The trouble with ALL the coils in the 80B is that the paper type
tube formers shrink and it becomes all but impossible to move the
slugs without destroying them. You can get lucky by puffing
locksmith's graphite powder down each end of the tubes and if you
can move the slugs back and forth a little bit each time, to pick
up the graphite, you might be able to save them.?
I bought extra slugs and so I was able to destroy a couple to get
them out. I then loaded the tubes with graphite and inserted new
slugs, which moved in and out nicely. For 80 and 160, I didn't
have any of the longer slugs, so I got by with two standard sized
ones in each tube.
Another thing is that the values of the silver mica capacitors
given in the manual do not match what is installed on the tuned
input board.
Yes, Lou is right. As per the manual, you have to adjust the
slugs while you are driving with a decent amount of RF. Give it
heaps.
The final thing is that you can adjust the 20/30 slug for either
band but not both, which had me scratching my head for ages until
I wrote to Ameritron and they told me about it. 20m is the natural
choice.
I wish you luck with the tuning. It's not a pleasant job.
73, Alek VK6APK.
On 2/05/2025 5:29 am, Louis
Parascondola via groups.io wrote:
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Hi Jim doing the sweep thing gets you in the ball park but you
need to be under full power to tune the input. ?You just need a
little tweaking for the 572b tubes. ?Ameritron never adjusted
those coils for 80 meters, the slugs were always way out of the
area of the coil windings. To zero in on the best match always
takes ending trial and error. ?I¡¯d say removing ?one turn and
trying it is a good start. ?You definitely want the slug in the
area of the coil winding for maximum ?adjustment. BTW, a flat swr
isn¡¯t necessarily going to be what forwards you maximum output.
?You may very well obtain maximum power at 1.2 to 1. ?It all
depends on the radio¡¯s filter network and how close it really is
to 50 ohms. ?I always shoot for maximum forward power while
adjusting the inputs. If the slug is in the coil winding and the
swr isn¡¯t all the way down now you¡¯ll need a cap value change. I
usually try around 20pf on the lower frequency bands readjust the
coil to see if I¡¯m going the right way, as a last resort try a
little less. ?Just be glad you don¡¯t need to do all of them. Lou
On
Thursday, May 1, 2025, 11:50 AM, AA7CL via groups.io
<aa7cl@...> wrote:
Lou,
Thank you for your suggestion. I was thinking of doing
the same. I have several capacitor values lying on the
bench to do just that.?
I've read that some have suggested using a VNA to
sweep the input by substituting a resistor for the
tubes. Have you found this to be effective? I believe
the post I read indicated 200 ohms for 572b tubes but
I don't know how accurate that is.? Not sure if you
pull the tubes, sub in a single resistor between the
cathode and ground and then sweep to see where the SWR
is at. Would of course need to operate the relay.?
Jim - AA7CL
The position of the slug tell you a big story.
?Ultimately you would?want the slug to Bering inside
of the coil windings when the SWR dips the lowest.
?That would indicate there¡¯s enough of leeway in the
inductance. ?So the positioning of the slug gives you
a clue as to what is needed. ?Say for instance the
slug is inside the winding area and you can¡¯t null the
swr at any position of the slug that tells you
probably there¡¯s not enough total inductance to the
coil and you would add a turn or two. ?If the slug is
totally nowhere near the windings and almost out of
the tube then that tells you that you have too much
and you would take one or two off. ?You got do a
simpler test by adding about 10pf to the existing caps
to see what happens if it gets worse go lower . ?At
some point trying to change the cap values and the swr
doesn¡¯t null then going to the coil may be necessary.
?Since ?you are using different tubes and since the
quality control on them is all over it¡¯s not
surprising you have this issue. ?As I recal from
working on hundreds of these amps, the 80 meter slug
is always quite far out. ?Good luck
Lou
On Wednesday, April 30, 2025, 10:50 PM, AA7CL via
groups.io <aa7cl@...> wrote:
I¡¯m working on a used
AL-811 for a friend. The 811 tubes were bad, so
I replaced them with some Penta Labs 572b tubes
(A whole another adventure). I also replaced the
tube sockets, and installed GDTs on the tube
sockets and added some bias diodes on the center
tap of the transformer. I made sure there were
not MOVs in the input board. The issue I¡¯m
having is high SWR (over 2:1) on 80 meters and I
can¡¯t get full drive from what appears as the
exciter is folding back. All the other bands are
fine.
?
I¡¯ve attempted to
adjust the 80-meter input coil with no effect on
SWR/PWR in. I¡¯ve checked the input switch for
continuity and from the input through the
80-meter input coil with no problems. Switch
alignment looks good. I remove and tested C4
(for 80 Meters) a 500pF cap and it tested good.
?I¡¯ve removed the 200-ohm padding resistor just
to see what effect it had on SWR and nothing.
Obviously, I get more output for the same amount
of drive. If I¡¯m at the high end of 75 meter
with the tuning slug all the way at the end of
the coil form, the SWR just starts to drop, and
I see an improvement in drive level. With the
padding resistor in circuit, I¡¯m showing about
50 watts for drive and 60mA of grid current.
With the amp in bypass the radio will do 120W no
SWR.
?
I¡¯m wondering about
values of C4 and if anyone else has experienced
this behavior after swapping to 572b tubes?
Don¡¯t know how the amp performed prior as the
tube replacement.
?
Thanks for any
productive comments in advance.
?
Jim ¨C AA7CL
?
