Re: Grid #2 in ultralinear mode



RdM wrote:

> Patrick Turner wrote in
> rec.audio.tubes<43D2EF4E.B73A21CA@xxxxxxxxxxxxxxxxxx>:
>
> > Electrons swirl around the cathode in a cloud like a swarm of bees,
> > and they are emitted and reabsorbed constantly by the cathode.

OK, try reading the old books about thermionic emissions from cathodes.

>From what i read electrons are emitted if the cathode temp and material is just
right,
and so what happens to an electron once emitted?

They have an escape velocity, but do they hover in space near the cathode?
They are vigourously on the move all the time, coming and going, and the
cloud density is slightly affected by the anode or screen voltages, and a lot
affected by the
g1 voltage; with a +ve going grid, electron stream away from the cathode, so
the
*net* emission increases, and it is a substantially linear phenomena.

>
>
> Reabsorbed? By what process?
> The heated cathode, boiling off electrons in a vacuum, is also reabsorbing
> them, you say?

Nothing is "boiled off", but rather the electrons are given enough heat energy
to
spin off atoms, breaking the force which attracts them to the cathode.
They slow down, and return, become re energised...
Its like bees in a swarm, some are leaving and some come back, so a net
effect is a cloud of bees on the move.


> What percentage of emitted electrons would you say are
> reabsorbed, thus reducing both the anode current and available electrons in
> the g-k space cloud? I don't think I've heard of this before ...

I am not a tube making expert, nor a molecular scientist, am a lay person
interested in tube
usage in tube circuits.

I have some mental idea about the electron rich cloud that surrounds a cathode
in a tube.

Without any plate current if g1 in negative enough, there is still a cloud of
electrons
coming and going from the cathode and acting in a state of equilibrium.

The cloud builds up to limit the density of electrons.
The speed at which changes to emission from the cathode can be effected is
extraordinarily
fast, allowing extremely high frequencies to be amplified.

The capacity for increasing the emissions from the idle state is usually many
times,
and a 300B cathode can emit an amp for a short time if the grid is made quite
positive.
But usually, the emission of 70mA +/- 60mA load current change is well within
the abilities
of the cathode to emit electrons so that the tube is substantially linear.
The negative charge held by the cloud of electrons around the cathode repel
electrons to some extent and prevent the charge build up to exceed a point of
equilibrium.

Of course things get more mysterious where you have a tube loaded by a CCS,
then *NO CURRENT* change occurs, and the tube operates as a pure voltage
division device.

Patrick Turner.




.

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