Alek,
The tuning coils in this AL-811 are fine and easy to turn.
Thank you,
Jim - AA7CL
On 5/1/25 16:35, Alek Petkovic via
groups.io wrote:
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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:
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
?
