[AMRadio] Pi-Net vs Link Couple


Brett gazdzinski Brett.gazdzinski at mci.com
Tue Sep 27 09:27:40 EDT 2005


On my big rigs, I use a choke, plus a panel mount pot (25 or 35 watts)
so I can adjust the screen resistance.
The rig seems to modulate better when I give it about 4000 ohms at the pot,
and drop
100 volts. The screen is also much more stable and protected from excessive
current.
The pot could be adjusted to 0 ohms and the screen voltage variac set to
the correct voltage (say if I wanted to use it as a linier amp), but you
have to be REAL
careful in tuning up/etc, otherwise the screen current gets excessive very
quickly.

Paranoia made me put in an overload relay also...

Checking with the mod monitor shows the rig modulates much better with the
resistance
high and the choke in line.
>From memory, I think I loose about 10 or 15% positive modulation without the
resistor
in line.

Brett
N2DTS



-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of Bob Bruhns
Sent: Tuesday, September 27, 2005 9:13 AM
To: Discussion of AM Radio
Subject: Re: [AMRadio] Pi-Net vs Link Couple


Plate AND screen mod.

When the plate is modulated, its voltage varies,
and the screen current varies inversely, so an
impedance in series with the screen will modulate
the screen.

The interesting thing is that this works with
either a reactance or a resistance. With a simple
varying current, an inductance would be expected
to produce much more modulating voltage at high
audio frequencies than at low audio frequencies.
But the mechanism that produces the screen current
variations is sensitive to the screen voltage, so
that doesn't happen.  It's as though the screen
and the impedance tend to produce the
instantaneous voltage that the screen wants.

  Bacon, WA3WDR

----- Original Message ----- 
From: <bcarling at cfl.rr.com>
To: "Discussion of AM Radio"
<amradio at mailman.qth.net>
Sent: Tuesday, September 27, 2005 8:09 AM
Subject: Re: [AMRadio] Pi-Net vs Link Couple


> Are you talking about plate AND screen
modulation?
> Or just screen mod?
>
> On 23 Sep 2005 at 1:06, Bob Bruhns wrote:
>
> > This is the screen-choke self modulation
> > technique.  Screen current is affected by the
> > modulated plate voltage, and the screen choke
lets
> > the screen vary above and below the screen
suply
> > voltage according to screen current.  If the
choke
> > is large enough (you need 10 to 20 henries),
the
> > screen can follow the modulating audio
waveform on
> > the plate, and the net result is a happy,
> > self-modulated screen grid.
> >
> > To really tweak this, some people have added a
> > small compensating capacitor from modulated B+
to
> > the screen, to act as a capacitive voltage
divider
> > with the screen bypass cap, and maintain
screen
> > modulation percentage at high frequencies.
> >
> > The choke will only allow audio variations, so
the
> > screen will be held at the DC voltage of the
> > screen supply.  When used along with some
fixed
> > control grid bias, this eliminates the need
for a
> > screen clamp-tube.
> >
> >   Bacon, WA3WDR
> >
> >
> > ----- Original Message ----- 
> > From: "W5OMR/Geoff" <w5omr at satx.rr.com>
> > To: "Discussion of AM Radio"
> > <amradio at mailman.qth.net>
> > Sent: Thursday, September 22, 2005 10:28 PM
> > Subject: Re: [AMRadio] Pi-Net vs Link Couple
> >
> >
> > > Darrell, WA5VGO wrote:
> > >
> > > > Triodes are great for grounded grid, but
for
> > class C, why fight
> > > > neutralization? Use a power grid tube.
Sure
> > you need a screen supply,
> > > > but consider the paybacks. No
neutralization;
> > at least on the lower
> > > > frequencies, and low drive requirements. I
> > realize modulating it is a
> > > > little trickier, but I've never had the
Eimac
> > system of placing a
> > > > reactor in the screen supply to fail to do
the
> > trick.
> > > >
> > > > Darrell, WA5VGO
> > >
> > >
> > > that's interesting, Darrell... a reactor in
the
> > screen supply.  Are you
> > > capacitivly coupling it, as well?  No, that
> > can't be - the screens need
> > > to be DC voltage.  However, the choke
effectivly
> > kills any stray
> > > capacitances...
> > >
> > > hmmmmm
> > >
> > > Great discussion, so far.  Looking forward
to
> > more.
> > >
> > > Hope you guys are hunkered down for the
> > hurricane.  Kingwood, TX may be
> > > on the west side of the storm, but it's
gonna be
> > a big'un.
> > >
> > > --
> > > 73 = Best Regards,
> > > -Geoff/W5OMR
> > >
> > >
> >
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