[AMPS] Final (??) Comments on B&W850A
Jon Ogden
jono@enteract.com
Fri, 12 Feb 99 09:46:30 -0600
Well, after reading the messages that others have sent to me regarding
the B&W850A, I think I have come upon a solution - at least on paper.
Some have suggested modifying the taps on the coil. That is fine, but
messy. Also, if I increase the inductance on 10M, it will demand a
smaller Ctune. The problem is that Ctune could become unacheivable with
a 10pF (min) variable plus the capacitance of the tube.
The best solution I have seen is what several people suggested and that
is to add a small coil between the blocking caps and the tune C. In
effect what you are doing is creating a low-pass L-network comprised of
the output C of the tube and the coil. This way, you can "match" the
high impedance of the 4-1000 to some lower value impedance that is easier
to tune with the pi-network as is. I am probably going to slightly
increase the value of the 10M coil on the B&W, but that won't really be
an issue any more since I will be matching a much lower impedance value
to 50 Ohms.
One additional advantage I see that might occur from using this topology
is increased stability. The coil will look like a high impedance at VHF
thereby isolating the anode circuitry from the effects of Ctune. This is
speculation and hypothesizing right now, but it seems to make sense.
Anyhow, I've not tried any of this yet. And I probably won't get to this
weekend either with Valentine's day and all (I've already committed most
of Saturday to the YL). But perhaps next week I'll get to try it out.
Thanks to everyone for their help.
73,
Jon
KE9NA
--------------------------------------------------------------------------
Jon Ogden
jono@enteract.com
www.qsl.net/ke9na
"A life lived in fear is a life half lived."
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