"Patrick Turner" wrote

Ah, but greater minds than yours or mine suggest there **is**
NFB in a triode.

Maybe greater than your mind. If any of them claim what you are
claiming, their minds are definitely not greater than mine. There is
dynamic equilibrium in a triode, that some may loosely, maliciously,
dishonestly, or foolishly call NFB. I have accepted all along that
this loose definition has currency, as you will see if you read my
posts.

But it is not feedback in the sense used by audio engineers. It is
not feedback in the sense used when you say that an amplifier has
12dB of NFB. A feedback control system must by definition, and quite
categorically, have a summing node at the input that combines the
input signal with feedback from the output. It is allowable (and
often necessary, as in most global nfb applications in amplifiers)
for the output signal to be transformed before being applied to
summing node. We tend to trim its bandwidth, and we may transform
current to voltage (if current is considered the output and voltage
is the input this is necessary because the feedback signal must
summable with the input signal). But whatever we do with it, it must
be summed with the input signal at the input.

Before you doubt them publicly, at least try to see what they are
saying.
I suggest you read a few more old books.

I know precisely what they are saying. You have misunderstood most
of them. They are using the loose meaning of feedback, not the more
precise meaning used by control system engineers for the purpose of
designing, for example, audio amplifiers. If they used the term
"dynamic equilibrium", then perhaps they would be less likely to be
misunderstood by wanabees.

I don't agree that **anything** can be contrived as a feedback
system.
Being a realist, the only thing that can be contrived to be a
feedback
system
is one which actually is such a system, and can be seen to be so
when the
concept
is examined without needing any maths to describe or proove it.


Everything *can* be if you make an effort, but I have counselled
against that tendency if you check (search for "cows" if you can't
find the place). It is up to an engineer to decide what form of
analysis is most useful and convenient in each case. In the case of
the triode, it is neither useful nor convenient. If it were to be
described correctly as a feedback control system, it would not be as
you have done.

It is you and your current crew who go on about imaginary devices,
and daft theories that you have no understanding of or use for.

But only when you have read some more will you be able to
comprehend
what I am trying to say.

I understand what you are saying, cheeky boy. Don't expect me to
understand what you are trying to say. You are possibly trying to
make sense, but I wouldn't bet on that.

Fresh students make your mistake more commonly than any other. Only
difference is, you aren't learning. While you have been nursing your
misapprehensions for the last several years, I have taught the truth
to hundreds of people who will go on to design stuff that you may
rely on in your old age.

Answer to John, and indeed most of my posts, deals with rest of your
questions [below]. Can't comment on all the imaginary stuff in your
head that you wrote about. [also below] It's your head, do what you
want with it.

Oh, that leaves this...

A diode with a screen grid would make a CCS.

Still a diode though. current would only flow one way.

er, yes. Wouldn't be constant if it didn't, would it? Your carping
has become pathological I fear.

cheers Ian


Its like the model of a tube.
We start with a voltage generator with a pair of zero output
impedance
terminals
and operated by an input voltage so the output is u x Vin.
Then we have a series R to act as the Ra, and voila, a model for
any
tube.
So with a pentode, u = 3,000, so for one volt in, we get 3,000 v
out at
this gene,
and then this is is applied to 1M of plate resistance before being
applied to the load of say 50k,
we end up with 142v at the 50k, and we have a tube gain of 142.
Now everyone knows that 3,000v doesn't actually occur in our
little
signal pentode, say a 6AU6,
but the concptual model I have described here allows us to draw a
complete curcuit in
using "equivalent models" to allow the everyday workings to be
described
better on the back of
an envelope as something real, and drawn up, so we understand.
The same simple approach can be taken with a 300B.

Normally, the model of a triode is also a u.Vin generator, and Ra
= the
plate resistance we observe,
but we couls also describe it as a current generator with gm x
Vin, and
a loop of feedback to give us
our low triode plate resistance.

I suggest one has to think outside the square and allow the
imagination
to work a bit to understand tubes,
mosfets, and transistors.

Its been done for about 90 years now.

There was a guy called Cathdode Ray, who wrote volumes in
magazines of
the 1950s and 1960s,
and I suggest you dig around to find samples of what he wrote.
Its all in language any diyer can understand, and none of it is in
what
we would now call
geek spiel, and he doesn't befuddle anyone with huge equations
that
nobody can follow.

I have a couple of his books.



2***For a system to be a feedback control system in the usual
sense,
as in "12dB of nfb", there must be summation of input and
feedback
control signals at the input. If a triode is driven with an
effectively zero input impedance to the grid, there can be no
feedback of control signal to the grid.

But beyond the grid, there is the applied voltage field inside the
tube,

and from the anode there is also an applied voltage field.
The two fields sum, and the resulting field is what the electron
stream
reacts to.

If one removes the field applied by the anode, by placing a screen
grid
in there
you only have the applied voltage field of the grid, so gain is
much
higher, also thd is higher,
because there is no FB from the anode.

The impedance tied to the input grid is of little consequence,
although
with triodes its best if the grid
is driven from a low impedance to avoid miller effects at too low
an F.

(The control signal is a
voltage, so any feedback signal to be summed with it must also be
a
voltage, but you can't sum voltages at the input if it is driven
with a zero impedance source.

You completely miss the point about the voltage field effects
within the
triode.

Such a source is easy to
arrange...amps designed for A2 operation use them.) If triode
behaviour really results from feedback to the grid, then this
would
defeat it. Yet it still behaves like a triode. Hence its
behaviour
cannot be explained in terms of feedback to the input. It is
therefore not a feedback control system.


But you know in your heart that John's estimate of nfb in a SET
amp
is ludicrous. You are being silly, all three of you, and you know
it. Not for the first time either.

John is about dead right about the 14 dB NFB for a 3k load for
300B.
I said it was about 18 dB for 5k.
Applied FB depends on open loop gain, and gain varies with load.




If you held the screen
at constant voltage and attached the load to the anode. And if
the
screen of a tetrode were called the anode, and the anode were
called
a collector, or sink, or somesuch, then ppl would not be so
confused
about the tetrode or pentode, which is constructed by putting
lots
of holes in the anode and adding another electrode behind it to
prevent the electricity gathering and molesting its
characteristics.

I am not confused by the conventional terminology thus far
contrived.