Some additional photos from my linear:
http://tzitzikas.webs.com/fet20.JPG
http://tzitzikas.webs.com/linear20.JPG
http://tzitzikas.webs.com/linear21.JPG
http://tzitzikas.webs.com/linear22.JPG
http://tzitzikas.webs.com/fet21.JPG
http://tzitzikas.webs.com/pompos20.JPG
--- 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, 2:35 AM
I've looked again at the jpg and I think you have used
trifilar windings, so, that's my first idea more or less gone.
Imbalance can be due to poor coupling between the windings themselves or
imbalance in the driving currents. With high voltage supplies, balance
error should be small (compared to say a 12V amplifier where a small difference
in voltages can show a large imbalance %). If you could measure the
current in each half of the primary that would tell you the whole story: you
will need a HF current probe, preferably 2 probes and dual trace scope which
will also tell you about cross-conduction.
Another thought: you have 6 transistors in parallel in
each half and emitter (sorry - source) degeneration. Did you try
matching the FETs for gain or phase delay? If not, it's conceivable that
one side is conducting harder or for longer than the other side and causing an
imbalance. I don't know if the source resistor is the optimum value for
mis-matched FETs, but a higher value would increase negative feedback and
help with this; you have oodles of gain to sacrifice. Along the same
lines, the input drive to each FET may need optimising for the same
reason. Check all your resistor values, one might be a
dud.
This is all armchair speculation, I don't have enough
experience for better detail. Someone with modelling experience could
probably do this quite quickly.
Alex has probably got the answer regarding core
material.
David
G3UNA
----- Original
And how can i repair this problem???
You might consider imbalance in each side of the
primary which will cause a net dc to pass, saturation and
over-heating. This may be rubbish but it could get others
thinking along a different path.
David
G3UNA
>
HI. i have constructed this linear amplifier 500w rms (2kw pep)
(50ohm) with 12 mosfet irfp360,
http://tzitzikas.webs.com/linear500w.jpg for 160m
band.
> When i gave 3watts of driving r.f power, it gave to
output only 190w at 106VDC (6A current). Two radio amateurs who have
construct this linear claim tha it gives 500w
> r.f power at
110vdc.
> But when i tried to give 4watts of driving r.f power
the ferrite Cores (43 material)
> of transformer T3 broken!
Which do you think is the problem??
>
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