Kevin, the input circuits are "kind of" a pi type on the higher bands as
well. The missing C is furnished by the input C of the tube grid to cathode
capacitance.
I say "kind of" because the input C connects through a piece of 50 ohm coax
to the box containing the input networks. That piece of coax will alter
the C value seen by the networks.
The rf voltages involved in the input circuit are not extreme, maybe 70 to
100 volts rms maximum , so no need for very robust insulation there.
73,
Gerald K5GW
In a message dated 12/6/2010 3:04:47 P.M. Central Standard Time,
knormoyle@surfnetusa.com writes:
I just got done redoing the input network on my old FL2100b (adjust
15m/10m coils and caps) and was
happy with the result..so I went back to the AL-1200 I've mentioned here
before to fix it's issues.
1) Opening up the front panel and exposing the AL-1200 input coils, yes
indeed, the ferrite had
fallen out of the 10m coil. The 15m ferrite had been turned in too far for
my hex tool to reach it.
2) 10M couldn't be adjusted. I noticed it had just 1 turn for the "coil"
...so I disconnected the
coil and replaced it with a wire (basically just across the bandswitch).
10M looks good now. (it had
been centered lowerbefore I lowered the L by removing the one turn coil)
I wonder if they had tuned 10M when they manufactured it..since it shipped
with the green wire
disabling 10m.
3) I was surprised at the poor way the wires had been arranged. The leads
from the input coils to the
bandswitch seemed excessively long and the wires weren't arranged to
minimize coupling. Also the wire
insulation seemed cheap pvc? (not teflon). I rearranged things, just
because I had seen how dressing
had affected 10M/15M on the fl2100b.
4) Surprisingly, the input network is a Pi network (C's shunting both
sides of the L) on 160/80/40,
BUT just an LC network on 20/15/10. I know an LC is perfectly fine but was
expecting Pi on all bands.
I looked hard at this, wondering if maybe someone forgot to install some
micas, but it appears to be
the design.
I looked at the al-1200 user manual, which lists the input network values,
and it does seem to show
Pi network for all bands. It shows 3-1/2 turns for 10M
from the user manual
160M 1300pf, 28t, 2000pf
80M 1300pf, 18-1/2t, 1500pf
40M 500pf, 11-1/2t, 470pf
20M 220pf, 6-1/2t, 180pf
15M 150pf, 3-1/2t, 120pf
10M 150pf, 3-1/2t, 180pf and 27pf? (not sure why they list two here
would think it should be
100pf or so)
So this is odd. My input swr looks good now, except maybe on 40M it could
be a little better (1.6:1).
So I'm kind of thinking what I have is what Ameritron ships, but it's
different than the above in the
manual/schematic.
But maybe the above is what it's supposed to be?
Has anyone looked at an AL-1200 input network in their amp recently? Have
you seen 10/15/20 as an LC
network, or different values for L and C compared to what the schematic
lists above?
I just got done fixing my fl2100b 10m and 15m input network and had to
remove/add 1 turn and adjust
one side of the Cs. So I'm curious about all this input pi network stuff.
-kevin
ad6z
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