I obviously meant the tube and socket is 27pF not uF.
On Sun, 24 Nov 2024, 19:08 , <flynth@gmail.com> wrote:
> Could you let me know where is it discussed, please? (the ham radio
> handbook by Orr? I've read it multiple times, but I don't remember that).
>
> I've added a coil in series that was stretched/compressed between 700nH
> and 1.2uH.same as in SimSmith simulation it only moved the resonance of the
> entire pi network LOWER. The opposite of where I need to go.
>
> I start thinking I simply built the whole thing wrong as there is no hope
> whatsoever of getting it to work on 10m with connections that are 10cm long
> (one between the blocking cap and the plate cap, the other between the
> plate cap and the load coil). I have 20~30pF of extra capacitance
> somewhere. I should've made all these connections 2-3cm long, but i didn't
> realise it matters at such "low" frequencies as under 30MHz.
>
> The whole thing resonates at 25MHz with no plate cap at all and with load
> coil on 0.9uH (where it should be for 29Mhz). The tube and socket is 27uF.
> SimSmith tells me I must have another 25pF somewhere to get this result.
>
> I'll probably try using thinner wire (less capacitance) as a last idea.
>
>
> 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
>
>
> Thank you Steve, yes, it was a silly idea, I simulated it and it looked
> nothing like it should.
>
> 73,
> F
>
>
> On Sun, 24 Nov 2024, 16:04 Victor Rosenthal, <k2vco.vic@gmail.com> wrote:
>
>> 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
>>>>>>> Amps@contesting.com
>>>>>>> http://lists.contesting.com/mailman/listinfo/amps
>>>>>>>
>>>>>>
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