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LDMOS SSPA

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MRFE6VP6300H 2m 500w Amp

MRFE6VP61K25H 2m 1Kw SSPA

MRFE6VP5600H 70cm 1Kw SSPA

XRF286 23cm 300w amp

 

MRFE6VP6300H 2m 500w Amp       

After becoming active again I started using a couple of old Tona solid state amps on 2m and 70cm both running about 100w.

I rapidly discovered that that wasn't going to be enough power so I started searching for something to take advantage of our new power limit of +30dbw. I discovered Ron VK4DD's web site where he had done a lot of experimenting with solid state amps on 2m. Well a couple of emails latter I had two of his experimental boards for the MRFE6VP6300H and all the information I needed to build them. I couldn't believe how easy it was to get these amps gong and in doing so discovered how robust these device are too. I inadvertently ran it with full drive into a short and one other time with an open circuit on one of the amps. On both accessions after a moment of, well that's done it, and correcting the problem I had full output again.

The amp was built into a DTL backup tape drive box measuring 235mm W X 130mm H X 300mm D which was surplus from my place of work. I have since build my 6, 2 and 70cm amps in to the same boxes and the 23cm amp was built into a slightly small version. These boxes have good builtin RF shielding and have a nice relatively blank front panel which lends its self to making a good looking finished amp.

Below are a few shots of the first amp which used two MRFE6VP6300H into a VK4DD combiner which produced about 500w of SSB. Because of the output matching circuit heating it was not possible to run much more than about 200-300w out in digital 100% duty cycle modes.

Here is the circuit of one of the amp boards as provided by Ron VK4DD.

This amp was very reliable and was a good testing ground for what was to come with the 1Kw amps I built.

Click on any image to see more detail.

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MRFE6VP61K25H 2m 1Kw SSPA

The first of the 1Kw SSPA's I built was for 2m from one of Jim's wonderful kits, you can visit his web site here. It took a while to get it really working well but now that I have it is a wonderful tool to have at my disposal on 2m.

This amp, like the one above, was built in a DLT tape drive box measuring 235mm W X 130mm H X 300mm D using the same basic layout. But only two sections of control and DC switching and RF deck and RF switching.

The front panel of this amp has some 5mm square LED bar graphs to show forward power and reflected power along with a multi function 3mm x 5mm display to show Voltage, Current and heat-sink temperature. At some stage Ill get a common CNC cut front panel made for all the SSPA's in the meantime Ill make do with the rough ones currently in use.

To measure current I'm using a ACS758LCB-050U-PFF-T Hall effect device by Allegro (date sheet here). This is Unidirectional devices and has a low output offset voltage of .6v which makes interfacing to the LED display very easy. I derive the temperature voltage off of the W6PQL sequencer board where it is buffered and then sent to the selection switch.

The amp also uses one of W6PQL's filter coupler boards which I mounter in its own enclosure with a small fan mounted on top. The filter can get warm when running full output with JT65 and the fan soon gets that under control. I replacing the mica caps used in the filter with small Russian door knob ones and also added intersection shielding which increased the stop band attenuation considerably.

I was not sure if the Heat Sink and fans would be enough to provide the cooling required but after long periods of JT65 at full output Iv had no problems at all. Getting the efficiency up to the 70% mark helps a lot too. The Heat Sink some of you may recognize as its part of a heat-sink recovered from an old NEC paging TX.

On the control side view of the amp you can see the Automotive relay to switch the 50v during TX/RX. You can also see one of W6PQL's FET DC switching boards which is used to switch pre amp power.

The amp is controlled with a amplifier control board from W6PQL with a couple of mods. Normally the fans come on when in TX regardless of the heat sink temperature but I have changed this by adding a series resistor in the fan supply so that the fans run at a low speed on the start of TX and then switch to full speed once the heat-sink reaches about 40C. The fan you can see in the pictures below mounter on the side of the heat sink is a hi speed one with air flowing along the heat-sink fins and then exits out through the back and front of the amp.

As these amps only require about 3-4 watts of drive for full output so some sort of attenuation is required when driving it from one of the 50w out radios around today. I'm using a 10db 50w attenuator in the input from Henry Radio which has the added function of providing a good match to the driver.

Click on any image to see more detail.

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MRFE6VP5600H 70cm 1Kw SSPA

Built in the same style case as the 2m amp the 70cm amp was arranged differently to accommodate the wider amp PCB as it is using two 500w amps side by side with output combined all on the one board.

The cooling fan is mounted on the bottom of the heat-sink and draws air form under the amp which then exits front and back of the amp after traveling along the heat-sink fins.

The front panel has the same layout as the 2m amp and the back panel is also setup with the same connectors.

The amp is working well and will produce 1Kw without much stress. I'm not able to run it at that level as long as I would have liked as the heat-sink is just that wee bit too small or maybe a bit more air across it would help.

This amp now compliments the others in the station on 6m and 2m and represents a very compact and stable way to generate good power on 70cm.

Below are the images of the 70cm amp and as you can see it has the same basic layout of three compartments one for RF, control and the amp pellet. This arrangement works well and its not to hard to work on any components of the amp as all interconnecting wiring is on connectors so a complete section can quite easily be removed for service.

Click on any image to see more detail.

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XRF286 23cm 300w SSPA

This amp followed on from my success with the LDMOS amps I built for 6, 2 and 70cm although its using much older devices which are now getting harder to find.

After finding some surplus Spectrian amplifier output boards on eBay and harvesting the XRF286's along with some nice 50ohm chip resistors I was ready to start the amp construction.

First the hardest part, mounting the recovered XRF286's onto there new copper heat spreaders. After a few nervous moments on the gas cooker and a big thick Aluminum plate it turned out not that hard at all.

You do need to have a good even heat and some nice .5mm solder as well as some liquid flux to make this all work.

Click on the images to see more detail.

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These amps are built on some of Jim W6PQL's boards and they just work with each pellet will produce about 180W+ and combined I'm seeing just under 380w at saturation so about 350w of usable power.

The most critical part of the amp construction is to make sure the input and output boards are nice and snug up against the XRF386 as even a small 1mm gap will make a big difference. Also don't be lured into buying those cheap XRF286 on the WEB as they don't work.

The black square coax relay I found on eBay and isn't the usual configuration for a transfer relay as it has two pair of ports that connect as below.

De-energized   A------B                Energized    A          B
                                                                  |           |

                    C        D                                  C          D
These coax switches are rated at 300w at 2Ghz and so far have operated faultless. The amp is controlled by a sequencer so they are switched RF cold and that is essential for long relay life.

In the RF section under the relay you can just see a coupler salvaged off an old VE1ALQ combiner board. This is used to monitor SWR and relative output power and also feeds the controller to enable hi SWR shut down.

Just like the 70cm amp the main cooling fan is mounter under the heat-sink but because of the lower power levels the cooling is sufficient to run full output in digital modes on this amp.

I made a number of mistakes in the construction of this amp but the most annoying one was mounting the output combiner around the wrong way. I had the to amps presented across the wrong ports of the combiner. So I had to rotate the combiner board 180 Deg. That meant the length of the .141 output coax cables had to be lengthened. It looked way much nicer the way I originally had it but that only produced about 120w!

Click on any image to see more detail.

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