[Amps] AL-1200 pi input network. is manual right for correct "stock" LC v...

TexasRF at aol.com TexasRF at aol.com
Mon Dec 6 14:35:33 PST 2010

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  
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.
Gerald K5GW
In a message dated 12/6/2010 3:04:47 P.M. Central Standard Time,  
knormoyle at 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 
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.


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