LAZY wrote:Can someone explain why the 1000uf capacitor is conected on the input power supply?

Good question. Because it can?

SOTS 87 7 wrote: The thing that threw me was using the MRF 186 which is essentially a UHF device, but it appears to work fine.

Really? ...There's amps for the HF bands 1.8-54MHz on sale designed for those transistors. That's one hell of a broad ranged device if it works down that low and it's designed for UHF?

http://www.ebay.co.uk/itm/1-8M-54MHz-30 ... 51c55c2613

UPDATE: Just checked the datasheet and it appears they're designed for 800MHz-1GHz.

http://pdf1.alldatasheet.com/datasheet- ... RF186.html

LAZY wrote:Can someone explain why the 1000uf capacitor is conected on the input power supply?

You always should have filtering capacitor at power supply input. 1000uF is a bit low value for this but if you connect it to already filtered power supply, it is ok. Usualy it goes along with 100nF and 10nF caps. This lowers low frequency (50Hz) that may cause problems and hum.

The instructions on construction are a bit to say the least, vague but this was the most helpful one as I believe Mr. Techniqs also bought one.

In terms of the frequency range, I believe it's mostly internal matching / compensation, which causes most problems. The MRF186 datasheet doesn't say it's compensated, but it is specified as a broadband device (800MHz-1.2G, as Techniqs said), so I guess it probably has some internal matching. If it does, I wonder if it may be that it's so far below the intended frequency, that the internal matching isn't doing anything much. If it's a series "L", shunt "C" type arrangement, I think that may be the case.

UPDATE: Been using this board now For the last 2 months WITHOUT fault. We reworked the LPF, my mate checked it on his spectrum analyser, all good.

Good for not much money just a sod to put together.

Nicely done Sots. Do you have any pics of the finnished amp? What values did you use for the LPF, number of coils, Dia Etc?

Lazy

LAZY wrote:Nicely done Sots. Do you have any pics of the finnished amp? What values did you use for the LPF, number of coils, Dia Etc?

Lazy

Hi Mr. Lazy, it is installed in a rack mount case and in use, lol, I will certainly try and upload some pics of the innards when I get a mo.
Amazingly this little amp when mounted on a decent heatsink doesn't even get hot. I recon the Chinese got that one right for a change. Feeding in Aprox 2 watts, with 98.7 watts out into a 50 ohm load at 87.7Mhz. Not bad at all.

Transistors won't really have a lower frequency, just a range of frequencies where they are easily matched. The amp seems like a bargain, obviously providing your own heatsink ads a little cost. Seems almost too good to be true!

Mine arrived with loads of parts missing. Useless cunts!

teckniqs wrote:Mine arrived with loads of parts missing. Useless cunts!

Yeh, funny you should say that, the second ''KIT'' I bought the the MRF 186 was missing. However, was lucky I had one anyway.

Here is one I put together using ''real'' components instead of those bloody SMD parts supplied. Here are the 2 ready fro testing and setting the bias voltage before fitting the MRF 186.

I've no idea why the first pic came out twice.

One day I'll learn how to use a computer. Hahaha.

Looks like you've gone with the original LPF inductor dimensions ?

Yes, at the moment, we are still in the ''playing around'' with it stage at the mo, obviously if we find something that basically works we will upload it and share the result. mind you, it seems ok as is oddly enough. Maybe an external LPF perhaps?

SOTS 87 7 wrote:Yes, at the moment, we are still in the ''playing around'' with it stage at the mo, obviously if we find something that basically works we will upload it and share the result. mind you, it seems ok as is oddly enough. Maybe an external LPF perhaps?

I had wondered how it would perform, with the original LPF. What I think you're perhaps seeing, are the combined effects of some useful inherent cancellation at 2f (due to the push pull design), with the LPF probably just starting to develop a reasonable amount of attenuation by 3f.
As you said, an external LPF is one way to improve performance. Or, I think component values from one of the "standard" Band II LPF designs (like the Stephen Moss design I posted earlier in the thread - just as one example), would transplant quite well onto the provided PCB; in place of the existing LPF. Turn each coil by 90 degrees, so that they're lengthwise across the gap, and still 90 degrees to each other, and it looks like you should be able to get the inductor length where you want it.

Just bought one of these. They look very similar to 300W pallet amps to me. I emailed the Chinese guy who designed it and he said the circuit can handle up to 600W, but didn't answer any specifics. I don't know if you could just drop in a 300W mosfet and have it working without any modifications. Anyone have a 300 they could try in this?

Blf278 could work if it can fit into slot. Also, since it is 50v device, maybe filtering caps should be changed. Actually, you can fit almost any twin fet (ldmos) device without board modifications. Maybe few additional components in input / output matching