Some weeks ago I bought a chinese amplifier kit on Ebay which came for 16 Euro / Dollar shipped. The kit took only 2,5 weeks to arrive and came in a simple plastic bag without any protection and it also came without a manual or instructions. But even if it does not work, the parts alone are worth the money. While waiting for the kit I found an interesing article from John, PD7MAA on his website. He does not only have the schematic available, he also shows how nice such DIY-projects can look like.

Especially the filter is worth a look. Building the kit was not too hard, but the surface-mount (SMD) parts are very tiny and you need a calm hand and good eyes. When I had everything assembled I tested it without a heatsink on the two IRF530N mosfets from IOR. It took not more than five seconds to kill both FETs while sending an AM carrier (1,5W drive). I ordered a pack of five new IOR IRF530N mosfets on Ebay which arrived today. After the QRL I soldered them in, and I drilled the holes into two pieces of an old heatsink harvested from an old PC power supply.

I did some power output and SWR measurements then. Numbers indicates the output and the numbers in brackets are "SWR-bars" on the FT-817 display:

 Input 80m 60m 40m 30m 20m 17m 15m 12m 10m
0,5W 60 (0) 55 (0) 40 (0) 28 (1) 20 (0) 20 (1) 13 (0) 10 (0)  8 (0)
1,0W 71 (0) 70 (0) 58 (1) 48 (2) 30 (0) 25 (3) 20 (0) 18 (0) 14 (0)
2,5W 89 (0) 79 (2) 72 (4) 58 (3) 60 (2) 52 (3) 58 (1) 60 (3) 40 (0)
5,0W 95 (1) 81 (3) 75 (4) 60 (4) 74 (3) 60 (4) 72 (0) 62 (3) 60 (0)

 

After the "measurements" I was able to work S57DX on 20m with 60W output (2,5W drive from the FT-817) and I also worked my friend Peter, DC2ZO. While in the QSO I pushed 5W from the 817 into the amp and with 3 SWR-bars on the dispay and 74W output I killed another pair of IRF530N mosfets. I will now mount a bigger heatsink, 2 fans and a case and then I will do some more measurements. As recommended by John, PD7MAA a drive of 2,5W seems to be ideal for me too.

Anton, HB9ASB already wrote an article about the 70W kit: FT-817 PA - Bausatz ohne Schema

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Youtube-video from KC2IRV:


Kommentare   
0 # A 2019-11-27 22:25
Generally the bias is not set as a distinct votlage on the gates. You set it by measuring the drain current
for the two mosfets. The correct current is in the range of 100-300ma per MOSFET for best linearity.
I'd think more than 2W is over drive.

The gate votlage isapproach is very unpredictable as it varies widely from one lot of devices to another.
Yes, The power tab (to220) devices do not cool well and will run hot anything under 80c at the top of the case is likely to survive.

A low pass filter is manitory as at best the 2nd harmonic is maybe -30dbc, and the third will be higher.
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0 # Hamad almezyan 2018-05-30 05:45
Hello sir thank you very much for explaining on the video but i want to ask you two questions it's change the frequency ( band ) automatically without switch? And the heat sink it's included with the kit ?
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0 # Henning Christof Wed 2017-11-26 21:05
mir scheint, dass der Verstärker bereits bei 0,5 W sine maximale Ausgangsleistung erreicht, bei der zehnfachen Eingangsleistung (5W) müsste auch die zehnfache Ausgangsleistung herauskommen--- sie tut es nicht! Unter diesn Bedingungen muss das Signal "fürchterlich" sein! IMD ???
Wo liegt der 1 dB Kompressionspunkt?
Freqeunzgang: immerhin 8,47 dB Unterschied = Faktor von 7,5.(80m - 10m)
73 von DK5LV
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