One of the problems that we QRP operators encounter is that we hear many stations but that frequently they do not hear us. We are just a tiny voice, barely audible in the QRM and amongst lots of QRO-hams. Although I am somewhat proud that I worked OX3KQ, OM Jesper, on or near the west coast of Greenland with my MKARS80 with 10 W PEP, a distance of 3500 km, I got a little frustrated from being a voice in the wilderness most of the time, so I decided to do something about it and to build a linear amplifier for my MKARS80.

It should be very simple, yield some 40 Watts at 13,8 Volts DC, use readily available components and cost about 30 Euros. In addition, it should not be necessary to modify the MKARS80 for it, so it should include a simple VOX-circuit to switch to transmit once RF power is applied to the amplifier.

After the first prototype was working successfully, I built a Mark 1 model of my amplifier and after having tested it for  a year or so I can say I am quite pleased with it. I now get heard amongst the big guys and my counterstations are often astonished that I am using home brew gear worth less than 100 Euros altogether.

The schematic/parts list with brief building instructions as well as the PCB lay-out (copper side and component side) can be found below. I trust the schematic is pretty much self-explanatory. The output MOSFETS are the same as the one used in the MKARS80 and they cost virtually nothing. The only part that needs comment is the BC547C used in the VOX-circuit. This should really be a C-type and not B or A, because of its amplification factor. With an A or B-type the VOX-circuit and thus the entire amp will not work! If you plan to take part in contests, if you hold lengthy QSO’s or if you live in a hot climate and certainly if you do PSK31 I would recommend or even urge you to use a large heat sink on the final MOSFETS. I used an aluminium TEKO box I had handy at the time. It does double duty as a heat sink and it does not get very hot during voice QSO’s. Anyway a solid Hammond 1590T diecast aluminium case, for instance, would probably be a better choice than the somewhat flimsy TEKO-case.

At 13,8 Volts DC, the amp’s power output will be over 40 Watts, at 20 Volts DC I obtained 70 Watts.  OK, it’s still not the usual 100 Watts, but on the other ham’s end the difference is barely noticeable.

Good luck building!

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Please note that the schematic has been  updated regularly; current version is 1.17. This version supersedes all previous ones.

This is Mark 1, click the picture to enlarge in a new window

Copper side view!

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The ready PCB, before being built into the enclosure. The two large holes on both sides of T2 are there for making it easier to bolt the IRF510’s to the bottom (or top) plate of the enclosure

The two IRF510’s are mounted below the PCB, wires bent upwards and soldered onto the copper side of the PCB. The IRF510‘s metal mounting parts are flush with the bottom of the metal stand-offs that support the PCB

90 mm

One of the things I wanted to do in 2010 was to build the linear amp in a better box, more attractive to the eye and with a better thermal capacity.

I was able to finish it early December 2010. Building it in the Hammond box # 1455J1201BK I bought for the occasion made it necessary to slightly modify the PCB. I made it a little larger, 75mm instead of 70, so that it would slide in the rails provided in the box. Since the box is low profile, I had to put all standing components, like the toroid coils and an electrolytic capacitor, flat on the PCB. Since I already had to modify the PCB anyway, I made an improved version of it.  The lay out file can be downloaded by right-clicking on the following link: MKARS_Lin_Amp_40W_2.1.2.LAY.

You need the sPrint 5.0  lay out viewer downloadable with the following link from ABACOM to view and print it.

The box has been provided with an extra radiator to make the amplifier more contest proof. Not that I do any contests with an MKARS80, but more cooling capacity never harms.

Power supply comes from a Toshiba Satellite Pro A100 P100 mains adapter rated at 15V 6A, more than adequate for this purpose and available for instance in the UK on eBay at less than 10 GBP.  For other countries: google “15V 6A” and see what comes up. At 15V my amp produces over 50W and draws about 5 Amps from the PSU.

Overall view of the amp. The case comes from Hammond. I took the black version. The black finish is not conductive, I found out, so I had to scratch it from the aluminium at spots where electrical contact is required. Next time I will use an unfinished box!

The front of the amp. I like to make neat front panels with clear and simple inscriptions. I use semi-gloss photo paper to print the decal for the front panel, the design was made with Corel Draw on the PC. The paper panel is finished with a clear PU varnish from a spray can to make it more scratch proof.

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The innards of the amp. I would make the PCB different and better the next time, but that is one of the problems of radio amateurism: we know exactly how our project should have looked like after we have finished it .....

The  rear panel; the bezels on the front and rear panels should be mounted behind the metal end covers according to the Hammond instructions, but I found it better to mount them in front so that they keep the printed front panels in place en protect them. It does not look all that bad this way. Also, the metal end panels now make contact with the body of the housing, which provides better shielding.