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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