Is the 27pF measured with the tube in the chassis? I've been surprised
by how much extra capacitance appears when it's assembled. If you
haven't already, leave the plate RFC and the first section of other
wiring from the plate connected and measure the total C.
Steve G8GSQ
On 24/11/2024 18:09, flynth@gmail.com wrote:
I obviously meant the tube and socket is 27pF not uF.
On Sun, 24 Nov 2024, 19:08 , <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>
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> 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> 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>
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> 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> 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> 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
On 23/11/2024 15:57, 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)
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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