[AMRadio] Pi-Net vs Link Couple


Bob Bruhns bbruhns at erols.com
Fri Sep 23 01:06:10 EDT 2005


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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