>
> I'm not sure I'm using the G3SEK spreadsheet correctly or if the advice is
not to my problem, but the Pi-L values for the plate capacitance are lower
Pi L2-Less values.
On default settings the pi-L circuit also shows much lower plate
capacitance for the Pi-L network.
Furthermore, if I increase tube anode impedance, this requires less, plate
capacitance, not more I think. So adding an inductor between the tube and
the input of the pi network is taking me in the opposite direction from
where I need to go.
Is this correct?
Any feedback on the series capacitor idea? I'm not seeing any options that
would work without a switchable element at 10m.
Regards,
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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>>>
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>>>
>>
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