Gary Coffman wrote:



snipped


But the key thing to understand here is that a resistance need not
be *dissipative*. A good electrical example is a triode tube, or *valve*
as the British called them. The control voltage on the grid changes
the current flow through the tube by modulating what we call the
plate resistance. But this isn't an actual dissipative resistance.
It is a *mathematical fiction* we use to model the plate current
valving action of the grid.

Not really Gary:

AC plate resistance is the "fictional one" and is defined as the dynamic ratio of
plate voltage to changes in plate current at a *constant* grid voltage.

DC plate resistance is the ratio of plate voltage to plate current and it is a
*dissipative* resistance. That's what makes the plate of a triode (or a diode,
tetrode or pentode tube) glow red hot if you "push it" too much.

Don't take it too hard Gary G I had to confirm my dusty memories of this stuff
at:

http://www.lh-electric.4t.com/vt_primer4.html

Jeff (Who burned his fingers more than once on those big black metal metal 6L6s he
couldn't see had a cherry red plate.)
--

Jeff Wisnia (W1BSV + Brass Rat '57 EE)

"If you can smile when things are going wrong, you've thought of someone to blame
it on."