Re: 12 KW CCS ON 160-15M....USING THE 3CX-6000A7..... PART 25
Here is the part 25 u tube link.?
https://www.youtube.com/watch?v=-BpADwJMbOk
|
12 KW CCS ON 160-15M....USING THE 3CX-6000A7..... PART 25
Here, the soft start is wired up. Paralleled 25 ohm, 50 watt resistors used. Since there is a total of 125 uf for the filter cap assy, the step start has to be done very carefully, with the requisite delay...typ 5-10 seconds. 12.5 ohms is used because if say 25 ohms used....the B+ will come up to almost max, but won't reach max, since the magnetizing current on the paralleled plate xfmr's primary's will still flow through any step start resistor...creating a V drop across the step start resistor assy....so you never get the full 240 vac applied to the xfmr's primary. Once shunted, you end up with a secondary surge. To minimize the secondary surge, a lower value is used for the step start resistor assy. Regardless of value, when T=0, you always end up with the full 240 vac dropped across the resistor assy.
Paralleled 4 ga teflon wire used. Mercury contactors used. I found a pair of 250 amp supercon connectors, cheap, on ebay, so notified Scott asap....and he snagged em. These go on the power cable ......and plugged into the mating (existing jacks) on the back of the HV supply. This supply gets a bit more complex, since the pair of identical 7.5 kva (5300 Vac) plate xfmr's will reside in a separate cabinet. The HV supply is getting there.....albeit slowly, due to time constraints.
Note you can turn on the close captioning...and also the entire transcript.
|
Re: Plate blocking cap in Alpha 76PA
Thank
you Peter and Chuck. ?I have discarded the Cera-Mite capacitor and ordered a .005 door-knob cap, with similar dimensions to the Cera-Mite, from Surplus sales. While waiting, I am finishing up
the current work on the Alpha 76PA and will close it up for testing.?
When the new cap arrives next week, I¡¯ll open the Alpha and retro-fit the new cap into the Amp.
?
73,
Dave, w6de
?
toggle quoted message
Show quoted text
From: [email protected]
<[email protected]> On Behalf Of Peter Voelpel via groups.io
Sent: 7 May, 2025 22:14
To: [email protected]
Subject: Re: [ham-amplifiers] 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?
|
FS: SPE Expert 1.5K-FA in exceptional condition
On behalf of the estate of local KE5BR, available is an SPE Expert 1.5K amp in excellent condition.
The amp was recently brought up to Series 2 specs with all factory updates being applied by Main Trading Company of Paris TX.
Includes original box and documentation.? $3500 cash, no trades.
Pickup near Kansas City.? Or for personal delivery at the Xenia Hamvention next week.? Sorry, no shipping.
All proceeds to the surviving XYL.
Should you have interest in this amplifier, kindly contact me directly off list.? And thank you for your consideration.
--
73/jeff/ac0c
alpha-charlie-zero-charlie
www.ac0c.com
|
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 ¨C AA7CL
?
|
HaL Mandel
