Yes Jim,
You are totally right, of course the electrons are always negative, if not
there would be no flow from anode to screen or either.
Its already a bit late ;-))
It is only the direction of electron flow which changes.
73
Peter
-----Original Message-----
From: Jim Garland [mailto:4cx250b@miamioh.edu]
Peter,
I may have misunderstood yout comment, and if so I apologize, but if not
then I believe you are harboring a misconception about charges and current
flow.
Electrons always have a negative charge, and there's nothing we can do to
them that will change that fact (except annihilate them with a positron,
which is a postively charged electron, and which turns them into a gamma
ray. Or we can combine them with a proton, which turns them into a
chargeless neutron. Neither of these processes are believed to occur in
vacuum tubes.) This means that if electrons are absorbed by the screen grid,
no matter where they come from, then that corresponds to a positive grid
current. The only exception is if the screen grid is floating (not connected
to any external circuitry), in which case no screen current flows and the
screen voltage becomes more negative, thus creating an electric field that
repels the approaching electrons until no more of them accumulate. The
bottom line is that electrons coming from the anode are indistinguishable
from electrons coming from the cathode, so far as their charge, mass, and
spin are concerned. They will differ only in their kinetic energy, which is
basically how fast they are moving.
If the screen grid is grounded, then there is no charge accumulation on the
screen. Electrons striking the screen from the cathode or anode flow
immediately to ground, with no charge buildup. If the screen is connected
to a positive power supply, then in equilibrium there will be a charge
deficit on the screen (resulting in an outward electric field), but this
charge will neither decrease nor increase as cuurent flows into the screen.
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