Ian Iveson wrote:

"Patrick Turner" wrote

In the case of a 300B, and all triodes, there is already an
internal electrostatic NFB loop.

What you call feedback is a matter of definition, not physics or god
or nature.

The usual definition would not normally include your idea of how a
triode works. If the output is taken from the anode, then the output
is a result of a self-regulating system with the point of dynamic
equilibrium modified by the Vgk input. There is no feedback to the
*input* (Vgk), which is why it would not fall under the usual
definition of a feedback control system.

Child, in Terman's 1937 book describes what he calls "self regulation"
in the triode. Maybe you should read it.

But it appears to me there is an electrostatic divider network between
the output, the anode
and grid, the input, and when there is no load at the anode, there is no
gain, so the gm
is at its maximum value.
But when a very high load is connected, or indeed a CCS, gm = 0.0 A/V
because there is no
current change in the triode with a CCS. That is the effect of the
voltage feedback.




According to your thinking, a resistor, or a pot, would equally be a
feedback control system.

Nope.



Until you get your head around this point, the rest is not worth
comment.

Fair enough.

About 2 years ago, Dennis Assenayev tried to convince everyone thet FB
existed in triodes.
He was such a clumsy teacher, but finally he convinced me.
The full discussion is there in the archives.

Dennis also maintained the FB in triodes was via a non linear transfer
mechanism,
since plate current varies with the voltage taken to the power of 2/3.
I'm not too clear about that, you'd have to check the old books for the
math.
Dennis believed that tubes working in pentode or beam tetrode mode with
CFB windings, like Quad II was
a better way to apply FB locally than rely on the triodic mechanism.
Trouble is then you have the parasitic reactances to worry over.
So one has to wind OPTs with low capaciatnce and leakage.
The screen is then taken to a fixed voltage, so some of the linear FB
is applied to the screen, and this tends to change the spectra of the
outcome towards triodes.
But I digress, we are focused on the FB in triodes, or the lack of it,
if perhaps you are to believed.

Patrick Turner.



cheers, Ian