"John Byrns" wrote in message
...

Hi Ian,

I have two problems with your view of "NFB".

More than two, in my view.

First were is it written that in order to have "NFB" you must sum
voltages?

The control input to a triode is a voltage.

I have seen analog "NFB" systems where the signals being summed
were represented by air pressure, so why not create "NFB" by
summing
electric fields? In the triode the input and output voltages are
transduced to electric fields, and those electric fields are then
combined
within the triode to create "NFB".

The control input of a triode is not air pressure, nor an
electrostatic field. You cannot sum a voltage with an air pressure,
nor with an electrostatic field, nor indeed can you sum an air
pressure with an electrostatic field.

Second, what is wrong with doing the summing within the tube, why
must the
summing be external?

I have not said that it is necessary for it to be external. I have
argued that none of the effects you speak of are internal as you
have claimed, but depend in part on external elements. I was arguing
that your claim that it is internal is false.

Taking your reasoning to the extreme, virtually no
audio amplifiers that exist today use "NFB", because the summing
is done
internally to the amplifier. By your reasoning, for an amplifier
to have
"NFB" the summing must be done in a circuit external to the
amplifier
housing, for example by a rat's nest of resistors hanging off the
input
and output terminals.

No. That is your lack of understanding, not my reasoning.

If anyone is still following this thread and wondering about the
truth of the matter, the best thing you could do would be to go to
your local college or university library, and find a recommended
text on feedback control systems. Read the introduction, look
through the chapter headings. You will almost certainly want to buy
it if you think you can handle the maths.

Because it is generic to all control system engineering, it will
allow you to map your knowledge and experience of systems in general
onto audio amplifier design. So if you are an experienced plumber,
for example, you can find out that ringing or oscillation in water
pipes happens for the same reason as it does in amplifiers, and can
be cured or designed out using the same principles. So an educated
builder could understand valve amplifiers through his experience of
building. Then he just has to learn about the particular components.
Real engineering is the same everywhere. Everything you know and
feel about the world applies to audio amplifiers.

Also, you will find out if you don't already know that mainstream
audio engineering is Fourier-fetishist. You may wonder what happened
to Laplace.

cheers, Ian

cheers, Ian