On Mon, 25 Nov 2024, 16:30 Victor Rosenthal 4X6GP, <k2vco.vic@gmail.com>
wrote:
That amp was good for more than 1500w on all bands. There are many
commercial and homebrew amps using tetrodes that can do this.
This is very good to know.
Neutralization isn't relevant to the problem that you are having.
The point with neutralisation was that it could cancel the anode
capacitance, but as your amp works without it, my should too.
The Q that we have been talking about in this discussion is the the Q of
the tank circuit, not the Q of the coil. This Q is determined by the L/C
ratio of the pi-network and the output impedance of the tube, not the
X/R of the coil.
Let's be precise. (Correct me if I'm wrong here). For resonance both the
reactances of the inductance and capacitance have to cancel out. Let's say
the tube output impedance is 2000 Ohm, if we take capacitance of 50pF that
at 29MHz has reactance of 109ohm. So a resonant circuit with this
capacitance would have a Q of ~18. This tells us this capacitance is non
optimal as we want to hit about 12 (which we do at about 27pF - exactly the
capacitance of the tube in the socket).
If we're fine with Q of 14 we can have capacitance of 38pF. That's what I'm
attempting to hit now. I'll make the plate cap about 5pF, add 5pF for stray
capacitance and it will hopefully work. Your comment that it works
encourages me to try it.
Also it certainly wouldn't take 1m of tubing to make a
1 uh inductor! I think a decimal point is misplaced here!
A decimal point was certainly misplaced in the email that went to you
before, but not where I talk about the length of tubing :-)
Somewhere around 70-90cm is what you could use if you want really high Q
coil. For example you could do 0.75m if you do 4 turns at 60mm diameter,
6mm copper pipe 36mm total coil length. But you can get a lot better
inductance per cm if you go even bigger, for example 11 cm diameter. 2.5
turns. 86cm long.
I belive Jim in previous email even mentioned half turn coils.
Let me try to summarize:
The problem in your amp, as far as I can tell from your description, is
that it's hard to get a LOW enough OVERALL Q of the tank circuit because
of the minimum capacitance of your tuning capacitor plus output capacity
plus strays. One solution to this is to add a small coil between the
plate and the input of the pi-network (probably less than 1 uh). This
transforms the impedance of the tube to a lower value that's easier to
match with a practical pi-network.
It's important to keep the leads between the coils, capacitor, and
bandswitch short so that the major part of the inductance is in the
coils and not in the leads. It's possible to build a tank circuit that
will not resonate at all on 10m if you are not careful about this (I've
done it).
If you're using the G3SEK spreadsheet, there is a place to specify the
inductance of the small coil. But you have to adjust it experimentally
for best results.
If your tube needs a parasitic suppressor, you can make the coil a
little bigger and solder a noninductive resistor across a section of it.
This also requires experiment to find the best spot to put the tap.
I'll send you the page you requested. But I think all the information
you need is here.
Thank you. I'll try it and see.
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