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@gmail.com> wrote:
From: Alex Eban <alexeban@gmail.com>
Subject: RE: [Amps] 160m mosfet linear amplifier problem with ferrite cores
To: "'sasas asasas'" <tzitzikas_ee@yahoo.com>, amps@contesting.com, "'David
Cutter'" <d.cutter@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@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of sasas asasas
Sent: Sunday, April 18, 2010 3:05 PM
To: amps@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@ntlworld.com> wrote:
From: David Cutter <d.cutter@ntlworld.com>
Subject: Re: [Amps] 160m mosfet linear amplifier problem with ferrite cores
To: "sasas asasas" <tzitzikas_ee@yahoo.com>, amps@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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