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

[Victor Rosenthal 4X6GP]

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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