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[AMPS] Another Stupid Question

To: <amps@contesting.com>
Subject: [AMPS] Another Stupid Question
From: alex@sandlabs.com (alex)
Date: Sun, 2 Dec 2001 03:19:28 -0500
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To fully understand the function of this cap, one must make a Scattering
Parameter Analysis of the stages, the generator and it's load. A simple
Smith Chart is just a beginning, a tool that can induce substantial =
errors
if the assumptions made about inputs and outputs are wrong. I will =
suggest
you to use MWOffice for a comprehensive analysis of the amp. You might =
have
to make the S parameter list for the tube or perhaps the manufacturer =
had
done so already.

Good luck
Alex
http://www.sandlabs.com
ssage -----=20
  From: Radio WC6W=20
  To: W8JI@contesting.com ; amps@contesting.com=20
  Sent: Sunday, December 02, 2001 1:47 PM
  Subject: Re: [AMPS] Another Stupid Question



  On Thu, 29 Nov 2001 19:12:35 -0500 "Tom Rauch" <w8ji@akorn.net> =
writes:
  >=20
  > >    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.
  >=20
  > Well let's see...
  >=20
  > If the anode operating impedance is 3000 ohms and we use the=20
  > somewhat incorrect but common method of Rp/Xc =3D Q, then we=20
  > would have 300 ohms Xc with a Q of ten. That means the blocking=20
  > cap would be 75 ohms, which is without any doubt needlessly=20
  > large since it is in series with a 3000 ohm source impedance!
  >=20
  > A blocking cap of even 500 ohms reactance would have a minimal=20
  > affect on anode impedance seen by the tank, so why make it so=20
  > large?
  > 73, Tom W8JI
  > W8JI@contesting.com=20
  >=20

  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











  *   =20

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<HTML><HEAD>
<META http-equiv=3DContent-Type content=3D"text/html; =
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</HEAD>
<BODY bottomMargin=3D0 bgColor=3D#ffffff leftMargin=3D3 topMargin=3D0 =
rightMargin=3D3>
<DIV>To fully understand the function of this cap, one must make a=20
Scattering<BR>Parameter Analysis of the stages, the generator and it's =
load. A=20
simple<BR>Smith Chart is just a beginning, a tool that can induce =
substantial=20
errors<BR>if the assumptions made about inputs and outputs are wrong. I =
will=20
suggest<BR>you to use MWOffice for a comprehensive analysis of the amp. =
You=20
might have<BR>to make the S parameter list for the tube or perhaps the=20
manufacturer had<BR>done so already.<BR><BR>Good luck<BR>Alex<BR><A=20
href=3D"http://www.sandlabs.com";>http://www.sandlabs.com</A><BR>ssage =
----- </DIV>
<BLOCKQUOTE dir=3Dltr=20
style=3D"PADDING-RIGHT: 0px; PADDING-LEFT: 5px; MARGIN-LEFT: 5px; =
BORDER-LEFT: #000000 2px solid; MARGIN-RIGHT: 0px">
  <DIV=20
  style=3D"BACKGROUND: #e4e4e4; FONT: 10pt arial; font-color: =
black"><B>From:</B>=20
  <A title=3Dwc6w@juno.com href=3D"mailto:wc6w@juno.com";>Radio WC6W</A> =
</DIV>
  <DIV style=3D"FONT: 10pt arial"><B>To:</B> <A =
title=3DW8JI@contesting.com=20
  href=3D"mailto:W8JI@contesting.com";>W8JI@contesting.com</A> ; <A=20
  title=3Damps@contesting.com=20
  href=3D"mailto:amps@contesting.com";>amps@contesting.com</A> </DIV>
  <DIV style=3D"FONT: 10pt arial"><B>Sent:</B> Sunday, December 02, 2001 =
1:47=20
  PM</DIV>
  <DIV style=3D"FONT: 10pt arial"><B>Subject:</B> Re: [AMPS] Another =
Stupid=20
  Question</DIV>
  <DIV><BR></DIV>
  <DIV></DIV>
  <DIV>&nbsp;</DIV>
  <DIV>On Thu, 29 Nov 2001 19:12:35 -0500 "Tom Rauch" &lt;<A=20
  href=3D"mailto:w8ji@akorn.net";>w8ji@akorn.net</A>&gt; writes:<BR>&gt; =
<BR>&gt;=20
  &gt;&nbsp;&nbsp;&nbsp; I think you'll find that if you do all the math =
that,=20
  for a&nbsp; typical<BR>&gt; &gt; loaded Q of 10-12 in the Pi-net, the =
coupling=20
  cap value should be&nbsp; 4X<BR>&gt; &gt; the value of the plate =
capacitor at=20
  the lowest frequency of&nbsp; operation.<BR>&gt; <BR>&gt; Well let's=20
  see...<BR>&gt; <BR>&gt; If the anode operating impedance is 3000 ohms =
and we=20
  use the <BR>&gt; somewhat incorrect but common method of Rp/Xc =3D Q, =
then we=20
  <BR>&gt; would have 300 ohms Xc with a Q of ten. That means the =
blocking=20
  <BR>&gt; cap would be 75 ohms, which is without any doubt needlessly =
<BR>&gt;=20
  large since it is in series with a 3000 ohm source impedance!<BR>&gt; =
<BR>&gt;=20
  A blocking cap of even 500 ohms reactance would have a minimal =
<BR>&gt; affect=20
  on anode impedance seen by the tank, so why make it so <BR>&gt; =
large?<BR>&gt;=20
  73, Tom W8JI<BR>&gt; <A=20
  href=3D"mailto:W8JI@contesting.com";>W8JI@contesting.com</A> <BR>&gt; =
<BR></DIV>
  <DIV>Hi Tom,</DIV>
  <DIV>&nbsp;&nbsp; This was an interesting enough query to prompt the=20
  installation of the old Smith chart program on the new computer.</DIV>
  <DIV>&nbsp;</DIV>
  <DIV>&nbsp;&nbsp; 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>&nbsp;</DIV>
  <DIV>&nbsp;&nbsp; At higher frequencies, where given a typical tube =
the plate=20
  C&nbsp;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=20
  &amp; impedance ratio.</DIV>
  <DIV>&nbsp;</DIV>
  <DIV>&nbsp;&nbsp; At lower frequencies, where the typical plate choke =
begins=20
  to become a influential part of the overall network, a similar effect=20
  occurs.</DIV>
  <DIV>&nbsp;</DIV>
  <DIV>&nbsp;&nbsp; And at some median freq where the tube's plate=20
  C&nbsp;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=20
  looks less disruptive.</DIV>
  <DIV>&nbsp;</DIV>
  <DIV>&nbsp;&nbsp; I think I'll&nbsp;remain with the traditional size =
coupling=20
  cap and avoid the extra math.&nbsp; :-)</DIV>
  <DIV>&nbsp;</DIV>
  <DIV>73 &amp; Good morning,</DIV>
  <DIV>&nbsp;&nbsp; Marv&nbsp; WC6W</DIV>
  <DIV>&nbsp;</DIV>
  <DIV>&nbsp;</DIV>
  <DIV>&nbsp;</DIV>
  <DIV>&nbsp;</DIV>
  <DIV>&nbsp;</DIV>
  <DIV>&nbsp;</DIV>
  <DIV>&nbsp;</DIV>
  <DIV>&nbsp;</DIV>
  <DIV>&nbsp;</DIV>
  <DIV>&nbsp;</DIV>
  <DIV>&nbsp;</DIV>
  <DIV>*&nbsp;&nbsp;&nbsp; </DIV>
  <DIV></DIV></BLOCKQUOTE></BODY></HTML>

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