6 & 2 meter LDMOS 1KW Linear Amplifiers

 

6 meter amp62 mtr amp20 6 meter amp8

(click on any photo for a larger view)

 

My interest in 2 meter EME made it obvious to me that I needed a bit more power than the usual 100 watts available from most radios on the market. For years, I had used a Henry Radio Tempo 2002 tube amplifier that uses a pair of no-longer-available 8874 triodes. While this amplifier treated me well over the past 12 years, I could see the writing on the wall and started looking into alternatives. I received an offer I couldn't refuse so I sold it (for more than I paid for it years ago!)

I ran across an article in the October 2012 issue of QST describing a modern 2 meter linear amplifier that developed over 1000 watts with a single LDMOS transistor. Wow! And it required less than 2 watts drive in order to do it (that works out to around 27 dB of gain in a single device). The amplifier was designed by Jim W6PQL (his website may be found http://www.w6pql.com/)

I found myself reading and re-reading that article over and over again as it became obvious to me that I had to have one! After finding Jim's website and looking around a bit, I discovered he offered a turnkey version of his amplifier for a cool $2,700! Well, as I have decades of experience in construction of electronic projects and as it turns out Jim offers kits of his various boards that are reasonably priced (why reinvent the wheel right?), I bit the bullet and ordered several of the boards (the 2 meter amplifier board kit, control board kit , FET switch, LPF/Directional Coupler kit and Bar LED kits) from him. I also ordered the copper spreader and machined heatsink from him so I wouldn't need to have the required machining of them done locally in order to speed my project along. I ordered the needed MRFE6VP61K25H transistor from DigiKey (they actually had the best price for single unit pricing at $244).

While I am pretty good with metal fabrication, I still lack some of the tools needed to make a professional looking enclosure so I turned to some outside resources to do the front and rear panels of my enclosure (http://www.protopanel.com). They even have a free CAD program with which to design your panels which you simply email them and they manufacture them for you. The bottom plate and cover I had done by a local metal shop. I did my own drilling (and tapping as needed) and other fabrications needed.

I made several changes from Jim's original design so as to fit my personal tastes and needs including:

  • Digital metering instead of analog
  • A 15 dB input attenuator so as to cope with my TS2K which can supply up to 100 watts on 2 meters
  • Elimination of the Power On LED as the digital meters' backlights make it obvious that power is applied

6 meter amp1

After I completed the 2 meter version, it become apparent to me that I needed to retire my converted Heathkit SB-200 and build a matching LDMOS amplifier for 6 meters. I purchased a second set of boards from Jim for the new project but, this time, got the copper spreader from a local source, heatsink direct from the manufacturer and 6 meter amplifier pallet kit from RF Ham in Europe. As I learned quite a bit about the construction needed for the project from my 2 meter build, I did most for the 6 meter amplifier machining myself but did have a friend with a mill do the surfacing and milling needed for the copper spreader. With the exception of the amplifier pallet itself, the interiors of both amplifiers look very similar so I've only included pictures from the 2 meter one. However, here is the 6 meter amplifier pallet so you can see the differences:

 

You may click on any image for a larger view

2 mtr amp2
 Here are the main boards mounted on the heatsink. The top left is the 15 dB input attenuator, below it is the control board and to its left is the FET high current switch. Also the top edge of the heatsink are the four 40mm 12 v cooling fans (wired in series and powered directly from the 50 volt supply). The complete 1KW amplifier "pallet" is in the center.

 

 

  2 mtr amp3

 

 

A closeup of the amplifier pallet, control board and input attenuator

 

 

 2 mtr amp4

 

Here's a view of the Low Pass Filter/Directional Coupler board mounted on the side of the heatsink. You can also better see the cooling fans mounted along the side of the heatsink using a piece of aluminum angle stock

 

 

2 mtr amp6

 

 

Putting it all together with the wiring of the heatsink mounted boards started, the output coaxial relay mounted (on the right), the input coaxial relay's coil is visible on the left side. The little board at the bottom right is the thermistor for the high temperature cutout circuit.

 

 2 mtr amp9

 

 

Here's a better view of the input coaxial relay. It is manufactured by Magnecraft

 

 

2 mtr amp11

A front view of the heatsink. As you see, most of the wiring is now done. The device mounted to the front is the ammeter shunt. You can also see the output coaxial relay wiring has been finished. Note the ferrite beads on the large red wire on the amplifier pallet. These beads keep damaging RF out of the FET switch to which it is connected

 

 

 6 meter amp12

 The front panel wiring. A Molex connector is used to make for easy servicing. Towards the left edge you see the 2 LED bar meters. The 2 digital panel meters are seen along the top edge. The perf board below the panel meters are the isolated power supplies for the panel meters because they cannot share a common ground with the circuit they're measuring.

 

 

 

2 mtr amp18

Here's the back panel of the completed amplifier. You can see the four 40mm cooling fans. Notice I used SO239s for the RF connections. Contrary to popular mythology that is prevalent in some circles, they really are quite acceptable for VHF use. Do notice the output connector is teflon however.

 

 

 

 

The amplifer requires 50 VDC at a maximum of 30 amps. My supply consists of four 12 VDC @ 30 amp supplies I bought through Ebay for $25 each. I adjusted each one to 12.5 volts output and wired them in series and then mounted them in a different enclosure whose primary AC power is controlled by the front panel ON/OFF switchs on the amplifiers but not directly! Instead I used a 9 volt battery to control a solid state relay in the supply that in turn controls the AC to the four supplies. As the solid state relay only draws 4 ma, the battery should last for quite a long time and I have no voltages within the amplifier enclosure higher than 50 volts. As both amps will never transmit at the same time, I share the single power supply between them both. Powering one up powers them both but as I said, only 1 ever gets used at any one time.

I couldn't be happier with the results, both in appearance and performance. With my TS2K set to 50 watts out (1.5 watts supplied to the transistor through the attenuator), output is slightly over 1000 watts at 144.200. Performance on 6 meters is similar with 1.2 watts drive producing over 1000 watts out. On-the-air reports have been good as well with clean sounding audio reported. All this from packages that weigh in under 15 pounds and measure 12"w x 6" H x 12"D each.

The block diagram may be found here

The Front Panel schematic may be found here