Re: Plate blocking cap in Alpha 76PA
Same here, used exactly
the same type to get more capacitance on 160m as a HT bypass capacitor
It exploded after a few
minutes.
I assume those capacitors
are not suitable for RF.
?
73
Peter, DJ7WW
?
?
?
Regarding replacing
the plate blocking caps….if it ain’t broke, don’t fix
it.?
?
I blew up one of
those red plate blockers by Ceramite.? It turns out that they are not
rated for significant RF current, so…
?
Keep the original
caps would be my suggestion…
?
-Chuck K1KW
?
?
I am doing some updating to my Alpha 76PA 3-tube
amplifier.? I have switched the T/R relay to a pair of vacuum relays and
changed the "relay" (AKA Send or PTT) request to send from 24 Volts
to be a compatible 12 Volts for modern transceivers.? I am about to
replace the original Alpha Plate Choke with a re-wound Ameritron style taller
choke.
While I'm at it I have a, new-but old, Cera-Mite 5000
PF, 15 KV door-knob capacitor.? I'm planning to replace the original plate
blocking capacitors with this door knob cap.? The original plate blocking
cap uses dual original 2000 PF, 6 KV ceramic disk capacitors.
I've has this door knob cap a long time and I'm no
longer planning on building a tube amplifier from scratch.? As you can see
I've fabricated two copper straps to place the cap in the 76pa amp.? I
fabricated these straps to put to door knob cap into a previous Alpha 76PA I
had.? And when fastened in place it is quite sturdy and has adequate
clearance from its surroundings.
This amplifier will be kept in my station as a back-up
for my Solid State PA.? I've found SSPAs to be less tolerant of operator
errors than tube amps.? But I still like the instant on and 6 meter
capability in my current SSPA (403A/Flex PGXL).
What are your thoughts on this replacement other than
it is overkill in this situation?
|
Re: Plate blocking cap in Alpha 76PA
Dear Dave,
?
The ‘PA makes a great project amp.
?
I went with dual 3CPX800A7’s and
slammed together an outboard power
supply. The power supply was way
heavier than the RF head.
?
I replaced the blocker with a single
HT58 ceramic doorknob and put in
a new plate choke with a tinier heavier
wire gauge and a doorknob at the
bottom.
?
?
Hal
W4HBM
|
Re: Plate blocking cap in Alpha 76PA
Regarding replacing the plate blocking caps….if it ain’t broke, don’t fix it.? ? I blew up one of those red plate blockers by Ceramite.? It turns out that they are not rated for significant RF current, so… ? Keep the original caps would be my suggestion… ? -Chuck K1KW ?
toggle quoted message
Show quoted text
From: [email protected] [mailto:[email protected]] On Behalf Of Dave w6de
Sent: Wednesday, May 07, 2025 12:26 AM
To: [email protected]
Subject: [ham-amplifiers] Plate blocking cap in Alpha 76PA? I am doing some updating to my Alpha 76PA 3-tube amplifier.? I have switched the T/R relay to a pair of vacuum relays and changed the "relay" (AKA Send or PTT) request to send from 24 Volts to be a compatible 12 Volts for modern transceivers.? I am about to replace the original Alpha Plate Choke with a re-wound Ameritron style taller choke. While I'm at it I have a, new-but old, Cera-Mite 5000 PF, 15 KV door-knob capacitor.? I'm planning to replace the original plate blocking capacitors with this door knob cap.? The original plate blocking cap uses dual original 2000 PF, 6 KV ceramic disk capacitors. I've has this door knob cap a long time and I'm no longer planning on building a tube amplifier from scratch.? As you can see I've fabricated two copper straps to place the cap in the 76pa amp.? I fabricated these straps to put to door knob cap into a previous Alpha 76PA I had.? And when fastened in place it is quite sturdy and has adequate clearance from its surroundings. This amplifier will be kept in my station as a back-up for my Solid State PA.? I've found SSPAs to be less tolerant of operator errors than tube amps.? But I still like the instant on and 6 meter capability in my current SSPA (403A/Flex PGXL). What are your thoughts on this replacement other than it is overkill in this situation?
|
Plate blocking cap in Alpha 76PA
I am doing some updating to my Alpha 76PA 3-tube amplifier.? I have switched the T/R relay to a pair of vacuum relays and changed the "relay" (AKA Send or PTT) request to send from 24 Volts to be a compatible 12 Volts for modern transceivers.? I am about to replace the original Alpha Plate Choke with a re-wound Ameritron style taller choke.
While I'm at it I have a, new-but old, Cera-Mite 5000 PF, 15 KV door-knob capacitor.? I'm planning to replace the original plate blocking capacitors with this door knob cap.? The original plate blocking cap uses dual original 2000 PF, 6 KV ceramic disk capacitors.
?
?
I've has this door knob cap a long time and I'm no longer planning on building a tube amplifier from scratch.? As you can see I've fabricated two copper straps to place the cap in the 76pa amp.? I fabricated these straps to put to door knob cap into a previous Alpha 76PA I had.? And when fastened in place it is quite sturdy and has adequate clearance from its surroundings.
?
This amplifier will be kept in my station as a back-up for my Solid State PA.? I've found SSPAs to be less tolerant of operator errors than tube amps.? But I still like the instant on and 6 meter capability in my current SSPA (403A/Flex PGXL).
What are your thoughts on this replacement other than it is overkill in this situation?
?
73,
Dave, w6de
|
I like to fit a C1 capacitor capable of withstanding the full EHT, and/or..... add an RFC between J1 and earth to protect the radio in case of flashover and C1 fail.
?
?
|
Jim, this worked great! I confirmed that the SWR reading on my meter was pretty close. I just used an antenna analyzer, and it read 1.8:1 with the slug all the way out. One of the oddities I noticed was that someone had used what appears to be a film cap on the padding resistor. I decided to change it using a ceramic. This really improved my drive level, and I now have enough loading control. SWR is a bit better also. Not sure it's worth the hassle of chasing it.
Film Cap:
?
Now I'm having issues with noise in RX and grid meter movement on the upper bands when the plate circuit is brought into resonance using the plate/load controls. I tried manipulating the parasitic choke inductor winding distances without change. Not sure if this is a biasing issue or some other component failure.?
?
Looking for ideas!
?
Jim - AA7CL
|
Clipperton V/144 MHz On 6M?
I've seen reference to a "6M Kit" for the Clipperton V 2 Meter amplifier (uses a single 4CX250B). Anybody have any pix or drawings of this conversion? It seems to me that this amplifier was essentially a copy of an article I saw in 73 Magazine way back in the late 60s or early 70s where the guy buillt a pretty compact amplifier using a 4X150A with a tank circuit that covered both bands, depending upon which output connector was connected to the antler; I think it was link-coupled on 144 MHz, but a conventional pi-net on 6M?? I don't rember what he did about the input circit. I have another 800W amp for 2M, so I'm thinking of converting this one to 6M instead. (And if I ever acquire an Alpha 8406, then I'll convert the Clipperton to 220! 8-)
TNX,
Steve K0XP
|
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
|
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.?
|
Hi Alex,
Will do , thanks for the tip , there is something wrong.
73 Philip 9H1PA?
I asked about the input cap because a couple of years ago, a
friend, VK6OX,had a problem with his dual GI7b 6m amp. It took
forever, for VK6OX, VK6JX and myself to track down but it ended up
being the input tune cap shorted due to a flashover in one of the
tubes.
In the article you quoted, input coupling cap, C1 is between the
input socket and the input tune network. In this configuration, if
input tuning variable cap C2 is shorted, it will ground the
cathode. The grid is already grounded, so you effectively have a
grid to cathode short via the capacitor.?
If C1 is on the cathode side of the network, this can't happen.
In that configuration, if somehow C2 is short circuit, the tube
will behave normally DC wise but your input RF will be shorted to
ground.
Anyway. Check that out.
73, Alek VK6APK.
On 3/05/2025 2:01 pm, Philip via
wrote:
Hi Alex,
Tnx for your reply. So the changes were not all at once. It
was over a long time, one by one to?
try and find the cause
Let me reply to your questions.
Have you hi pot tested the GS35b between cathode and
grid? Are you able to swap the old tube back easily?
I tested the Old tube and New tube with Heater On and saw a
good Grid to Cathode current as a Ham instructed
me to do. If I remeber correctly he told me a very good
valve will have about 8 mA current after about 30 mins, and
good
one will have around 4 mA, under 3 mA the vacuum in the
tube would have started to go soft and can be repaired with
some prolong running of Heater but not always. New tube is
nearly 5 mA, old ones little over 3 mA.
Have you checked the heater feed through cap?
Yes I have changed the feed through caps with new ones same
problem.
Also changed HT feed through. The HT Supply is used also
for another
GS35b Amp and works 100% OK, so I eliminated the power
supply from the equation.
Was the heater transformer a like for like
replacement??
The old transformer was changed as it was an Oil Filled one
and was leaking
New one was from another GS35 amp and voltage adjusted to
12.6V .
Is there a DC blocking cap between the input network
and the cathode? If not, then check the input tune
variable capacitor/s for short/s.?
Yes there is 1nF Blocking Door Knob capacitor leading to
the input tuning network. I have not checked anything there, I
was terminating
the input while applying HT.
Hope I have explained myself well, if anything not clear
please let me know. Thanks for the help.
73 Philip
9H1PA
Seems like you've changed too many things at once.?
A few guesses off the top of my head.
Have you hi pot tested the GS35b between cathode and
grid? Are you able to swap the old tube back easily?
Have you checked the heater feed through cap?
Was the heater transformer a like for like replacement?
Is there a DC blocking cap between the input network and
the cathode? If not, then check the input tune variable
capacitor/s for short/s.
Good luck, Alek VK6APK.
On 2/05/2025 5:07 am, Philip via wrote:
Hi All,
I am new to the Group, a friend of mine suggested
to ask my problem here as there are many capable Hams
on this group.
I have a GS35 on 2m and has been working for years,
I have changed the tube during it's life and blown a
bias transistor here and there.
The problem I am having now is one I have never
seen. When I apply HT to the Amp usually the Amp is
cut-off with the cut-off resistor and no current
flows. Now I get Negative Grid current and Anode
current starts to flow, and the more I increase the
Anode say from 1000v to 1500v the current increases.
The things I have changed in the Amp:
Tube - New tested good tube installed.
Bias Board - tested adjustable from 0 to 55 v
Filament Transformer
HT & Heater feed-thru cap.
Usually when the Grid Bias transistor goes - I am
using a BDX66B from DL4MEA design (?
) the amp does not do anything so not sure that Bias
is the problem.
I anyone might have come across this negative grid
current problem please let me know.
73 Philip
9H1PA
|
Re: Vernier Drive Replacement - Ameritron AL series amp
I tackled the problem by taking the amp apart, removing the capacitor and the ball drive, cleaning and lubricating both. The front panel of the amp will have to be removed to get the ball drive out. Both Tune and Load capacitors have to come out to get to the screws that hold the capacitors and the ball drives in. Not for the faint of heart. You can try a shot or 2 of WD-40 with an extended spray attachment to get some lube into the ball drive at a minimum. New ball drives are available on the Internet for ~ 45.00.
Jackson Bros. in England make them. You may want to get someone to help you with this, or seek out a repair shop.
?
73' Dan W2NO
|
I asked about the input cap because a couple of years ago, a
friend, VK6OX,had a problem with his dual GI7b 6m amp. It took
forever, for VK6OX, VK6JX and myself to track down but it ended up
being the input tune cap shorted due to a flashover in one of the
tubes.
In the article you quoted, input coupling cap, C1 is between the
input socket and the input tune network. In this configuration, if
input tuning variable cap C2 is shorted, it will ground the
cathode. The grid is already grounded, so you effectively have a
grid to cathode short via the capacitor.?
If C1 is on the cathode side of the network, this can't happen.
In that configuration, if somehow C2 is short circuit, the tube
will behave normally DC wise but your input RF will be shorted to
ground.
Anyway. Check that out.
73, Alek VK6APK.
On 3/05/2025 2:01 pm, Philip via
groups.io wrote:
toggle quoted message
Show quoted text
Hi Alex,
Tnx for your reply. So the changes were not all at once. It
was over a long time, one by one to?
try and find the cause
Let me reply to your questions.
Have you hi pot tested the GS35b between cathode and
grid? Are you able to swap the old tube back easily?
I tested the Old tube and New tube with Heater On and saw a
good Grid to Cathode current as a Ham instructed
me to do. If I remeber correctly he told me a very good
valve will have about 8 mA current after about 30 mins, and
good
one will have around 4 mA, under 3 mA the vacuum in the
tube would have started to go soft and can be repaired with
some prolong running of Heater but not always. New tube is
nearly 5 mA, old ones little over 3 mA.
Have you checked the heater feed through cap?
Yes I have changed the feed through caps with new ones same
problem.
Also changed HT feed through. The HT Supply is used also
for another
GS35b Amp and works 100% OK, so I eliminated the power
supply from the equation.
Was the heater transformer a like for like
replacement??
The old transformer was changed as it was an Oil Filled one
and was leaking
New one was from another GS35 amp and voltage adjusted to
12.6V .
Is there a DC blocking cap between the input network
and the cathode? If not, then check the input tune
variable capacitor/s for short/s.?
Yes there is 1nF Blocking Door Knob capacitor leading to
the input tuning network. I have not checked anything there, I
was terminating
the input while applying HT.
Hope I have explained myself well, if anything not clear
please let me know. Thanks for the help.
73 Philip
9H1PA
Seems like you've changed too many things at once.?
A few guesses off the top of my head.
Have you hi pot tested the GS35b between cathode and
grid? Are you able to swap the old tube back easily?
Have you checked the heater feed through cap?
Was the heater transformer a like for like replacement?
Is there a DC blocking cap between the input network and
the cathode? If not, then check the input tune variable
capacitor/s for short/s.
Good luck, Alek VK6APK.
On 2/05/2025 5:07 am, Philip via wrote:
Hi All,
I am new to the Group, a friend of mine suggested
to ask my problem here as there are many capable Hams
on this group.
I have a GS35 on 2m and has been working for years,
I have changed the tube during it's life and blown a
bias transistor here and there.
The problem I am having now is one I have never
seen. When I apply HT to the Amp usually the Amp is
cut-off with the cut-off resistor and no current
flows. Now I get Negative Grid current and Anode
current starts to flow, and the more I increase the
Anode say from 1000v to 1500v the current increases.
The things I have changed in the Amp:
Tube - New tested good tube installed.
Bias Board - tested adjustable from 0 to 55 v
Filament Transformer
HT & Heater feed-thru cap.
Usually when the Grid Bias transistor goes - I am
using a BDX66B from DL4MEA design (?
) the amp does not do anything so not sure that Bias
is the problem.
I anyone might have come across this negative grid
current problem please let me know.
73 Philip
9H1PA
|
Hi Alex,
Tnx for your reply. So the changes were not all at once. It was over a long time, one by one to? try and find the cause Let me reply to your questions.
Have you hi pot tested the GS35b between cathode and grid? Are you able to swap the old tube back easily? I tested the Old tube and New tube with Heater On and saw a good Grid to Cathode current as a Ham instructed me to do. If I remeber correctly he told me a very good valve will have about 8 mA current after about 30 mins, and good one will have around 4 mA, under 3 mA the vacuum in the tube would have started to go soft and can be repaired with some prolong running of Heater but not always. New tube is nearly 5 mA, old ones little over 3 mA.
Have you checked the heater feed through cap? Yes I have changed the feed through caps with new ones same problem. Also changed HT feed through. The HT Supply is used also for another GS35b Amp and works 100% OK, so I eliminated the power supply from the equation.
Was the heater transformer a like for like replacement?? The old transformer was changed as it was an Oil Filled one and was leaking New one was from another GS35 amp and voltage adjusted to 12.6V .
Is there a DC blocking cap between the input network and the cathode? If not, then check the input tune variable capacitor/s for short/s.? Yes there is 1nF Blocking Door Knob capacitor leading to the input tuning network. I have not checked anything there, I was terminating the input while applying HT.
Hope I have explained myself well, if anything not clear please let me know. Thanks for the help.
73 Philip 9H1PA
Seems like you've changed too many things at once.?
A few guesses off the top of my head.
Have you hi pot tested the GS35b between cathode and grid? Are
you able to swap the old tube back easily?
Have you checked the heater feed through cap?
Was the heater transformer a like for like replacement?
Is there a DC blocking cap between the input network and the
cathode? If not, then check the input tune variable capacitor/s
for short/s.
Good luck, Alek VK6APK.
On 2/05/2025 5:07 am, Philip via
wrote:
Hi All,
I am new to the Group, a friend of mine suggested to ask my
problem here as there are many capable Hams on this group.
I have a GS35 on 2m and has been working for years, I have
changed the tube during it's life and blown a bias transistor
here and there.
The problem I am having now is one I have never seen. When
I apply HT to the Amp usually the Amp is cut-off with the
cut-off resistor and no current flows. Now I get Negative Grid
current and Anode current starts to flow, and the more I
increase the Anode say from 1000v to 1500v the current
increases.
The things I have changed in the Amp:
Tube - New tested good tube installed.
Bias Board - tested adjustable from 0 to 55 v
Filament Transformer
HT & Heater feed-thru cap.
Usually when the Grid Bias transistor goes - I am using a
BDX66B from DL4MEA design (?
) the amp does not do anything so not sure that Bias is the
problem.
I anyone might have come across this negative grid current
problem please let me know.
73 Philip
9H1PA
|
Seems like you've changed too many things at once.?
A few guesses off the top of my head.
Have you hi pot tested the GS35b between cathode and grid? Are
you able to swap the old tube back easily?
Have you checked the heater feed through cap?
Was the heater transformer a like for like replacement?
Is there a DC blocking cap between the input network and the
cathode? If not, then check the input tune variable capacitor/s
for short/s.
Good luck, Alek VK6APK.
On 2/05/2025 5:07 am, Philip via
groups.io wrote:
toggle quoted message
Show quoted text
Hi All,
I am new to the Group, a friend of mine suggested to ask my
problem here as there are many capable Hams on this group.
I have a GS35 on 2m and has been working for years, I have
changed the tube during it's life and blown a bias transistor
here and there.
The problem I am having now is one I have never seen. When
I apply HT to the Amp usually the Amp is cut-off with the
cut-off resistor and no current flows. Now I get Negative Grid
current and Anode current starts to flow, and the more I
increase the Anode say from 1000v to 1500v the current
increases.
The things I have changed in the Amp:
Tube - New tested good tube installed.
Bias Board - tested adjustable from 0 to 55 v
Filament Transformer
HT & Heater feed-thru cap.
Usually when the Grid Bias transistor goes - I am using a
BDX66B from DL4MEA design (?
) the amp does not do anything so not sure that Bias is the
problem.
I anyone might have come across this negative grid current
problem please let me know.
73 Philip
9H1PA
|
Re: Vernier Drive Replacement - Ameritron AL series amp
I'll take another look in there but looks like a b____ to get to it. I could have done this in my earlier years but eyesight is not what it used to be. How did you tackle the problem?
?
73,
Garry?
|
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
?
|
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
?
|
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
?
|
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
|
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 Mandel
