much to those who have gone out and bought their kilowatt to power cheat with, or for the guy who bought a amp pallet off the interwebs and assembled a kit. But for me, someone who has looked at lots of schematics, tried to understand each design decision the designer made and why they made it, then went and captured the schematic, laid out the PCB and had boards made, 50W is something of an achievement.

And don’t get me wrong here, 50W is what WA2EBY was getting out of 2 IRF510 and I am using 4 of them to make that same amount of power and still struggling to contain the magic smoke within the epoxy that is covering the silicon.

I was hoping for a little more out, 70W or there abouts would have been really nice. I do have room to push the amp a lot harder than I currently can. I can give it more drive, more bias and more voltage, with the current limitations being 5w drive from a CW transmitter i home brewed, 2V gate bias, and 30V and 10A from the lab power supply.

Along the way many parts were broken. I call this pile My Shame. Actually its not as bad as it looks, the first 4 fets to go were Ebay specials from China. I think they are fakes, they were never happy and just ran into uncontrolled oscillation.

The next 4 were all my fault. I gave it all the amps and they gave up the ghost after about 40 seconds of full key down madness.

This is the current mess of what is my work bench. Plenty of crap on the bench. A certain old guy home brew legend would say my bench is to clean LOL

Nothing like burned flux to get the soldering fan boys to go wild. There is nothing fundamentally wrong with the board, its is a little bit pedantic though. Its window of oportunity is quite narrow. Its kind of all or nothing. I had to change the input transformer and its ratio, the bifilar inductor feeding power needs to be changed as it gets to hot. Everything else is ok. Mechanically its easy to change out the fets on detonation. That is a plus.


Using the CW transmitter I built a while back to drive it.

This is the schematic. I probably should learn about temperature compensated biasing to make it a little more bullet proof. Other than watching the duty cycle and keeping it to say 50% or less, its ok and I have not harmed anything yet. It really could do with a bigger heatsink and some forced cooling to really crank it up. But for now, I am happy that it works.

I did key it up on 40m and sent some CW with it. Listening on KiwiSDRs i could hear myself in VK2,3 and 4. Not that, that is a challenge, I can do that with 5w. But i know its working and sounds fine. So that’s the end of this journey. I am calling this one won and done and now its on to the next thing.

Better add this image as well. Fiddy Whats. Oh one last thing, 5w in and 50w out, is 10dB gain. It is what it is.

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6 thoughts on “The Fine Art Of Making Power

  1. Weird, aren’t they? They seem so bloody simple, until you actually try to build one from scratch.

    “They” say FETs don’t need temp compensation, as long as the bias is well regulated. I don’t actually know, I’m still trying to get my 15m amp working.

    Nice job on these… Nice, clean circuit boards look so much better than ugly construction!

    1. Thanks Thomas, I do agree, while technically simple doing it yourself from the ground up is much harder than it looks. QRP labs has a push pull IRF510 amp they get 50W out of and its only 2 fets. I should be seeing more power than that, being parallel should give me another 20W or so easy. But im not getting that and I do not really understand why just yet.

      So what is happening with your 15m rig and what is the trouble you are having with the amp for it?

      1. Not sure yet what’s going on with the 15m. I can simulate ridiculous power output, but in real life I get a watt or two. Haven’t had a lot of time to work on it lately. I suppose could call it a 1 watt transmitter and be dome with it, but I’m stubborn!

        1. I can relate to being stubborn LOL 6 years I have been home brewing and I still have not built a transceiver worth using.

  2. Congrats! I’ve achieved far less at far lower power…

    Did you ever get an answer to your question about input/output capacitance when paralleling mosfets?

    1. Actually I did, my question was about impedance not capacitance, but the theory should hold for both I assume. I just have to do the experiments and make the measurements to confirm what I read. When you push pull the impedance should be in series, when you parallel the impedance should be in parallel. This would be more of an issue with BJT’s where the output impedance is orders of magnitude higher than with mosfets where both in and out impedance are rather low.

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