Ian Iveson wrote:

"Patrick Turner" wrote

I am not confused at all, its you.

The current flow in any tube, including a triode does not have any
feedback effect at all.

The anode ***voltage*** changes in a triode have a profound effect
on
the current flow.

Think of the triode set up at idle with no input signal.
[etc. etc....see below]

Well that's all very well but it is not feedback in the usual
meaning of the term. Where is the input summed with the feedback
loop? Quantity of argument doesn't substitute for simple fact.

cheers, Ian

The input and output signal is summed within the triode.

If the applied +ve voltage at the grid of the triode acts to increase
the current,
the anode voltage change, a -ve voltage, tries to reduce the current.
The sum of the two applied voltages (or electrostatic fields ) is what
actually
changes the current flow.
There is a summing of voltage fields. The difference between the
fields is what is amplified, and its no different to
the two signals going into a differential amp at the input of most amps.

Patrick Turner.






Now you are being cheeky as well as daft. Source impedance is the
answer to your question. Source impedance is not part of the
valve.

The Miller effect is not what Patrick was talking about. For him,
the characteristics of the triode are shaped by what he calls
feedback. He is confusing the effect of the load current on Va,
and
hence on the output, even at DC.

Apply a +ve signal to the anode from some low impedance source.
The increase in voltage increases the current flow.
The voltage change divided by the current change = the plate
resistance
in ohms.
Its usually a lower figure than the load with which is to be used
with
the triode.

But with a pentode, the current change is perhaps 20 times less
for the
same voltage change at the anode,
and that's because the anode voltage change is prevented from
affecting
its own current flow.
So Ra of the pentode is much higher, 12kohms with an EL34, as
opposed to
1,50 ohms
when the tube is strapped in triode, and the anode charge is
allowed to
affect the
electron stream just as if the screen was a metal cylinder anode
of the
same dia as the screen.

The differences between a signal pentode Ra and trioded Ra in a
6AU6 is
far greater than in most power tubes.

Patrick Turner.





You say that

Yes there is NFB. Consider this: The grid of the triode is
fed
from a
non-zero source impedance.

The feedback due to the miller effect is independent of the
source
impedance, or it would not be a characteristic of the valve. The
*effect* of the feedback does depend on the source impedance.

A triode fed from an effectively zero source impedance would
still
display the effect that Patrick is talking about, presumably,
because he doesn't mention source impedance. But there would be
no
Miller effect under such external conditions.

I remember when Patrick was mugged and dragged kicking and
screaming
into this foolish and misleading notion. He was right and they
made
him wrong. I am trying to help him escape. You are muddying the
water.

Read his post and come back to me if you can make sense of it.

cheers, Ian