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Hello,
I just received a tubeless, cherry looking SB200 as a first amplifier and
have a few questions:
1. A friend told me I can run 811's in it with no mods. I've noticed the 811
max plate voltage rating is only 1500v, while the SB200 plate supply is
about 2400v. Is this a problem for the 811's on SSB?
2. Someone loaned me a pair of 572B's to check the amp out with. We noticed
that my transistorized exciter had trouble tuning into the SB200, especially
on 75 meters. I've sifted through the AMPs archives, and have read Rich's
articles on the SB220 regarding input tuning and increasing the Q of the
input networks. In his article he states that the Q of the input (PI)
network is Zin/Xc = 50ohms/Xc. While intuitively this seams plausible (since
infinite Xc would cause zero Q ie: 50ohm load), I wonder where this
definition comes from. It's not obvious to me that Q is calculated this way
for the PI network. Can someone point me towards a reference on that? Also,
for modeling purposes of the PI network and Q, I'd like to use Q=Fc/BW
(center freq/bandwidth). In this case, what load impedance do I assign to
the tube? (I know it changes throughout the drive cycle). Finally, since the
SB200 has different tubes than the SB220, I assume that the PI networks
would be different for the two. Has someone determined optimum values for
the SB200 using 572B's?
3. I want to buy a new pair of 572B's. From what I can gather, the
Svetlana's may have a bit higher gain than the Taylors or Cetrons. That may
make them more prone to VHF parasitics. Is this true? Are the Svetlanas
recommended or should I stick with the Taylors? I've noticed that this amp
has been modified and does have parasitic suppression circuitry in the plate
connections. Please, I do not want to reopen the great parasitc debate etc.
4. Finally, Ive notice that one other possible mod has been made. An
uninsulated silver looking wire has been installed in series with the
filament choke. It is wrapped around two insulative standoffs near the
filament choke and is about 2" long. What is this? Should it be there?
thanks in advance for any comments,
Dave
K0QE
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<TITLE>SB200 questions</TITLE>
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<P><FONT SIZE=3D2>Hello,</FONT>
<BR><FONT SIZE=3D2>I just received a tubeless, cherry looking SB200 as =
a first amplifier and have a few questions:</FONT>
</P>
<P><FONT SIZE=3D2>1. A friend told me I can run 811's in it with no =
mods. I've noticed the 811 max plate voltage rating is only 1500v, =
while the SB200 plate supply is about 2400v. Is this a problem for the =
811's on SSB?</FONT></P>
<P><FONT SIZE=3D2>2. Someone loaned me a pair of 572B's to check the =
amp out with. We noticed that my transistorized exciter had =
trouble tuning into the SB200, especially on 75 meters. I've sifted =
through the AMPs archives, and have read Rich's articles on the SB220 =
regarding input tuning and increasing the Q of the input networks. In =
his article he states that the Q of the input (PI) network is Zin/Xc =
=3D 50ohms/Xc. While intuitively this seams plausible (since infinite =
Xc would cause zero Q ie: 50ohm load), I wonder where this definition =
comes from. It's not obvious to me that Q is calculated this way for =
the PI network. Can someone point me towards a reference on that? Also, =
for modeling purposes of the PI network and Q, I'd like to use =
Q=3DFc/BW (center freq/bandwidth). In this case, what load impedance do =
I assign to the tube? (I know it changes throughout the drive cycle). =
Finally, since the SB200 has different tubes than the SB220, I assume =
that the PI networks would be different for the two. Has someone =
determined optimum values for the SB200 using 572B's?</FONT></P>
<P><FONT SIZE=3D2>3. I want to buy a new pair of 572B's. From what I =
can gather, the Svetlana's may have a bit higher gain than the Taylors =
or Cetrons. That may make them more prone to VHF parasitics. Is =
this true? Are the Svetlanas recommended or should I stick with the =
Taylors? I've noticed that this amp has been modified and does have =
parasitic suppression circuitry in the plate connections. Please, I do =
not want to reopen the great parasitc debate etc.</FONT></P>
<BR>
<P><FONT SIZE=3D2>4. Finally, Ive notice that one other possible mod =
has been made. An uninsulated silver looking wire has been installed in =
series with the filament choke. It is wrapped around two insulative =
standoffs near the filament choke and is about 2" long. What is =
this? Should it be there?</FONT></P>
<P><FONT SIZE=3D2>thanks in advance for any comments,</FONT>
<BR><FONT SIZE=3D2>Dave</FONT>
<BR><FONT SIZE=3D2>K0QE</FONT>
</P>
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