MOSFET linear amp – A New Home

Hello all!

Here’s the new home for the RF MOSFET linear amplifier:


It’s been a while since I made any posts about my MOSFET amplifier for the low bands. I decided to re-house the project in a much bigger cabinet because things are just too confined, and getting rid of the heat is just too much of a problem within the confines of the original Pentium 2 computer case that I used. Bear with me, it takes me hours and hours these days to perform a simple task due to severe health limitations. So what should take me an hour takes a whole evening. If I exhaust myself, I will break off and take a few days off and come back to it later. So who knows when the amp will be completed!

I must say I am absolutely ASTOUNDED by the cost of a decent aluminium (or even steel) enclosure these days! So, ideally, though I would like to put the linear in a nice 4U rack enclosure, I’ve had to settle for simply a much bigger computer tower case instead. I toyed with the idea of buying the parts needed to make a nice 4U case for this project, but even buying aluminium angle, sheet and so on from various eBay places soon added up to a total of over 80 quid, and I would still have to make the box! Eighty fuckin’ quid! Think I’ll use the ol’ PC case, then!

I decided to blog it as I go along in case you are interested in building one yourself.

First job: clear out the IT paraphernalia from the computer tower, and install plenty of fans. Check the current consumption of all the fans running to get an idea of how robust to make the fan supply:


From past experience, I know that good airflow is a must for high-power, inefficient linear amplifiers such as this one. So I installed a total of eight case fans (common or garden pc fans) that draw in air at the bottom of the enclosure, blow it up and onto the huge heatsink for the FETS, and drag it out of the upper rf compartment.

As you can see, the fans will take 1.25 ish Amps, so I will need to find a robust way of easily delivering that current at this voltage (12v). I will likely keep it separate from the main amplifier supply (for reasons of rx noise) and I will only power the fans when the heatsink reaches a certain temperature. (More on this later when I explore the biasing of the MOSFETs — I will use heat-sensing feedback voltage that stabilizes the current to feed to a comparator that only supplies voltage to the fans as required.

Here’s a picture of the fans running and gives a better idea of the plan I have for this old tower case: it seems to be naturally divided into two halves: I am going to use he bottom section for the power supply, and the top half for the RF stuff.

Here you can see the fans “glowing” at the top of the computer case (not my choice, just what happened to be in stock at the time)!


Next job: get the guts of the amplifier installed in the top RF compartment. What a job! The heat-sink has been liberated from a very high power, but scrap audio amplifier. The heat-sink fins are super sharp and cut my hands to pieces!


Fag papers box added to give some idea of scale. If you are not a smoker, and do not visit Family Bargains (more or less our local pound shop), then let me tell you that the fag papers packet is about the same size as a packet of 20 cigarettes. The actual amplifier ‘brick’ is quite literally ‘one I made earlier’, before I was ill, so I’m hoping it doesn’t need too much more work. You can see the rough schematic here:

cheap mosfet linear amplifier circuit
cheap mosfet linear amplifier circuit


Did I tell you? My metalwork skills are shite! Really should put that grille on the inside of the case, but I got tired. At least the airflow is improved this way!


OK, had enough for now. Will continue this later…

You can read more about the ham radio MOSFET linear amplifier here.

Related articles on the same subject: MOSFET linear amplifier Power Supply

Cheap MOSFETS in ham radio linear amplifiers

My amp works!

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