------------ ORIGINAL MESSAGE ------------(may be snipped)
On Tue, 18 Oct 2016 05:44:13 -0600, W8ZR wrote:
>Positive screen current means that current flows into the grid, and negative
>screen current means that current flows out of the grid.
REPLY:
Be careful with this "into" and "out of" business. He is talking about
the so-called conventional current, which exists only in the
imagination.
In the real world, positive screen current (electrons) flows from the
cathode to the screen and out to the positive power supply.
Likewise, negative screen current means the screen is actually
emitting electrons as if it were a cathode itself, a phenomenon caused
by secondary emission where arriving electrons actually "knock off"
electrons from the screen and it appears to be emitting electrons of
its own. See Wikipedia discussion:
https://en.wikipedia.org/wiki/Secondary_emission
Because of this phenomenon, the screen supply must be capable of
supplying electrons as well as receiving them. This is most easily
accomplished with a resistor from screen to ground, whose value is
such that current through at idle equals or exceeds the value of
expected negative screen current.
I wish the concept of "conventional current" would disappear from the
textbooks since it purely imaginary. Electron flow is real.
73, Bill W6WRT
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