Re: Grid #2 in ultralinear mode



Phil Allison wrote:

> "mick"
> >
> > In a 6L6 g1 and g2 are wire spirals wound in the same direction and
> > in-line, allowing a straight line through from the cathode, through
> > the grids, between the beam-forming plates to the anode.
>
> ** Same construction method used in beam tubes and power pentodes like the
> EL34 and EL84.
>
> The screen wires are sheltered " down wind " of G1 where electron density is
> low.

Screen grid wires are placed so most electrons miss hitting the screen;
hence screen current is below 10% of the total anode and screen currents.

The screen inserts the presence of a fixed voltage field between g1 and the
anode,
the screen has a mighty presence, but is not designed to absorb
many electrons like the anode.

>
>
> > I don't know
> > about the diameter of the wire used, but it could be an indication of why
> > the max g2 dissipation figure given in the spec is quite low.
>
> ** Heat radiated from G2 is all collected by the anode anyhow. At least in
> UL mode, the AC component of screen current adds to the output power -
> unlike pentode mode where it is entirely wasted.

The output power contribution by the screen to OPT primary is tiny
compared to the anode contribution.
UL power output is usually 10% or more less than pentode or beam terode
connection,
and the power "wasted" at the screen is also negligible. The "wasted"
screen power is really an investment of power to maximise plate power.


>
>
> > This form of construction *could* be why 6L6s seem to be quite good when
> > triode-connected. In a normal tetrode or pentode the electron stream will
> > hit g2 or g3, causing dissipation in those grids. g2 does operate as an
> > anode, with the true anode merely being an electron collector.
>
> ** Huh ?
>
> " with the true anode merely being an electron collector "
>
> What drivel.

Its not quite as drivelly as you think.

If you had a 6L6 with a plate structure which was the same diameter as the
screen grid winding, and there was no screen, then the tube would act
exactly like a 6L6 in triode, but it would be a real triode.
But with a real 6L6 strapped in triode, the screen voltage acts to inject a
plate voltage closer to the
control grid and cathode
than where the plate is actually located, which is further out beyond the
screen,
so the anode can be a larger diameter structure, and this possibly allows
slightly higher dissipation for a given amount of plate metal.

>
>
> > The 6L6, on
> > the other hand, still uses the true anode as the electrons will mostly
> > tend to miss g2, and there is no g3 to get in the way.
>
> ** Make up your mind.
>
> Compewter geeks........

Tube operation is messy.

Electrons swirl around the cathode in a cloud like a swarm of bees,
and they are emitted and reabsorbed constantly by the cathode.
The electrons can slightly feel the presence of the +ve charges on the far side
of the control grid1,
and if g1 goes a bit less -ve, ie, swings slightly more +ve, then some sneak
past g1,
and accelerate fast towards whatever +ve charge is beyond.
Most miss hitting g2, and continue on towads whatever else is positive,
which is the anode, and they belt into it generating the heat
= Ea x Ia in watts.
But quite a few bounce off the anode because they didn't find a soft landing
into orbit on an atom to fly into.
This is secondary emission. But in pentodes a suppressor grid is fitted,
and its charge is at 0V, and it repels most of the electrons bouncing back from
the anode towards the g2
and with much reduced speed, so they are forced to return to the anode where
eventually they are absorbed.

The beam forming plates in a beam tetrode
repel electrons on two sides of the electron flow and squeeze the broad stream
that occurs in a pentode into
a narrow beam and thus creating a space charge between the g2 and anode
that suppresses the secondary emission by forcing bouncing low velocity
electrons back towards the anode.

Patrick Turner.


>
>
> ....... Phil

.

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