|
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 – 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
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 – AA7CL
?
|
Jim,
?
You might need to add about 10pF off of
the cathode pin(s).
?
Do you have a VNA? They are swell gear
to expose tuning glitches.
?
Hal
W4HBM
|
Hal,
Yes, have a nanoVNA and spectrum analyzer with tracking gen.?
Can you please explain the methodology you propose? I'm curious to know how you use your VNA to test the input circuit of tube amp. I have read about using small non-inductive resistors between the cathode and grid to approximate load impedances and making
adjustments to the input coils but I have not done so.?
How and why would 10pF (bypass?) be necessary off the cathode pins in this case? And how would that not affect the other bands?
What is strange to me is that on 80 meters, the SWR does not change at all, even with varying drive/output levels. It also makes me wonder if I'm seeing harmonics in the input.?
Sorry for the silly questions. I'm just trying to get a better understanding of what is going on here. I'm not making sense of it.?
Thank you for your suggestion!
Jim -? AA7CL?
toggle quoted message
Show quoted text
Jim,
?
You might need to add about 10pF off of
the cathode pin(s).
?
Do you have a VNA? They are swell gear
to expose tuning glitches.
?
Hal
W4HBM
|
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
toggle quoted message
Show quoted text
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 – AA7CL
?
|
Jim,
?
In my a-building Grid-Driven 2-holer Tetrode the Breuhne
bridge used for Neutralizing the tubes has a small roller
inductor and both a fixed and variable bread-slicer.
?
Since the Input network is using a 9:1 balun transformer
it will be necessary for me to connect a 450 ohm
Non-Inductive (Dale 50Watter, Green metal case)
resistor between one of the tube socketses G1 pin
and chassis ground.
?
The VNA connects to the RF Input port and the
T/R Input relay will be picked special:
?????????????“Tune for the Least Smoke”
?????????????????????????????? Right?
?
Hal W4HBM
|
Jim,
?
For Joy!
?
Connecting the SpecAn/TG to the amp’s
output spigot, once the Input network
is run through a VNA plot set on the
input (with the N.I. resistor going...)
you will then be able to detect the
spectral purity details.
?
BUT!!!!!
?
You MUST remember to first gauge the
oomph behind the signal you wish to
analyze so that you don’t blow-out the
analyzer completely.
?
Make sure you actually recollect the
SpecAn’s maximum hold-off, and
make sure you are confident the
attenuator Pile is working like it
should. Do you have a trusty ol’
DMM that can tell you amplitude in
dBm that’s terminated with 50 ohms?
?
Boy-o-boy-o-boy.? Nerves of steel for
the man.
?
Hal
W4HBM ?
|
Hal,
?
I'm relatively new to tube amplifiers and I certainly don't know all the jargon. I assume you are meaning a " Bruene Bridge" as in Warren Bruene's capacity-bridge from the 50's? Bread-slicer as in air-variables?
?
Not sure how this applies to my questions. I believe we are talking about two different amplifier/designs.? I'm not clear if you are providing me with advice and I'm not tracking you.?
?
I think I get the concept of placing a non-inductive resistor across the cathode/grid in the amplifier that I'm working on, then enabling the TR relay and sweeping the input. My understanding is that the 572b - would require roughly a 220-ohm resistor across each tube socket. I may attempt this later today.?
?
Am I close or am I missing something? Thanks again..
?
Respectfully,
?
Jim - AA7CL
|
Dear Jim,
?
While I was describing the method for employing a
non-inductive resistor in place of the tube circuit
in a grid-driven tetrode scheme, the methodology
carries into basically the same steps in tuning
in the input circuit of a triode.
?
Does your amplifier have a way to fine-tune
the RF Input?
?
Then, yanking the tubes and soldering in
an N.I. is the least destructive method of
starting to pinpoint the sweet spot.
?
For now, leave off the SpecAn completely.
?
The VNA looking into the amplifier input
port will provide a “notch” in the VNA
linear display such as determining the
Return Loss. The reason I mentioned
adding something like a 10pF silver mica
is for you to see the result of this both
in the linear return loss graph and on
the Smith Chart display.
?
Adding tiny amounts of capacitance or
inductance will cause these displays to
change shape sharply, and may give you
a clue as to what may be needed to
move on to the 50 ohm, J-zero location
on the VNA Smith Chart.
?
A method I find useful is to actually wire
up a Pi filter on the input circuit and use
some sort of reflectometer, like a Fwd/Refl
SWR meter, and to tweak the amp input in real time,
like a “double tune-up,” but all this takes
so much time while the ionosphere is boiling
away at the speed of light.
?
Hal
W4HBM
|
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 – AA7CL
?
|
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:
toggle quoted message
Show quoted text
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 – AA7CL
?
|
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:
toggle quoted message
Show quoted text
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 – AA7CL
?
|
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:
toggle quoted message
Show quoted text
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 – AA7CL
?
|
Hal,
Thanks for the clarification. Yes, the AL-811 does have an
adjustable tuned input. This is why I mentioned the fact the slug
was at the end of the coil form.
I'm very familiar with the VNA and can use it sweep the input and
find/make the necessary adjustments.?
This amp is a 3 tub amplifier with 572b tubes. From what I have
read, this is what i understand the steps to be:
- Remove power from the amp
- Remove the tubes
- Solder in a N.I. 220-ohm resistor between the grid and cathode
pin on each tube.
- Attach a 50 ohm load to the output port of the amp
- Attach a calibrated VNA to the input
- Provide temp power to the TR relay
- Measure / adjust the 80 meter tuning coil for best SWR
Here is another question:
The load provided by the resistors are going to purely resistive.
How can I compensate for reactances associated with the tube
loading??
I assume the new tubes I installed have different loading from
the 811s originally installed in the amp and the reason I'm unable
to properly tune the input on 80 meters. Also it begs the question
as to why the other bands are fine and only 80 meters is affected?
Again, Thanks for all of your help!
Jim - AA7CL
On 5/1/25 12:14, HaL Mandel via
groups.io wrote:
toggle quoted message
Show quoted text
Dear Jim,
?
While I was
describing the method for employing a
non-inductive
resistor in place of the tube circuit
in a
grid-driven tetrode scheme, the methodology
carries into
basically the same steps in tuning
in the input
circuit of a triode.
?
Does your
amplifier have a way to fine-tune
the RF
Input?
?
Then,
yanking the tubes and soldering in
an N.I. is
the least destructive method of
starting to
pinpoint the sweet spot.
?
For now,
leave off the SpecAn completely.
?
The VNA
looking into the amplifier input
port will
provide a “notch” in the VNA
linear
display such as determining the
Return Loss.
The reason I mentioned
adding
something like a 10pF silver mica
is for you
to see the result of this both
in the
linear return loss graph and on
the Smith
Chart display.
?
Adding tiny
amounts of capacitance or
inductance
will cause these displays to
change shape
sharply, and may give you
a clue as to
what may be needed to
move on to
the 50 ohm, J-zero location
on the VNA
Smith Chart.
?
A method I
find useful is to actually wire
up a Pi
filter on the input circuit and use
some sort of
reflectometer, like a Fwd/Refl
SWR meter,
and to tweak the amp input in real time,
like a
“double tune-up,” but all this takes
so much time
while the ionosphere is boiling
away at the
speed of light.
?
Hal
W4HBM
|
Hi Jim,
?
If you detune the front end the reflected or
returned Smith chart will show greatly, yes?
?
Only by running the entire amp together is
it possible to get the most bang for buck.
?
As many as there are of builders there will
be separate and distinct philosophies about
how to Tune the Whole Thing.
?
The problem with using a SpecAn with the amp
going is, as we talked about earlier, dealing with
Sampling Attenuation. If an analyzer’s only rated
at let’s say 15dBm, then hitting it with 13dBm is
still calling it close, but nevertheless the practice
might be the best way to see what’s hiding in them
thar hills.
?
I use a stack of Inmet model 2N-3dB N-connector
attenuators on the analyzer Port 1, but also run a
bench DMM with good ability to portray dBm,
because, Jim, I don’t trust anything in RF to not
be lyin’.
?
Once the tubes are humming and your exciter is
driving the amp a good sampling pile and your SpecAn
would be perfect. All you need do then is to tweak the
input coils and watch the ‘scope.
Hal
W4HBM
?
|
Lou,
Thanks for your reply. Yes, I've noticed from pictures and videos
that the 80 meter coil slug is always at the end of the coil form.
Very strange!
The coil is not easy to get to without a bunch of disassembly. I
would like to be very confident it the need to add or remove a
winding before I attempt to do so. I realize the effect loading
has on the input impedance. Any drive level changes effect loading
etc. Adjust the input, see an increase in drive level, readjust
the amp, re-measure PWR/SWR input. Oh, your comments regarding
measuring power vs SWR is spot on! When attempting to adjust at
max power before, it was much easier to view power out of the
exciter rather the SWR.
In thinking about this issue, there is something that keeps
coming to mind. I noticed while troubleshooting this issue,
measured SWR/Power out of the exciter did not change at all with
respect to amplifier loading or tuning coil adjustment. That seems
very strange to me. The SWR measured roughly 2.4:1 and exciter
drive was limited to 50 watts. I'm assuming the exciter was
folding back. I may connect another radio today with an on-board
antenna tuner to see how it reacts.?
Also, along with the tube upgrades to this amp, I installed some
Bias diodes from CTR engineering. This would no way affect input
impedances correct? If so, I would need someone to explain that!
Thanks again Lou!
Jim - AA7CL
On 5/1/25 14:29, Louis Parascondola via
groups.io wrote:
toggle quoted message
Show quoted text
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 – AA7CL
?
|
No bias diodes won’t affect the input tuning. ?On higher bands if the neutralization is off you’ll notice varying input swr with the load control, but you may still get drive with no foldback. ?That’s a different story altogether than on lower frequencies. ?Those amps are kinda crazy, I work on them all the time. ?The latest one showed perfect neutralization on 10 meters and on all other bands except 15. ?15 made normal power so I left it. ?Generally if 10 meters is neutralized the rest are too days W8 JI. ?Additionally I’ve found that the neutralizing plate might want to be exactly where the input rod is. ?Sometimes I cut down the plate. ?Lou
On Friday, May 2, 2025, 11:44 AM, AA7CL via groups.io <aa7cl@...> wrote:
Lou,
Thanks for your reply. Yes, I've noticed from pictures and videos
that the 80 meter coil slug is always at the end of the coil form.
Very strange!
The coil is not easy to get to without a bunch of disassembly. I
would like to be very confident it the need to add or remove a
winding before I attempt to do so. I realize the effect loading
has on the input impedance. Any drive level changes effect loading
etc. Adjust the input, see an increase in drive level, readjust
the amp, re-measure PWR/SWR input. Oh, your comments regarding
measuring power vs SWR is spot on! When attempting to adjust at
max power before, it was much easier to view power out of the
exciter rather the SWR.
In thinking about this issue, there is something that keeps
coming to mind. I noticed while troubleshooting this issue,
measured SWR/Power out of the exciter did not change at all with
respect to amplifier loading or tuning coil adjustment. That seems
very strange to me. The SWR measured roughly 2.4:1 and exciter
drive was limited to 50 watts. I'm assuming the exciter was
folding back. I may connect another radio today with an on-board
antenna tuner to see how it reacts.?
Also, along with the tube upgrades to this amp, I installed some
Bias diodes from CTR engineering. This would no way affect input
impedances correct? If so, I would need someone to explain that!
Thanks again Lou!
Jim - AA7CL
On 5/1/25 14:29, 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 – AA7CL
?
|
LEAVE the tubes in their sockets? when doing your input swr tests.? ?The tube's? grid to cathode C? is directly in parallel? with the C2? cap of each PI tuned input. (5.9 pf PER tube).?
If this is the 3 x tube amp, input Z? is? 220 /3 = 73.33 ohms.? ?You only require just one resistor that's close to 73 ohms...and temp wire it in between the grid + cathode of just ONE of the 3 x tubes? ( any tube, but just one tube).?
If it's the 4 x holer, it's just 220 / 4 =? 55 ohms.? ?Same deal.? ?55 ohms from grid to cathode of any one tube.?
You don't require a SA or anything other test gear.? ?The vna? or any other analyzer is all that's required.?
|
On 5/3/2025 5:40 AM, Jim VE7RF via groups.io wrote: You don't require a SA or anything other test gear.? ?The vna? or any other analyzer is all that's required. Not even that, if you have a late-model radio with built-in SWR meter: simply turn your drive all the way down, like to 5 watts or less, then set your S-meter to read SWR. If the 572 filaments aren't lit, they can't draw grid current, so no worries about blowing out the grids. Steve, K0XP
|
HaL Mandel