Thought I'd try one of these.....

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rettimsnarT
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Re: Thought I'd try one of these.....

Post by rettimsnarT » Thu Jan 12, 2017 12:55 am

MRF186 good for 120w maybe this with MRF9180 push 200w

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Re: Thought I'd try one of these.....

Post by Maximus » Mon Feb 06, 2017 5:43 pm

Made a start with the amp. Then hit a wall as the kit was missing a length of 25ohm coax, which isn't available locally. So it'll have to be eBay.

No expense is spared. Had about 5mm spare wire after winding the coils.

There was also a 10ohm and 1k resistor missing.

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Re: Thought I'd try one of these.....

Post by Maximus » Thu Mar 02, 2017 10:11 pm

The amp will easily do 200W+ on only 24V! The gain on this design is insane!


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MC Spanner
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Re: Thought I'd try one of these.....

Post by MC Spanner » Sun Apr 16, 2017 1:07 am

Maximus wrote:
Thu Mar 02, 2017 10:11 pm
The amp will easily do 200W+ on only 24V! The gain on this design is insane!
Be careful, that FET is fragile - don't breathe on the input! That board is OK for the price but could be improved. The bias is critical and varies significantly with temperature and the output circuit is quite basic and would benefit from techniques to improve linearity. It's not as linear as the manufacturers suggest and not uncommon in amplifiers of this type for around 10dB more 3F than 2F. For this reason, and the look of the filter, I'd be surprised if your measured 200W is all on Band II.

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Re: Thought I'd try one of these.....

Post by Maximus » Sun Apr 16, 2017 4:29 am

Yes I found out the hard way with the bias.

Fitted a Freescale transistor and the thing is rock solid.


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Re: Thought I'd try one of these.....

Post by Maximus » Sun Apr 16, 2017 4:32 am

Some more pictures

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Re: Thought I'd try one of these.....

Post by OgreVorbis » Sun Apr 30, 2017 3:02 am

I have a version of this board made for the MRF186. Can I just swap in the MRF9180 for more power output? It seems to run at the same voltage and matching on these devices seems pretty generic, so it should work right? Maybe just some bias adjustment?

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Re: Thought I'd try one of these.....

Post by Maximus » Sun Apr 30, 2017 11:14 am

Yes AFAIK the boards and components are the same. The bias may need adjusting though.


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Re: Thought I'd try one of these.....

Post by OgreVorbis » Sun Apr 30, 2017 10:37 pm

So I found these boards on ebay and it looks like they're using the MRF9180. They are using an extra coil between the two drains and also two resistors. Do you think adding that on your design might improve things? Surprised they only say 150W though.

Anyway I just bought some MRF9180s to replace the MRF186 to see if I can get more power. What should I adjust the bias voltage to?

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Re: Thought I'd try one of these.....

Post by Maximus » Mon May 01, 2017 3:24 am

I think the extra coil and resistors are in place to stop the thing 'taking off' if the bias or drive would suddenly develop a mind of its own. Probably isn't critical.

Can't remember the exact bias voltage, but I know the variable pot barely adjusts it.

Also had different results from the transistors. The supplied (used) Motorolas were absolute crap and kept on popping. The Freescale versions are very reliable and rugged.

You can easily push 200w into a good matched load without harmonics. I wouldn't recommend going much over 150w as the transistor runs cool and can easily survive a big mismatch (using Freescale).



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Re: Thought I'd try one of these.....

Post by Maximus » Mon May 01, 2017 3:34 am

With a few modifications, the amps have many other uses. Especially DAB, when combined with some good filtering on the output.


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Re: Thought I'd try one of these.....

Post by steven30gr » Sun May 21, 2017 9:33 am

hello from me guys!few questions for 9180 mdifications!how much amps needs?any changes of materials in output filter?or the same values?bias voltage?cables of 25ohm doesnt burns?

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Re: Thought I'd try one of these.....

Post by sinus trouble » Tue May 23, 2017 12:11 am

Hello Mr Steven!
As far as I know? the 9180 is interchangeable with the 186 with no modifications? Teflon based coax will be fine for this design!
As mentioned several times! Bias voltage cannot be specified for any given design! The important part is the drain quiescent current, When the transistor goes from an off state to conduction? This varies with devices and would normally be around 100ma for class B operation but as it is dual FET 200ma would normally be seen?
You need to monitor the drain current (with no RF drive) and set the bias voltage accordingly!
I am as stupid as I look! :|

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Re: Thought I'd try one of these.....

Post by steven30gr » Wed May 24, 2017 6:49 pm

sir sinus
thannks for responce and answer some of my questions!if i put dirctly 9180 vs 186 what i take from this change with the same volts,ampes?

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Re: Thought I'd try one of these.....

Post by sinus trouble » Wed May 24, 2017 11:27 pm

Hello Mr Steven! Glad to help!
I could just tell you the answers? But you would not learn anything that way! :)
To get a grasp of what you are dealing with? The datasheet is always a good start! One thing you have to bear in mind is the manufacturer will deal with absolute maximum ratings in a perfect environment! In your application this will not be the case, it will likely be half of their figures!
Take a look at this regarding bias!
mrf9180.PNG
When we look at the ON state characteristics? we can see that the Vgs(th) in a common source configuration @ Tc=25c is between 2 - 4v? But its Vds is only 10v (supply voltage)
Now the max voltage I would expect you to use is 24 - 26v supply! I would still stick to an absolute max gate bias of 4v just to be safe!

Now how much current will the whole assembly consume in operation assuming the Vds stays constant? And you want 170watt out? In a fantasy world, the transistor would be 100% efficient and only draw 170watt from your PSU?
The reality is that at best the transistor will only be 75% efficient? Therefore your transistor will be dissipating around 215watts!

215w divided by 24v = 8.95A So you would need atleast a 10A PSU to achieve anything near that output!
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