Oops. I was talking about the AL-80B amp. I didn't read the
header.?
If the 811 has the same tuning tubes, you'll have the same
problems.
Alek.
On 2/05/2025 7:33 am, Alek Petkovic via
groups.io wrote:
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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:
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
?
