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

Tue Apr 20 10:50:53 PDT 2010

hi. The manufacturer and another radio amateur claim that this linear amplifier which they have constructed works perfectly and stable, and gives 400w minimum r.f power at output stage (with 110vdc power supply-13A the current). i don't know spice simulation.
The linear which i have construct gave to the output stage (at 50hm dummy load) 190w r.f power with perfect modulation with 106vdc power supply. the current in this test was 6A.
The problem witch i am trying to solve is to find why my linear gives only 190watt at the same voltage, and why every time i was trying to increase the input r.f power, up to 4 watts, the circuit get into oscillation and the ferrite broken. Something goes wrong but i can't find what exactly.
The only difference between my amplifier and the prototype is that i use 6 ferrite beads at output transformer and the designer of the prototype uses only 4, as you can see in photos.
The other difference is that i am using Siliconix IRFP360 fets and the designer of prototype using International Rectifier fets. Moreover his operate frequency is 1650khz and my operation frequency is 1431khz. So i worry if i must use another ferrite material of 43, something like 77 material. 
What do you think? 

--- On Tue, 4/20/10, Boye Christensen <oz7c at cool.dk> wrote:

From: Boye Christensen <oz7c at cool.dk>
Subject: Re: [Amps] 160m mosfet linear amplifier problem with ferrite cores
To: "sasas asasas" <tzitzikas_ee at yahoo.com>
Date: Tuesday, April 20, 2010, 9:47 AM

Hi
Have u tried to run a simulation  e.g with spice
to se if this works ???

You can dowload LT spice free to try ?

As far as I can se this fet with 2 x6 pcs will not work

Ciss 4.5nF * 6
Crev 490pF *6
Cout 1100pF *6

a lot to  control !

http://www.vishay.com/docs/90292/sihfp360.pdf

73 Boye  OZ7C

----- Original Message ----- From: "sasas asasas" <tzitzikas_ee at yahoo.com>
To: <amps at contesting.com>; "'David Cutter'" <d.cutter at ntlworld.com>; "Alex Eban" <alexeban at gmail.com>
Sent: Tuesday, April 20, 2010 5:09 PM
Subject: Re: [Amps] 160m mosfet linear amplifier problem with ferrite cores

> 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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