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On Thu, 29 Nov 2001 19:12:35 -0500 "Tom Rauch" <w8ji@akorn.net> writes:
>
> > I think you'll find that if you do all the math that, for a
typical
> > loaded Q of 10-12 in the Pi-net, the coupling cap value should be 4X
> > the value of the plate capacitor at the lowest frequency of
operation.
>
> Well let's see...
>
> If the anode operating impedance is 3000 ohms and we use the
> somewhat incorrect but common method of Rp/Xc = Q, then we
> would have 300 ohms Xc with a Q of ten. That means the blocking
> cap would be 75 ohms, which is without any doubt needlessly
> large since it is in series with a 3000 ohm source impedance!
>
> A blocking cap of even 500 ohms reactance would have a minimal
> affect on anode impedance seen by the tank, so why make it so
> large?
> 73, Tom W8JI
> W8JI@contesting.com
>
Hi Tom,
This was an interesting enough query to prompt the installation of the
old Smith chart program on the new computer.
If the blocking cap is made as small as you propose, it becomes part
of the matching network rather than just a bypass and some interesting
things happen depending on the frequency of operation.
At higher frequencies, where given a typical tube the plate C
represents a significant part of the plate tune cap, the effect is to
significantly raise the plate impedance (as seen by the Pi-Net input) and
the balance of the pi-net would need to be adjusted to maintain the
desired Q & impedance ratio.
At lower frequencies, where the typical plate choke begins to become a
influential part of the overall network, a similar effect occurs.
And at some median freq where the tube's plate C effectively cancels
out the plate choke shunt L (i.e. the plate circuit is purely resistive
at that point), the lowered value of the coupling cap looks less
disruptive.
I think I'll remain with the traditional size coupling cap and avoid
the extra math. :-)
73 & Good morning,
Marv WC6W
*
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<DIV>On Thu, 29 Nov 2001 19:12:35 -0500 "Tom Rauch" <<A=20
href=3D"mailto:w8ji@akorn.net";>w8ji@akorn.net</A>> writes:<BR>> <BR>&=
gt;=20
> I think you'll find that if you do all the math that=
, for=20
a typical<BR>> > loaded Q of 10-12 in the Pi-net, the coupling =
cap=20
value should be 4X<BR>> > the value of the plate capacitor at =
the=20
lowest frequency of operation.<BR>> <BR>> Well let's see...<BR>=
>=20
<BR>> If the anode operating impedance is 3000 ohms and we use the <BR>&=
gt;=20
somewhat incorrect but common method of Rp/Xc =3D Q, then we <BR>> would=
have=20
300 ohms Xc with a Q of ten. That means the blocking <BR>> cap would be =
75=20
ohms, which is without any doubt needlessly <BR>> large since it is in =
series=20
with a 3000 ohm source impedance!<BR>> <BR>> A blocking cap of even =
500=20
ohms reactance would have a minimal <BR>> affect on anode impedance seen=
by=20
the tank, so why make it so <BR>> large?<BR>> 73, Tom W8JI<BR>> <A=
=20
href=3D"mailto:W8JI@contesting.com";>W8JI@contesting.com</A> <BR>> <BR></=
DIV>
<DIV>Hi Tom,</DIV>
<DIV> This was an interesting enough query to prompt the=20
installation of the old Smith chart program on the new computer.</DIV>
<DIV> </DIV>
<DIV> If the blocking cap is made as small as you propose, it=20
becomes part of the matching network rather than just a bypass and some=20
interesting things happen depending on the frequency of operation.</DIV>
<DIV> </DIV>
<DIV> At higher frequencies, where given a typical tube the =
plate=20
C represents a significant part of the plate tune cap, the effect is =
to=20
significantly raise the plate impedance (as seen by the Pi-Net input) and =
the=20
balance of the pi-net would need to be adjusted to maintain the desired Q &=
amp;=20
impedance ratio.</DIV>
<DIV> </DIV>
<DIV> At lower frequencies, where the typical plate choke =
begins to=20
become a influential part of the overall network, a similar effect occurs.<=
/DIV>
<DIV> </DIV>
<DIV> And at some median freq where the tube's plate=20
C effectively cancels out the plate choke shunt L (i.e. the plate =
circuit=20
is purely resistive at that point), the lowered value of the coupling cap =
looks=20
less disruptive.</DIV>
<DIV> </DIV>
<DIV> I think I'll remain with the traditional size =
coupling=20
cap and avoid the extra math. :-)</DIV>
<DIV> </DIV>
<DIV>73 & Good morning,</DIV>
<DIV> Marv WC6W</DIV>
<DIV> </DIV>
<DIV> </DIV>
<DIV> </DIV>
<DIV> </DIV>
<DIV> </DIV>
<DIV> </DIV>
<DIV> </DIV>
<DIV> </DIV>
<DIV> </DIV>
<DIV> </DIV>
<DIV> </DIV>
<DIV>* </DIV>
<DIV></DIV></BODY></HTML>
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