[Amps] Question about average anode current in tetrode amplifiers on higher frequencies.

Mon Nov 25 09:23:16 EST 2024

Is the 27pF measured with the tube in the chassis? I've been surprised
by how much extra capacitance appears when it's assembled. If you
haven't already, leave the plate RFC and the first section of other
wiring from the plate connected and measure the total C.

Steve G8GSQ
On 24/11/2024 18:09, flynth at gmail.com wrote:
> I obviously meant the tube and socket is 27pF not uF.
>
> On Sun, 24 Nov 2024, 19:08 , <flynth at 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 at 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 at 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 at 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 at 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 at 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 at 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 at 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 at 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 at contesting.com
>>>>>>>>> http://lists.contesting.com/mailman/listinfo/amps
>>>>>>>>
>>>>>>>> _______________________________________________
>>>>>>>> Amps mailing list
>>>>>>>> Amps at contesting.com
>>>>>>>> http://lists.contesting.com/mailman/listinfo/amps
>>>>>>>>
>>>>>>>
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