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

[Steve Thompson]

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 at 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 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
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