Re: Tube Coil, impedance matching

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jlnance-at-intrex-dot-net>

On Mon, Aug 13, 2001 at 08:15:56PM -0600, Tesla list wrote:
> Original poster: "Sundog by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<sundog-at-timeship-dot-net>

>  When it comes to impedance matching, I'm a dimwit.  The only radio amatuers
> handbook I could find talked down about tube gear like it was inferior or
> some crap, I got the idea the author just didn't know squat about tubes, or
> was too lazy to study them well.

>  So, I suppose you'd have to start from the beginning....  I'm sure there's
> more non-mathheaded tube coilers like myself who are itching to figure this
> out too :)

Well, I dont know exactly what part of the circuit you are trying to match,
so I will talk about impedance matching in general.  Impedance is the ratio
of voltage to current (volts/amps).  For DC circuits it is the same thing
as resistance, and I am going to pretend that its the same for AC circuits
too, at least for this discussion.  Thats not always true, but its good enough
for now.

Now lets talk about Thevinin's Theorem.  Thevinin figured out that if I have
ANY circuit composed of voltage sources, current sources, resistors or any
other linear component, and I connect 2 wires to any two points in that
circuit, at the other end of the wires it looks like I have a voltage source
in series with a resistor.  To summarize with a picture, any linear circuit
with two connections to the outside world looks like this to the outside
world:

        +-----------------\/\/\/-----o
        |                  Rth
      /---\
     |     | Vth
     |     |
      \---/
        |
        +----------------------------o

Now you have this circuit with two terminals.  Let say you want to connect
a resistor across these terminals and you want this resistor to absorb as
much power as possible.  What value should this resistor have?  If you work
through the math, you will find that you tranfer maximum power when the value
of this resistor has the same value as Rth.  This is the essence of impedance
matching.  You are trying to transfer maximum power from one circuit to
another, and you do this by ensuring that their Thevinin resistances are
equal.

Now lets get back to tubes.  A tube is not a linear component, so if we put
tubes in a circuit, it is not strictly correct to represent it as a Thevenin
equivalent.  We can however approximate it as a Thevenin equivalent, and it
is extreamly useful to do so.

I got to go to work now.  Ill write some more latter.  Let me know if this
is useful, and tell me exactly what you are trying to match.

Thanks,

Jim