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Written by Rasmus SA5RJS on 2024-12-29 at 15:24

⚒️ Build thread! 🧵

Follow along when I build a MINIPA70 amp, a $20 linear "70 watts" amateur radio transmitter amplifier kit for HF.

[#]AmateurRadio #HamRadio #electronics

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Written by Rasmus SA5RJS on 2024-12-29 at 15:36

2. Why am I building this? My BITX40 transceiver puts out about 7 watts of power, and I have not been very successful getting contacts with it. Other amateur radio operators have said that QRP SSB is hard, but that 20 watts (like that from a Xiegu G90) is plenty enough.

So that's a perfect excuse for me to buy and build yet another kit! I hope this kit will provide linear amplification (needed for SSB) and give me 20-50 watts of transmit power.

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Written by Rasmus SA5RJS on 2024-12-29 at 15:48

3. The kit is not complete. For instance, it does not come with a heatsink or an enclosure. Instructions and a schematic were not included either. And I know the "70 watts" in its name is not realistic.

But I think the kit can be a useful first PCB for me to experiment with HF power amplifiers. There are a lot of tinkerers who have built this before and it's easy to find blog posts and videos showing how to build and test it. So this is a well beaten path and I'm in good company!

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Written by Rasmus SA5RJS on 2024-12-29 at 15:54

4. I would like to thank @quantensalat and @SA6JKK here on Mastodon for sharing their builds of a scratch built and a MINIPA70 amp. You inspired me to try this too!

https://astrodon.social/@quantensalat/111039741265686012/

https://mastodon.radio/@SA6JKK/111344946727316480

NA5Y on YouTube also has a nice build video: https://youtu.be/JgiakIxMjfI?si=eJ8XpO3NIKfLXAIN

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Written by Rasmus SA5RJS on 2024-12-29 at 16:15

5. The first step was to break off the PCB parts used to form the input and output transformers and file the edges. It only took a minute to make all the edges super smooth and nice to work with.

Next I chose a heatsink: an old CPU heatsink salvaged from a trashed computer. Then I drilled and tapped M3 holes. First time for me tapping holes in metal, and drilling in copper. The drilling was harder than I expected. The holes ended up a millimeter or so from where I planned, but it doesn't matter.

[#]AmateurRadio #HamRadio #electronics

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Written by Rasmus SA5RJS on 2024-12-29 at 21:35

6. RF choke.

This is made with two turns of 1mm enameled copper wire around a binocular core. It was difficult to bend and thread the thick wire through the core and I accidentally scraped away some enamel in the process. The turns were stiff and didn't touch, so I didn't worry too much about it. I fixed everything in place with hot glue later.

The inductance of the choke turned out to be 4 μH. The reactance becomes roughly 180 Ω for 7 MHz and 350 Ω for 14 MHz. The 50 Ω output impedance of the output transformer secondary looks like 0.6 Ω from the primary (assuming a 1:3 turns ratio, and thus a 1:9 impedance ratio). So in the current divider formed by the choke and the output transformer, most of the current goes to the output transformer (more than 99% I think?) because 0.6 is much smaller than 180.

Actually, the amplifier is a push-pull design with the output transformer doubling as a combiner. So my back of the envelope calculation is probably off by a factor of two in some direction. If you know how to reason about the output impedance of a push-pull amp like this, please feel free to explain it to me!

[#]AmateurRadio #HamRadio #electronics

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Written by Rasmus SA5RJS on 2025-01-01 at 16:48

7. Electrolytics, relay, and DC power cord.

I soldered the remaining components that would be obstructed by the transformers if not soldered before. I also soldered a 5.5/2.1 mm DC power cord (another dumpster find) so that could test the bypass relay by shorting the PTT pins. It went "click" as expected!

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Written by Rasmus SA5RJS on 2025-01-01 at 17:20

8. Input and output transformers: assembly and mount.

The transformers were fun to assemble. It tries to solder the PCB end caps with the bottom edges as flat and aligned as I could. I put the standing on one end and added a little solder to the other end. Then flipped them over and soldered the other end. Then flipped them back again and soldered properly.

It took a lot of heat to melt the solder and I found it useful to move the soldering iron in circles around a tube's edge to melt all solder consistently.

Next I soldered the transformers to the PCB. This also took a lot of heat and some time. Now the project is much more three-dimensional and has a lot more heft. I look forward to firing up the amp (not literally, I hope, although that will surely happen anyway)!

[#]AmateurRadio #HamRadio #electronics

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Written by Rasmus SA5RJS on 2025-01-05 at 08:57

9. Inductance check of tube windings (input secondary and output primary)

I measured both half-turns together in series:

Output transformer: 5.57 uH

Input transformer: 0.97 uH

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Written by Rasmus SA5RJS on 2025-01-06 at 21:20

10. Added transformer wires and performed lots of measurements

The measurements can be seen in this spreadsheet (screenshots added to this post for convenience): https://docs.google.com/spreadsheets/d/1481s1TLiWwiSgU1UrxgSDXqTYwXUrslObALEj7EQkOw/edit?usp=sharing

Most of these measurements were performed on yesterday's live stream on YouTube: https://www.youtube.com/live/wQLbCVtNCCQ

I measured both transformers at all thee windings with all the other windings open and shorted. I also measured the two "tube" windings in series as separate entries.

Any interesting patterns? Tell me if you find any!

So far I've seen that the one tube doesn't "see" whether the other tube is shorted or not, except when the wire joining the tubes is shorted. Others in this thread has argued that the coupling between the two tubes are so low that the halves of each transformer could be seen as its own transformer, only connected in series by the wire winding.

[#]AmateurRadio #HamRadio #electronics

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Toot

Written by Rasmus SA5RJS on 2025-01-06 at 21:46

11. Calculated Coupling Coefficients

I added calculated coupling coefficients to the spreadsheet in the previous post. "Coupling 1→2" means "Measuring at winding one with winding two opened and then closed an applying k=sqrt(1 - (Lshort/Lopen))".

EDIT: I'm not sure how much this calculation makes sense with open and short circuits. In the real circuit, the output transformer will be terminated with 50 ohms on the wire side and with something like 1-5 ohms (my impression of what the FET output impedance is for the IRF530) on each tube. Maybe scattering parameters with properly terminated windings would make more sense.

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