https://www.ebay.co.uk/itm//223271118164

I noticed these amps on eBay, the design looks very similar to the MRF186 and MRF9180 kits with just a slightly different layout and more components added, with a good clean drive there is no reason why this amp wouldn't perform well.

....And it says 360w on their Bird power meter which seems pretty good, so continuous 300w operation shouldn't be a problem with good cooling and a decently matched aerial.

I have been looking at this one which is is very similar but with an on board LPF and only £40 more
From what I can make out they use > SD2931 Mosfet

https://www.ebay.co.uk/itm/75-110Mhz-30 ... SwH8ta3vox

BriansBrain wrote: ↑Tue Mar 03, 2020 11:35 am

I have been looking at this one which is is very similar but with an on board LPF and only £40 more
From what I can make out they use > SD2931 Mosfet

https://www.ebay.co.uk/itm/75-110Mhz-30 ... SwH8ta3vox

300W RF Pallet - SD2931 - inc LPF.jpg

I was actually supposed to be posting up the version you posted above ^ with the LPF included and was looking at 3 different types, but I posted that one by mistake.
It would be worth the small extra cost for the ready built Broadband LPF to be included, it also includes adjustable SWR protection rail.

Yes I expect you are probably right, the transistors look to be either SD2931, MRF151 or BLF177 devices.

...A few years ago I got 220w out of a single BLF177 without even trying!

The problem I always find with these units is finding a suitable heatsink

For the 150W versions I use a 200 x 69 x 37mm @ +/- £11 on ebay with free post
With two 12V 2.2W fans mounted on top.
Hint >> Just search for 200x69x37mm

However...
For the 300W I think something a little bigger would be needed

A friend of ours bought one of these a couple of weeks ago. He brought it over so that we could test it.

It needed nearly 10 Watts of drive on 98 MHz to get 300 W out. The second was only -45dBc, so their lowpass filter isn't great. We bolted it to a very big "Redpoint" heatsink (0.07°C/W) and added a couple of fans for good measure. After half an hour at 300 W into a 50Ω dummy load. two of the output filter caps exploded, suggesting that they really weren't up to the job. The PA continued to work, and there wasn't much change to the harmonic content of the output!

We rebuilt the output filter, using Arco and Semco caps, and adding screens between the coils.

The PA was now giving about 325 W out for 10 Watts drive. Backing the drive off to about 8.5 Watts gave us exactly 300 W. We left it overnight into the dummy load and examined it again in the morning. The bias had altered slightly during the night, and the output power had dropped slightly (285 Watts). Tweaking the bias got us back to 300 Watts.

Measuring the supply current and output power suggested an efficiency of about 71%, which is pretty good for a broadband circuit. We tested just how broadband it was by switching the drive to 90 MHz (310 Watts) and 105 MHz (297 Watts).

After 24 hours, we switched off and checked that everything was still secure on the board (I've had cheap PAs that unsolder themselves over hours of use!). The coating on the transformer coax was soft - it had obviously got pretty hot in use - and another of the original output capacitors was discoloured, prompting me to replace all the Chinese output capacitors with good quality ones.

The whole mess was mounted into a 3U 19" box, and assembled into a rack with a 10 Watt driver, stereo coder, composite modulation limiter, audio limiter and RDS panel, for a pretty good, cheap 300 Watt stereo rig.

Albert H wrote: ↑Fri Mar 06, 2020 11:07 pm The whole mess was mounted into a 3U 19" box, and assembled into a rack with a 10 Watt driver, stereo coder, composite modulation limiter, audio limiter and RDS panel, for a pretty good, cheap 300 Watt stereo rig.

As I am thinking of getting one of these boards myself...

That was very comprehensive information Mr. H ... thanks

BB
One other thing - we found that we had an RF ground loop through the screws screwing the board to the heatsink. Cutting away the copper around most of the screw holes solved this one. The heatsink was connected to the PCB at just one point.

Hey Brian!
I have just got back from Lanzarote, I tried my best to listen to your station but I just could not get you? was you on air?

sinus trouble wrote: ↑Sun Mar 08, 2020 12:55 am Hey Brian!
I have just got back from Lanzarote, I tried my best to listen to your station but I just could not get you? was you on air?
20200305_150124.jpg

Sorry... Brian operates at 150W - so not possible to reach Lanzarote at 236km away

It might get to Morro Jable if you put it on one of those masts up that whacking great mountain a bit!

300 Watts christ you'll need to make sure you have a whopping 17amps on your supply with about 17volts to get that 300 out lol

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thewisepranker wrote: ↑Mon Mar 09, 2020 12:03 pm It might get to Morro Jable if you put it on one of those masts up that whacking great mountain a bit!

Yes...
If I could afford it I would have Brian on the top of Pico de las Nieves.
It's the highest peak on the island of Gran Canaria.
Its height is 1,949 metres (6,394 ft) above sea level.
I can see it from my location now.
That is where all the big boys have there antennas
There are two locations available with hut, power and mast base available.
I guess they would wan't +/- 1000 Euros a Month rental + electricl.

Brian if you give them your wife they'll be happy to give you a hut only joking lol.

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Electronically wrote: ↑Mon Mar 09, 2020 3:53 pm Brian if you give them your wife they'll be happy to give you a hut only joking lol.