[Amps] 160m mosfet linear amplifier problem with ferrite cores

[sasas asasas]

the manufacturer who construct the prototype and use mica to the fets, propose to me. to not use mica, but to take the drain for the 6 fets from the heatsink. He told me just to use some thermal silicon paste for low thermal resistance.

--- On Tue, 4/20/10, Alex Eban <alexeban at gmail.com> wrote:

From: Alex Eban <alexeban at gmail.com>
Subject: RE: [Amps] 160m mosfet linear amplifier problem with ferrite cores
To: "'sasas asasas'" <tzitzikas_ee at yahoo.com>, amps at contesting.com, "'David Cutter'" <d.cutter at ntlworld.com>
Date: Tuesday, April 20, 2010, 2:32 AM

First of all the guy INSULATED the FET's from the heatsink!
You can see the mica insulators of the side plate where the transistors are mounted.
He also used a WIDE  copper strip to connect the drains; parallel connected FET's are notorious for breaking into oscillations using the stray inductance of interconnecting wires as a resonating circuit. I've seen a pair of them running off merrily at about 80 MHz , in a circuit supposed to work at DC! 
It's probably more an issue of the way it's built than a real electrical problem. If stray coupling causes oscillations, then it can also explain the cracked cores in the output transformers.
My recommendation is that you rebuild it along Tzitzika's model, with good short wiring, insulated transistors and good bypassing.
It's like cooking: first time copy the recipe exactly. Improvise on the second run! At these power levels it takes very little to go from a working circuit to a catastrophe!
Alex        4Z5KS


-----Original Message-----
From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On Behalf Of sasas asasas
Sent: Sunday, April 18, 2010 3:05 PM
To: amps at contesting.com; David Cutter
Subject: Re: [Amps] 160m mosfet linear amplifier problem with ferrite cores

The linear have been constructed by me (if you mean the manufacter of linear amplifier).

The prototype have been constructed by schematic designer and you can see a photo here:

http://www.tzitzikas.webs.com/linear_first.jpg 


Also, this linear have  been successfully constructed by another radio amateur. You can see photos here:

http://www.tzitzikas.webs.com/linear_working3.jpg


http://www.tzitzikas.webs.com/linear_working1.jpg


http://www.tzitzikas.webs.com/linear_working2.jpg    
            

Both of these linear are using International Rectifier IRFP360 mosfets.
I use this time Siliconix IRFP360 mosfets. Both of amateurs claims that their linear amplifier gives at least 450w r.f power at the outupt stage.
--- On Sun, 4/18/10, David Cutter <d.cutter at ntlworld.com> wrote:

From: David Cutter <d.cutter at ntlworld.com>
Subject: Re: [Amps] 160m mosfet linear amplifier problem with ferrite cores
To: "sasas asasas" <tzitzikas_ee at yahoo.com>, amps at contesting.com
Date: Sunday, April 18, 2010, 4:56 AM



 
 


Are you using the same manufacturer as your friends?  

 
If so, yours should be the similar performance unless something is wrong.
 
David



      
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