Cecil Moore wrote in
t:

Owen Duffy wrote:
These are not necessarily the same value. In fact, the dynamic source
resistance is usually much higher than the required load resistance,
and the ratio is usually higher for a pentode or tetrode than for a
triode operating at the same voltage and current.

So, immediately, there is an apparent conflict with the proposition
that the dynamic source resistance and the load resistance are the
same.

Does that take into account the step-down transformation?

Cecil,

The two previous paragraphs that you have omitted in your quote provide
the context for the paragraphs that you did quote. The context is in the
anode circuit of the PA being discussed.

The "source load" that results in the "source load line",

I don't really understand the concepts of a "source load" or "source load
line". Perhaps your meaning is the load in the anode circuit of the PA, I
will read on with that interpretation.

is not the physical load in the system. It is the physical
load in the system transformed by the transmission
line, the filters, the tank circuits, and the transformers.
In short, it is the transformed load seen directly
*by the source - at the source*.

Ok...


For instance, a source may have a dynamic source
resistance of 1000 ohms. A 20:1 tank circuit
transformation takes it to 50 ohms. The load line
for that amp has a slope of 1000, not 50.

I am not comparing apples with oranges, not comparing impedances on
different sides of the pi network.

To expand the first example with the details:
-if Ql is 10 and Ra is 1400 ohms, a 1% decrease in the extenal 50 ohm
load results in a 0.26% decrease in the anode load impedance

Rl=50, |Za|=1400.0;
Rl=49.5, |Za|=1396.4, a 0.26% decrease in |Za| for a 1% decrease in Rl.

You cannot think of a PI coupler (and the original post was discussing a
PI coupler) in this application as an idealised symmetric n:1
transformer, whilst this coupler has an apparent ratio of 28:1 (1400/50),
incremental impedance changes are in a quite different ratio.

A PI network is not in the general case symmetric, your example of a 20:1
"tank" circuit (and I would argue that "tank" is usually used to mean a
parallel tuned anode circuit, typically link coupled) is not symmetric
and the point of my post was to say that Zin/Zout is not a straight line,
and general analyses based on a fixed ratio are likely to be flawed.

Owen