Regarding the length of the leads, I had the same problem in my 4CX1000A
amplifier
<https://www.qsl.net/k2vco/4CX1000A/K2VCO%204CX1000A%20Amplifier.html>
and I had to completely rebuild the tank circuit in order to get a low
enough inductance to resonate on 10m with the correct amount of
capacitance. My problem was the length of the leads between the
bandswitch and the coil. The blocking capacitor can be mounted directly
on the tuning cap. Then you only have to worry about the lead from the
plate to the blocking cap, by way of the plate choke.
The additional inductance is needed to get a reasonable Q with an
achievable amount of capacitance. This will require a lower inductance
in the tank coil.
You do not want to use thinner wire. You should use flat strap (6 mm
wide for the bandswitch, 10mm for the plate to blocking cap) to reduce
the stray inductance as well as RF resistance.
73,
Victor, 4X6GP
Rehovot, Israel
Formerly K2VCO
CWops no. 5
http://www.qsl.net/k2vco
On 24/11/2024 20:08, flynth@gmail.com wrote:
Could you let me know where is it discussed, please? (the ham radio
handbook by Orr? I've read it multiple times, but I don't remember that).
I've added a coil in series that was stretched/compressed between 700nH
and 1.2uH.same as in SimSmith simulation it only moved the resonance of
the entire pi network LOWER. The opposite of where I need to go.
I start thinking I simply built the whole thing wrong as there is no
hope whatsoever of getting it to work on 10m with connections that are
10cm long (one between the blocking cap and the plate cap, the other
between the plate cap and the load coil). I have 20~30pF of extra
capacitance somewhere. I should've made all these connections 2-3cm
long, but i didn't realise it matters at such "low" frequencies as under
30MHz.
The whole thing resonates at 25MHz with no plate cap at all and with
load coil on 0.9uH (where it should be for 29Mhz). The tube and socket
is 27uF. SimSmith tells me I must have another 25pF somewhere to get
this result.
I'll probably try using thinner wire (less capacitance) as a last idea.
On Sun, 24 Nov 2024, 16:57 Steve Thompson via Amps, <amps@contesting.com
<mailto:amps@contesting.com>> wrote:
You certainly don't want a small series C anywhere at the tube end of
the matching, it pushes everything in the wrong direction and you end up
with higher loaded Q.
Steve G8GSQ
Thank you Steve, yes, it was a silly idea, I simulated it and it looked
nothing like it should.
73,
F
On Sun, 24 Nov 2024, 16:04 Victor Rosenthal, <k2vco.vic@gmail.com
<mailto:k2vco.vic@gmail.com>> wrote:
No, the coil goes in series with the input to the pi network.
I misspoke when I said it increases the impedance of the tube. The
impedance doesn't change; you are just converting the pi network
into one that can match the impedance of the tube to 50 ohms with a
smaller tuning capacitor. You can think of the coil as canceling
some of the tube's output capacity if you wish.
This is a well-known method of dealing with the problem of excessive
Q on the higher bands. It's discussed in the handbooks and I've used
it myself with good results.
Victor 4X6GP
On Sun, Nov 24, 2024, 16:48 <flynth@gmail.com
<mailto:flynth@gmail.com>> wrote:
I do understand the coil goes on the input of the pi tank(on the
tube side), but the series coil increases the impedance of the
tube as source (as seen by the pi tank). So it goes in the
opposite direction I need I believe.
Did you mean the extra coil is connected in parallel? This I can
understand. This will effectively lower the tube output
impedance and should allow the use of higher capacitance.
I'm just trying to simulate this with NecSIM right now. It seems
like it might work.
Is this what you were proposing?
On Sun, 24 Nov 2024, 15:39 Victor Rosenthal,
<k2vco.vic@gmail.com <mailto:k2vco.vic@gmail.com>> wrote:
I'm suggesting an L-pi not a pi-L. A pi-L is useful to
reduce needed output capacitor size and to reduce harmonics,
particularly on lower frequencies. A pi-L has an additional
inductor at the output of the network; I'm suggesting one at
the input.
I think I recall a place in the spreadsheet for this. But I
found it hard to do this analytically. You can get it into
the ballpark, but then you have to adjust the inductance
experimentally.
Victor 4X6GP
On Sun, Nov 24, 2024, 14:56 <flynth@gmail.com
<mailto:flynth@gmail.com>> wrote:
I'm not sure I'm using this spreadsheet correctly, but
the Pi-L values for the plate capacitance are lower than
normal Pi values. I'm attaching the spreadsheet, but
also pasting a screenshot using default settings as I'm
not sure if such attachments are allowed here.
On default settings the pi-L circuit also shows much
lower capacitance for the Pi-L network:
73,
F
On Sun, 24 Nov 2024, 12:07 , <flynth@gmail.com
<mailto:flynth@gmail.com>> wrote:
Thank you for your reply. I'll calculate this. I was
considering adding a small (12pF) series capacitor
that would be shorted on all other bands than 10m.
Are there some hidden dangers with this idea? It
seems it would allow me to work on 10m with minimal
modifications.
73,
F
On Sat, 23 Nov 2024, 15:32 Victor Rosenthal 4X6GP,
<k2vco.vic@gmail.com <mailto:k2vco.vic@gmail.com>>
wrote:
The Q is too HIGH, not low. 15 pf minimum
capacity on the vacuum cap is
high, when you add in the output capacitance of
the tube and the strays.
I suggest you think about adding a small
inductance (1 uh or less,
usually) between the plate and the pi net input,
converting it to an
L-pi network. This is discussed in the ARRL and
Bill Orr handbooks. It
is also in the G3SED spreadsheet. That will
increase the output
impedance of the tube so as to make it easier to
match with a practical
pi network.
If you do this, be sure to check carefully for
VHF parasitics, since it
can introduce instability.
73,
Victor, 4X6GP
Rehovot, Israel
Formerly K2VCO
CWops no. 5
http://www.qsl.net/k2vco <http://www.qsl.net/k2vco>
On 23/11/2024 15:57, flynth@gmail.com
<mailto:flynth@gmail.com> wrote:
> Hi,
>
> I've built a GU-43B based amp roughly
following Pa0fri design (shown here
>
https://pa0fri.home.xs4all.nl/Lineairs/Frinear1500/FRI1500eng.htm
<https://pa0fri.home.xs4all.nl/Lineairs/Frinear1500/FRI1500eng.htm>)
>
> My modifications are: better PSU for the
screen(thanks to this group) , no
> 160m band therefore no toroid switched in
series with the Pi tank coil. And
> instead of a 1500pf variable load cap I have
20~600pf variable plus a
> selection of extra capacitances on a switch.
My plate cap is a 15~500pF
> vacuum Jennings. Connections between
components are made with 0.2mm x 12mm
> wide coil strip and where not possible (coil
taps to band switch) 3mm
> copper wire is used.
>
> The coil is wound with 6mm coper pipe and has
two diameters. First is
> approximately 40mm, has only 6 or so turns
stretched to 80mm, then 12 turns
> on 90mm (if I remember correctly) quite close
to eachother (2~3mm apart).
> I've used my NanoVNA to set up coil taps for
the bands. The entire coil
> measures 9uF at 100kHz and works great at
80m. Anode voltage is 3200V
> falling to 3050V under load.
>
> Here is the problem. I suspect due to stray
inductances of 3mm wire used to
> connect coil taps the first tap (under 1 uH
if u remember correctly) is
> only half a turn from coil start. I didn't
think this will cause a problem,
> but I'm seeing this:
>
> On all bands up to and including 20m if I
increase my drive power to about
> 10W (CW) the amplifier consumes near 0.45A of
current (measured with a
> normal amp meter and a panel meter). It puts
out somewhere in the region of
> 1000W. If I increase the drive slightly it
goes up to 0.6A and power out is
> 1500W.
>
> The screen current is zero until drive power
reaches about 10W then it goes
> negative to go back to zero at about 13W. If
I increased it more it would
> increase rapidly and activate the protection.
This is on all bands up to
> 20m.
>
> Today I tried 10m for the very first time. I
tuned normally (peaking power
> with plate cap at very low drive, then
increase power until I see screen
> current move or it gets to target anode
current and set the load cap just
> below the peak power, same place screen
current is a little bit positive).
>
> But, I increase the drive, the amplifier
consumes the required current, but
> output power is very low. Only about 200W at
0.6A. Tuning behaves normally
> as well as screen current. I tried to see if
something is heating up with a
> thermal camera, but nothing is.
>
> I suspect the problem is somewhere in my PI
circuit. Perhaps the Q factor
> is too low?
>
> Can someone, please give me some tips where
to look in troubleshooting
> this? Is there some way I can verify this
issues as existing/resolved with
> a nanovna?
>
> Also, my drive power and input match is fine
as my driving rig has a built
> in ATU. I see a confirmation of the low
output power on my station monitor
> (oscilloscope like device).
>
> Many thanks,
> F
> _______________________________________________
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> Amps@contesting.com <mailto:Amps@contesting.com>
>
http://lists.contesting.com/mailman/listinfo/amps
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