Re: There is no feedback in a triode

robert wrote

Depends on how you define "feedback".

Quite so.

A triode's plate current is influenced by the voltage on its plate. And if
you have a resister on the plate to B+, the resistor will have an amplified
signal voltage on it, which variance influences plate current some more.
Maybe that's feedback, maybe not, your call.

Same could be said of a potentiometer.

I have argued the point formally and in detail before.

I take my definition of feedback from standard control systems engineering,
which is the genus of which audio engineering is a species. It is the same
methodology adopted by all branches of real engineering, and the one assumed by
RDH4, for example.

As I have said before, it is possible to apply control systems theory to
anything, including resistors. You can use it to arrive at Ohm's law, even. That
doesn't mean that resistors have "internal feedback". In just the same way, you
can apply it to a triode, and with a fair amount of contrivance, you can derive
part of Child's law. But systems theory is not necessary for either derivation,
although it *is* necessary for understanding and using the feedback we apply in
our circuits.

The Miller effect accounts sufficiently for the direct effect of the anode
voltage on the grid. Even there, it is only significant if external impedance is
added to the grid.There is no summing point where the output voltage is compared
with the input voltage ahead of the forward transfer function.

Those who invent imaginary internal circuits made out of vacuum can never prove
that they are real, because they are not. They are mathematical contrivances and
have never, ever, been found to be useful in practice, either in circuit design
or in valve design. It is not up to me to prove that imagination is not real. It
is up to them to show it is useful.

It has been suggested, preposterously, that the writers of RDH4 didn't know
about systems theory. Actually the book cites several standard texts on control
systems engineering for background reference. It is where all the stuff like
Bode theory, feedback equations, Fourier, Laplace, Nyquist, etc etc comes from.

It has also been argued, equally desperately, that the writers of RDH4 didn't
know how triodes work. Also that the manufacturers of valves didn't know how
they worked. In fact they knew pretty much the same as we do, as far as it is
necessary to know anything at all.

It should be clear to everyone that the notion of internal triode feedback is a
source of stupid confusion. How many times have you come across an argument that
starts like:

A: I like SETs because I don't like to use feedback

B: But triodes have internal feedback.

Ya de ya, blah blah.

Often some useless debate ensues about the difference between one kind of
feedback and another, as if "internal" somehow is better than "external" or
"added" ...blah blah, whatever.

Let's keep it simple. For engineering purposes, the meaning of "feedback" could
not be more clearly defined. It is as tight as you could wish. Check out any
recognised general text on control systems engineering. You will find the same
strict definition, pretty much word for word, in any such reference.

I won't get round to replying to everyone, and I won't have arguments with those
who simply refuse to abandon the illusions they propagate for commercial or
other dishonest purposes.

You have put your finger on one essential point. If there is no delay, then in
the real world there is no feedback. Also, if there is no delay, then there is
absolutely no reason to resort to systems theory. None of that stuff...Bode,
Laplace, etc...is useful if there is no delay. Whatever may or may not be bad
about feedback therefore cannot be bad where there is no delay.

Not keen on analogies, but this may be a way of looking at it:

You want a car that has a linear response to the accelerator pedal. You find
that wind resistance introduces a problem of sagging. Wind resistance is not
feedback in your system because the throttle, which is your input, is not
sensitive to it. You can deal with the effect of the wind load in two ways. You
may add negative feedback, which is what cruise control does, or you can
tailgate, which is a bit like what a pentode's so-called screen does. Tailgating
is not feedback, and neither does it magically remove some kind of mystical
internal feedback inside your car.

cheers, Ian



.

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