A quick question, does anyone on this list have a working amplifier that
gets to it's rated output on 10m with a grid driven tetrode with no
neutralisation? If so, can you post a picture of its pi tank?
73,
F
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
>
> On 24/11/2024 14:10, 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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