Home made BLF188 QRO

During big cw contests a good QRO will improve overall score.

Many hams use tubes, others use transistors, home made or factory made. The bad, the good and the ugly. One should balance the price, resilience, size, noise and the heat that QRO produces.I have elected to build a home made  QRO using BLF188 LDMOS transistor about 200 Euros give or take for 1 piece.Oh yes, I’ve burned some.My wallet went thin after 3 pieces and I wanted to drop the project.

I burned last LDMOS when antenna  has fallen down to the ground while transmitting and no protection  implemented at that time.

But a good friend send me another BLF188 and since then, I decided to rebuild it from the ground zero, protection first then testing LDMOS afterwards.


9 November 2020 Status Update :

Got my own design new pcb’s delivered from JLCPCB , all in one like amplifier board and bandpass filters.

Got a huge copper plate like 300x200x10 mm size from ebay..

YO8ROO helped me big time drilling and CNC (sort of) on his garage tools.

Lots of cheap thermal paste used between aluminium  heatsink and copper plate.

Expensive thermal paste Arctic MX4 used between BLF189XRB ( instead of BLF188)  and copper plate with no soldering, also on dallas 1 wire temperature probe.

Initial heat test was made using laundry ironing station over the copper plate, of course sending my wife to mall shopping before that not willing to see me doing that (sneaky trick aint it ?).

Tefal ironing got heater error after 30 minutes, copper plate was literally sucking the heat off, so no more than 60 celsius measured over Dallas 1 wire probe.

Soldered all components onto new PCB, new coax cables rewiring and at last, using old trusted Solomon soldering station, put the mighty BLF189 on it.Measured input impedance thru all bands and triple checked all before touching OPERATE button.

Emotions,a bit stressed out but started my PA using 1.5 ohms power resistor series with 50 V rail.

Set bias current, and pushed Icom 7300 into it set at 0 % which means some milliwats, sort of.

Wattmeter shown output, then bit by bit increased input power watching all parameters on my large 7 inches touch screen. All good and at just 10 W input via 10 dB ATT I have seen just about 1400 W out on dummy load.

Stopped there as I do not want to push limits, set tcvr to 7 W and minus 10 dB, Voila ! 1Kw output at just 0.7 input at the gates.Copper plate is fast spreading the heat so I am happy. Sat down and waiting next weekend for some extra heat stress tests.

73 ‘s ,Gabriel yo8rxp


  • Aluminium 1.5 mm was used. From mouser dot com I got SMD components ,  Laird toroids and 1Kv mica silver capacitors
  • An old Eltek Flat Pack 1800 was used as power source and another cheap 12V SMPS  + lm7805 for relays and controller.
  • Aluminium heatsink 300x190x50 mm  without copper heat spreader. 2 PWM high speed fans also from junk box.
  • EasyEDA online software to design PCB’s and JLCPCB manufactured it.Also for SWR bridges I used DJ0ABR design and approach with AD8307 fast log detector
  • W6PQL was the source for tech info but I changed a lot in his design, but still having problems in 21 and 28 Mhz, so for the moment those bands are disabled temporary.

I built the case using plain aluminium 8×8 mm rectangular bars and 3mm screws + nuts .Front panel has a large official 7 inches touch screen for Raspberry Pi and the back plate houses the UHF connectors, Ptt jack and power jack + fuses. Aluminium shield plates were used between power amp and swr bridges, also Raspberry Pi has separate shielding.

Installed  linux Kivy OS on Rpi and start writing pure python ontop of Kivy framework which is a real piece of art, congratulations to Kivy team !

Built a separate protection board using LM339 comparators and CD4013 flipflops based on YO4HFU design but also altered for my needs.

This protection once properly calibrated, it keeps the bad out of BLF188 which is not so resilient as producer advertise it, simply because on CW (not pulsed) if transmitting on 28 mhz with 3.5 mhz filter set, then bye bye , another 200 Euros burned away.

This protection board measures 4 protection stages : wrong filter selected , more than 2:1 SWR , drain over current and input overdrive power.

Why not using Raspberry Pi for protection ? Coz it aint realtime, no matter what bad or good code I write. Raspberry pi in this case is just for design, filters selection, and visual alerts / indicators.

Protection board in realtime will cut off 50 V rail by flipping BTS50085 enable pin and disabling  PTT  T/R relays, then sending to RPI informations about fault occured. I do like building things, but if one wanna replicate it without good knowledges, antenna analizer, good oscilloscope and other lab tools then I advice to stop here and purchase a factory one.Why ? The quantity / price of  LDMOS burned will overcome the price for a factory made one.You have been warned !!!!!

Total cost for this project about 500 – 600 Euro give or take, without burned LDMOS’es, and not counting emotional drama seeing a very nice flame / bang coming from precious BLF188 burning away into oblivion when mistakes are made.

Next step is to matte black paint it and some nice text on the front panel;

Here is a photo gallery, in the future will post schematics and  the python code is here to download. 73’s Gabriel

My gratitude goes to W6PQL and PA0FRI for their impressive work on LDMOS’es amps. Guys, you rock !!

2 thoughts on “Home made BLF188 QRO

    1. Hi
      Well, about RF module is exactly as https://www.w6pql.com/1_kw_sspa_for_1_8-54_mhz.htm website.
      SWR bridges are same as https://www.dj0abr.de/index_english.html website.
      Protection is made using clasic comparators as https://www.qsl.net/yo4hfu/Files/LDMOS_2m/LDMOS_protection.pdf website but modified for my needs.
      The only part I developed is Raspberry pi Touch screen software and relays sequence but unfortunatley I do not have the drawn schematics as it is in my head.
      I do not use it anymore as I have found the tube amp is less expensive (I have burned quite a few LDMOS’es) and more resilient during CW contests. 73

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