[Patrick Turner]

HP wrote:

"Andre Jute" wrote in message ups.com...
My point is a little more subtle. I see an effect which looks like NFB
happening between the terminals of a triode; I'm happy to call it
internal or natural NFB until someone else offers an explanation that
justifies giving it another name; no one offers such an explanation,
merely negative objections to naming it NFB; until they pull their
finger from their arse and say something positive (and more cogent)
than Patrick, who *does* have a case, I shall not change my mind just
because they're "agin".

The effect you see is the change in plate current with plate voltage. This
is exactly the definition of plate resistance, which is both the name and
the explanation that have been in standard use for the past seven decades
or so.

In actual fact there is no external observation you can make of the triode
that proves, or even suggests, there is internal negative feedback at work
inside the tube. Everything you see can be more simply and economically
explained by the Thevenin or Norton models.

Thevenin, Norton, and the NFB conjecture are all just fictional models
that help to describe or predict the tube's external behavior. None of
them represents the actual physical processes at work inside the tube,
and it is a mistake to claim otherwise. The Child-Langmuir equation,
which is based on the distribution of electric fields and charge density
inside the tube is another model, and arguably as good an explanation
of tube behavior as we need. There's nothing in it that depends on
negative feedback, either.

Professor child called the mutual effect of anode AND grid voltages
in a triode a form of "self regulation".
See Radio Engineering, by Terman, 1937.

He does not call it NFB probably because it would have confused ppl at the time
who were used to NFB ONLY being some external networking to
get an amp of device to display lower Ra thd etc than otherwise it would
without NFB.

The observation of a triode as a mystery 3 terminal device could be
argued to contain NFB.

When you have a 300B with Ia = 70mA, Ea = +400V,
then if it acted like a resistor without NFB the R value
would be 400 / 0.07 = 5,700 ohms.
But where one maintains the grid bias voltage and fixes the cathode voltage,
abd simply raises the anode voltage by say 80V,
then one soon finds that the Ia change = 100mA, and
it is because the Ra = 800 ohms, and not the 5,700 ohms of quiescent Ea / Ia .
How can this be? Its because the anode has a massive effect on the
flow of electrons from the space charge around the cathode to anode despite
grid voltage's supposed control of current.

One simply cannot deny that a huge anode feedback effect exists,
and where you have a grid also able to control the Ia
with a gm of about 5mA/V then there MUST be a summing of the anode and grid voltage effects
according to some formula ( upon which Professor Child throws more light than I do ) .

So its very easy to see that NFB is alive and operational in a triode.

Because its self contained, there is no need to always represent the triode with symbols
to represent the NFB network that exists, it merely is represented by
the normal symbol but considered with Ra, µ, and gm WITH NFB being effective;
Thge triode cannot be considered without its NFB unless one makes a mental
model of a perfect pentode with NFB.
The perfect pentode BTW is an unusual animal because it'd assume Ra would be infinite,
and since µ = gm x Ra, then perfect pentode µ also would be infinite,
so that a mild external loop of NFB would render such a pentode circuit free of any THD
because the gain reduction with NFB would also be infinite.
But alas, no such perfect pentode exists. The anode voltage still always
has some effect on the Ia even with a screen present.



This is fundamentally an engineering debate and the arguments are
necessarily framed in engineering terms. The fact that technical language
is used to express a point doesn't make the point true by default. The
assumptions and inferences have to be grounded in fact and logic to be
valid. The challenge is to define who's qualified to decide what is factual
and logical in an engineering argument. An engineer perhaps?

I am not a qualified university educated triodologist.

But my powers of observation reveal NFB in triodes to me.

But by exactly what process in purely technical terms would you use to
allege NFB does NOT exist in triodes?



I would be delighted to debate this subject, point by point, in front of
a qualified and neutral judge (or panel of judges) from the tube audio
engineering community. How about Steve Bench, John Atwood, Lynn
Olson, or Morgan Jones?

The above list may not wish to join our little discussions because the
morons and arsoles wait impatiently with buckets of ****
to drench any attempts of true intellectual discourse and democracy.

Patrick Turner.


-Henry