I built a class ab amplifier kit to use with my qrp radio. 2 watts in creates 50+ out.
This was the pa 70 amplifier kit.

It is a kit with no instructions, but a whole suite of internet videos and articles.

I built mine using an MRF 9120 transistor. There was a comment on a thread somewhere that said use 4 turns on the output transformer instead of three when using the 9120 chip. That will help yield full power.

How can I test which is indeed best? I would think more turns changes the output impedance, but by how much. I am wondering how to best analyze the setup I have.

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  • $\begingroup$ I'm confused. To me as a wireless comms engineer by trade, 50 W is incredibly much; is that really "QRP"? $\endgroup$ Sep 6, 2022 at 14:36
  • 1
    $\begingroup$ @MarcusMüller I wish to take my QRP IC705 and temporarily use this amp to make it Non QRP for when conditions are poor. The MRF 9120 chip might be able to make 100 Watts, but I see about 50Watts on a SWR Meter/Dummy load when providing a 2 Watt RTTY signal. $\endgroup$ Sep 6, 2022 at 19:00
  • $\begingroup$ ah that makes a lot of sense! $\endgroup$ Sep 6, 2022 at 19:56
  • $\begingroup$ Do you have a dummy load and a power meter that you could just test both and see which gives you more power? I guess you run the risk of blowing your finals then if the load becomes badly matched. You could add current limiting to the finals to preserve them in that case. $\endgroup$
    – byl
    Sep 7, 2022 at 17:40
  • $\begingroup$ @byl I have done that, it seems that there is maybe more power with 4 turns. It definitely did not jump from 50 watts to 100 watts. The datasheet for the mrf 9120 says it normally operates at 890 mhz or so and 24 volts (120 watts rf), i am running it at 7 mhz and 12 voltes. $\endgroup$ Sep 8, 2022 at 13:41


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