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
>>>>> > _______________________________________________
>>>>> > Amps mailing list
>>>>> > Amps@contesting.com
>>>>> > http://lists.contesting.com/mailman/listinfo/amps
>>>>>
>>>>> _______________________________________________
>>>>> Amps mailing list
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>>>>>
>>>>
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