[John Byrns]

OK, unlike yesterday when I was not in the mood for ASCII art, today I
am happy to do an ASCII sketch of the triode equation used in my model.

Please remember that you must use a fixed pitch font in your news reader
to properly view this sketch.

Here is the equation for the triode model as I first posted it several
days ago.

Ipk = c * (u * Vgk + Vpk) ^ 1.5

For the purposes of this sketch I am going to replace the constant "c"
with a new constant "k" which is related to "c" by the following
function.

c = k / (u ^ 1.5)

Making this substitution we get the following equation which I will
sketch.

Ipk = k * (Vgk + Vpk / u) ^ 1.5

OK, finally here is the sketch showing how the negative feedback works.


__________
| |
| divide |
+----------------| by |---------------------+
| | u | |
| |__________| |
| |
| |\ |
| |\ | \ |
| | \ __________ | \ |
+--| \ | | | \ |
| \ | 3/2 | | v-c \ | Output
| sum |---| power |---| transcon- |---+-------
Input | / | function | | ductance / (plate)
--------| / |__________| | (k) /
(grid) | / | /
|/ | /
|/


I hope this sketch makes the negative feedback in the equation, and the
spice model derived from it, clear to all. This does not imply one way
or the other whether or not there is actually negative feedback involved
in the operation of a triode vacuum tube.

To properly understand this sketch it is necessary to understand that
the transconductance block is connected such that a positive input to
the transconductance block causes positive current to flow from the
output of the model into the transconductance block. This may be easier
to understand if you consider the value of the transconductance to be -k
rather than k.

I apologize in advance if this sketch doesn't reproduce properly on your
computer even when using a fixed pitch font.


Regards,

John Byrns