Adding an inductor between the tube and the pi network creates a new
network which can perform the necessary impedance transformation with a
more practical value for the tuning capacitor. If it helps, you can think
of it as canceling out some of the output capacity of the tube.
What I said about raising the impedance of the tube was incorrect. I don't
know what I was thinking.
Victor 4X6GP
On Sun, Nov 24, 2024, 16:13 <flynth@gmail.com> wrote:
> >
> > 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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