[AMRadio] The Viking II is FINALLY starting to work


Jose HF Silva zehelmer at yahoo.com
Thu Feb 7 00:49:32 EST 2008


See this:

«The coils should be "along side" the resistor in
parallel, not wrapped around the body of the resistor»


http://www.kk5dr.com/ampbuilders.htm

Parasitic chokes: In fig.16, you see a parasitic
choke. Much has been written about these chokes, much
debate, and much conjecture as to what makes an
effective choke to suppress parasitic oscillations in
an HF amplifier. The basic theory of choke operation
is as follows; The HF fundamental energy passes freely
through the choke turns, but any VHF energy "sees" a
very high impedance in the coil, and must then pass
through the resistor which rapidly attenuates the RF
to a level that can not sustain an oscillation in the
tank circuit.  I have experimented in the types of
coil material and found that Ni-Chrome® is a little
TOO lossy when used on 10 meter band, but it does
prevent parasitics. I found that Stainless Steel wire
is a good less lossy  material to use, and will not
"rob" the amp of  10 meter RF energy.

 The "old" practice of using flat copper strap, is a
total waste of time, and will not prevent parasitic
oscillations at all. Don't use it! Another "old"
practice is to wrap the coils around the body of the
resistor this is also a poor approach. 

My design of parasitic choke is; About 4 turns of
stainless-steel 18ga wire, about 1/4 - 3/8" inside
diameter. Silver solder the coil across the a 2-3
watts carbon resistor, 50-150 ohms, or a 100-200 ohm
"flame-proof" metal film resistor. The coils should be
"along side" the resistor in parallel, not wrapped
around the body of the resistor. The reason for this
is that I believe the VHF RF "sees" a higher impedance
than the old method. I have tested this theory, and it
seems to hold up. The wire I used is what is known as
"Aviation safety wire", used to secure parts on
aircraft. This wire is very flexible and very high in
Nickel content. If the coils heat up to much on 10
meter operation, remove one turn from the coil.


--- sbjohnston at aol.com wrote:

> 
> 
> Jose wrote:
> 
> >Do you really mean
> 
> ->Tinned or enameled?
> 
> Tinned, but either would work.
> 
> ->Wrapped or wound?
> 
> Wrapped, but wound expresses it well.
> 
> ->Times or turns?
> 
> Now I think you're kidding me.
> 
> 
> Steve WD8DAS
> 
> sbjohnston at aol.com
> http://www.wd8das.net/
> ------------------------------------------------
> Radio is your best entertainment value.
> ------------------------------------------------
> 
> 
> -----Original Message-----
> From: Jose HF Silva <zehelmer at yahoo.com>
> To: Discussion of AM Radio in the Amateur Service 
> <amradio at mailman.qth.net>
> Sent: Sun, 3 Feb 2008 9:14 pm
> Subject: Re: [AMRadio] The Viking II is FINALLY
> starting to work
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> Hi,
> 
> Do you really mean
> 
> ->Tinned or enameled?
> 
> ->Wrapped or wound?
> 
> ->Times or turns?
> 
> 73 de Joe / CT1AXG
> 
> 
> --- sbjohnston at aol.com wrote:
> 
> >
> > I noticed a type-O mistake in my earlier comment
> > about parasitic
> > suppressors.  I wrote:
> >
> > >I haven't found it to be critical.  I generally
> use
> > the solid tinned
> > >wire that radio shack sells on a small white
> reel,
> > wrapped three or so
> > >times around a 2-watt carbon resistor.  The
> > resistor is usually 47-ohm
> > >or thereabouts.
> >
> > but I meant to write:
> >
> > >wrapped three to six times around a 2-watt...
> >
> >
> > Steve WD8DAS
> >
> > sbjohnston at aol.com
> > http://www.wd8das.net/
> > ------------------------------------------------
> > Radio is your best entertainment value.
> > ------------------------------------------------
> >
> >
> >
> >
>
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