[Amps] Question about average anode current in tetrode amplifiers on higher frequencies.
Victor Rosenthal
k2vco.vic at gmail.com
Sun Nov 24 13:54:39 EST 2024
ARRL Handbook, 2010 ed., 17.9.5 (p 17.27):
"A third possibility is the use of an additional inductance connected in
series between the tube and the tuning capacitor. In conjunction with
the Cout of the tube, the added inductor acts as an L network to
transform the impedance at the input of the pi-L network up to the
2200-ohm load resistance needed by the tube. ... Since the impedance at
the input of the main pi-L matching network is reduced, the loaded Q for
the total capacitance actually in the circuit is lower. With lower Q,
the circulating RF currents are lower, and thus tank losses are lower."
They continue with calculations using an 0.5 uh inductor. They show that
the required total capacitance at 29.7 MHz in the example is 67 pf as
opposed to 36 pf without the inductor, which is easier to realize with a
practical capacitor.
This applies equally to a plain pi (not pi-L) network, obviously.
73,
Victor, 4X6GP
Rehovot, Israel
Formerly K2VCO
CWops no. 5
http://www.qsl.net/k2vco
On 24/11/2024 20:08, 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).
> tube and
> 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
> <mailto: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
> <mailto: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
> <mailto: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 <mailto: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
> <mailto: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
> <mailto: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 <mailto: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 <http://www.qsl.net/k2vco>
>
> On 23/11/2024 15:57, flynth at gmail.com
> <mailto: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 <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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> >
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