1200W 6m – 2m power amplifier based on the motorola AR313

I needed a power amplifier for 6m EmE, so after looking around I found that my father TF3T had 4 unassembled AR313 kits from Communication concepts lying around, he also had a PCS4 splitter/combiner. This he bought some 15 years ago. They are based on the MRF141G “gemini” tranisistor. http://www.communication-concepts.com/ar313.htm

As I needed QRO asap this summer, I decided to build two modules and combine them with a combiner I already had. Also I did not have a powersupply to drive 4 modules at the time. The amp was built and worked well on 6m. It was not until this winter I got interrested in 2m EmE, the plan was changed and now I would get the amp working on 2m as well. This really just meant building a small relay board to switch between two low pass filters.

Power supply:

basic specifications:

  • Input voltage: 220-240V (jumpable)
  • Output voltage: 4x 28v
  • Output current: 4x 25A max (limited at 23A)
  • Maximium power rating: 3KW
  • based on 723 voltage regulator + 20x 2n3055 output transistors.
  • 150ms slow start is accomplished using control from a PIC 16F877A microcontroller. (using 10Ohm resistor)
  • LCD display that shows current and voltage per channel and cuts out the PA and Power supply if over voltage or over current status is detected. It’s bit slow as the processor is running at 5Mhz so the reaction time is around 10ms.

So basically this is a 3KW linear power supply built around a 3KVA toroidal transformer woven by Airlink transformers (UK based). It’s a heavy transformer, 17Kg and I got it delivered to Newcastle where I was on a business trip, bought a small cabin-case for it and took it through BA flight to London and then to Iceland. There is a loophole in the on-board luggage rules at BA, only dimensions, no weight restrictions.

As the power amplifier has 4 PA modules, each operating at 28v/22A I decided it would be easier to build the power supply with 4x 20A outputs, rather than 1x 80A output.

I’m using the standard 723 regulator with a 2n3055 in a Darlington pair configuration, 1 driver and 5 bypassing.

The PSU has a slow start using a relay that shorts a 10 Ohm after around 200ms

The PSU is controlled by a PIC control board that uses it’s 8 AD inputs to monitor voltage and current. I use a PIC 16F877A as I just happened to have few lying around. Current is measured using a standard op amp in a differential configuration.

I decided to use a inductor on the output and use the resistance in the 16AWG wire as the shunt, both for current sense and current limitation.

for now, here are some pictures, I will update this later perhaps with schematics if someone is interested :

The amp is based on the AR313 . I have already the amp working at 600W, but I’m adding two more modules to give a combined power of 1200W.

here are pictures of the current work:

It’s not pretty, but in tests it puts out 1200W on 6m. I don’t have the ability to fully drive the amp on 2m , but it’s going up to 600W, but I see it’s not very efficient on 2m – that worries me.

The 2m low-pass filter can been seen there in the back, it uses RG142 coax cable as capacitors, close to 1pF per 1cm, so each of the capacitors is 18cm of RG142. It was modelled in Elsie, and used a miniVNA analizer to test it.

The amp is being built from the ugly design of my first 600w 6m amp,  so I look at this as a test bed for the moment. I have a nice chassis that I want to mount the amp in later. I’m also working on a nice sequencer to control relays and the pre-amp. That is coming – but I think I will test the amp on some nice EME over xmas. 😉

update: 22/12/2010:

Tested the amplifier on 6m – 1200W without saturation, can overdrive to 1500W. Power supply heats up well and after a 50 second xmit the transistors are cool, only thing worrying me is the 30 ohm balancing resistors are heating up -(50-60°C). Luckily I have 10x 25ohm 50W on it’s way from ebay (russian made). I could only drive the amplifier up to 200W on 2m but the Yaesu 723R is just to weak to drive the amp, I need around 40W on 6m to fully drive the amp, but I only get about 6w to the amp due to some mismatch in the splitter on 2m.