Hi Ian,

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

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

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


Regards,

John Byrns


In article , "Ian Iveson"
wrote:

"Patrick Turner" wrote

[below]

I don't believe you are trying to understand, Patrick. As Dylan put
it, you try to hide what you don't know to begin with. Glad to see
Positively 4th Street is on his latest "best of" cd.

Oh, and before I forget...not a knife, but a razor. Occam's, I
think. Can't remember where it fits in though...

One more time, two points.

1***Mathematically, any system can be contrived to be a feedback
system. That means everything, including every part of an amplifier,
including the chassis and the shelf it sits on, including the sun
and the moon and the stars and all the fishes in the sea.

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. (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. 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.

A diode with a screen grid would make a CCS. 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.

cheers, Ian

The input signal is to the grid, referenced to ground, not to the
anode. Of course a change in Va effects the current, but it does
not
impinge on Vg normally except via the miller capacitance.

The grid remains a high impedance input port of what is a
differential
amp
if you consider the triode as a perfect pentode with an internal
loop of electrostatic NFB.
The electrostatic FB network works rather like a resistance shunt
FB
network between the
anode and grid, but its unseen and inside the tube, and works like
an
imaginary resistance divider,
but it has no real resistance, since if it did, you'd have input
currents, but we don't,
all we have is an input voltage field, and then the field from the
anode
sums with the applied grid field
to make a net field which controls the flow from the cathode.



A diode works the same way. The effect is between cathode and
anode,
essentially. It is modified by the grid in the case of the
triode,
but the anode voltage only effects the *grid* voltage, which is
the
input, via the miller capacitance.

Its no use talking about diodes. We have a triode to deal with.


Just wondering BTW if there are such things as diodes with
screens?

You could build one.
Just build a tetrode without a control grid.
But its easier to just use a pentode and connect the grid to the
cathode,
and the "screen" then can be used as a control grid for what will
be a high plate resistance triode with low U, low Gm, and
pretty useless, except some folks have done just this to great
effect,
Tim DeParavicini designed an SE pentode amp with screen drive, and
lotsa
NFB,
and got good reviews.


Anyway, how can you say your nfb is internal when it requires an
external load? Without a load it disappears.

The FB is internal, and it depends on the signal changes at the
anode
to get any FB.

Its the same as any amp, short the output, and there is no FB.
No sound either.


And conversely, if I use an effectively zero input impedance at
the
grid, then there can be no feedback to it, and the triode will
still
behave like a triode. Your so-called nfb will not disappear then.
Only the miller effect will disappear.

Well, with a grounded grid in a normall biased tube, what you have
is a
diode.
The only thing that changes Ia is the change of Ea.
By keeping G1 at various fixed selected values of voltage and by
forcing
the anode to change its voltage,
and measuring the current,
we get the anode curves for the data sheets.

Diodes have the same curves as triodes, but with a diode, there is
only
one curve,
since there is no grid.

Pentodes on the other hand act like there is very little
electrostatic
feedback,
but let's not complicate the discussion any more than it is.



cheers, Ian



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