With an L network if the tube impedance is lower than the driver impedance
then there will be a coil to ground at the cathode rather than the needed
capacitor.
If the cathode impedance is higher than the driver impedance by a small
(even 3:1) amount then there will be a capacitor on the cathode side of the
L network but it will be too small and the Q will be too low for proper
flywheel effect.
With a tube that has 150 ohms input impedance using an L network to match to
50 ohms will have a capacitance of around 428 pf on 80 meters providing a Q
of only around 1.3.
Bill Orr recommends 13 pf/ meter for input capacitance. That would require
1040 pf at the tubes cathode for proper harmonic current transfer to ground.
Proper Q and proper shunt capacitance at the cathode can not usually be
obtained with an L network. It requires a Pi network to control those
factors. A parallel tuned tank would also work ok.
With an L network at the tube you can get an impedance match to keep the
driver happy but you might just as well employ the tuner in the driver to do
the same.
73
Gary K4FMX
> -----Original Message-----
> From: Bob Maser [mailto:bmaser@tampabay.rr.com]
> Sent: Wednesday, March 22, 2006 11:56 PM
> To: garyschafer@comcast.net
> Subject: Re: [Amps] Tuned input board on SB-220
>
> Not true
> ----- Original Message -----
> From: "Gary Schafer" <garyschafer@comcast.net>
> To: "'noddie'" <noddie@comcast.net>; "'Ed Dial'" <diale@mindspring.com>;
> <amps@contesting.com>
> Sent: Wednesday, March 22, 2006 11:22 PM
> Subject: Re: [Amps] Tuned input board on SB-220
>
>
> > There was some discussion on this board awhile back and wasn't it
> decided
> > that the tuner is actually an L network and not a PI? That would make it
> > unsuitable as a tuned input network.
> >
> > 73
> > Gary K4FMX
> >
> >
> >> -----Original Message-----
> >> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com]
> On
> >> Behalf Of noddie
> >> Sent: Wednesday, March 22, 2006 10:10 PM
> >> To: 'Ed Dial'; amps@contesting.com
> >> Subject: Re: [Amps] Tuned input board on SB-220
> >>
> >>
> >> The AT-100AMP is what I am going to use on my SB-221, I wish it had
> been
> >> around when I build a tuned input board for my Clippertron L, all the
> >> other
> >> tuned input boards do not work on the WARC bands and cost 150 dollars
> >> compared to 125 dollars for the AT-100AMP and this board is automatic
> >> with
> >> memories. The only thing you will have to watch out for is that your
> RF
> >> choke does not resonate on the WARC bands. I changed my choke to the
> RF
> >> Parts one at a cost of 25 bucks, it works for 3-500's and 572b's and
> will
> >> not resonate on the WARC bands.
> >>
> >> Mike
> >>
> >>
> >> Gentlemen,
> >>
> >> I need opinions from the experts that live here. I have an SB-220
> >> that does an excellent job on the pre-WARC bands, however it doesn't
> >> like 12 and 17 meters very well, thanks to the tuned input circuit
> >> from what I understand. I noticed that the good folks at LDG
> >> Electronics is now selling a stripped version of their AT-100
> >> autotuner as the AT-100AMP, marketed as an RF amplifier tuned input
> >> board for homebrews and the like. My question is, could this board be
> >> fitted into the SB-220 without much pain? Would this solve the WARC
> >> problem?
> >>
> >> If so, would it work in the SB-200 as well? I have no clue with 572s.
> >>
> >> Lazy and looking for the simple way out -
> >>
> >> Ed
> >> WA4RYW
> >>
> >>
> >> _______________________________________________
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> >> Amps@contesting.com
> >> http://lists.contesting.com/mailman/listinfo/amps
> >>
> >> _______________________________________________
> >> Amps mailing list
> >> Amps@contesting.com
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> >
> >
> > _______________________________________________
> > Amps mailing list
> > Amps@contesting.com
> > http://lists.contesting.com/mailman/listinfo/amps
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