On Fri, 18 Sep 2009 15:43:48 -0700, arb wrote:

Brenda Ann wrote:
"ian field" wrote in message
...

As I understand it, the electron beam is aimed slightly off centre and
the beam trajectory is corrected by an external magnetic field (the ion
trap) the ions being heavier than electrons are not deflected and
collide with the glass wall of the tube neck.

It would be interesting to know how you cantered the picture without
the ion trap.



Actually, the ion trap (there was none on the CRT when I got the set)
doesn't seem to do much with the centering, mostly it just makes the
picture darker (about 70% or so darker). The other (parts) set I have
had the ion trap, so I used the one from that set. On closer
observation, I don't think there ever has been one on this CRT, which
is why I'm wondering if it is a replacement that doesn't require one.
The tube is in exceptionally good condition for a set this old.



If the electron gun is aimed straight down the neck of the CRT,
instead of being quite noticeably at an angle, then the tube is an
aluminized screen version and does not require an ion trap. It would
probably be a 12LP4A rather than a 12LP4.
Regards, Alan

According to my tube manual, a 12LP4A requires a double ion trap, two
magnets. One TV that I have that has a 12LP4 in it (I don't know if its
an A or not) does indeed have a double trap with two magnets and two
bands around the neck.

Also note that not all tubes that needed an ion trap had a bent gun.
That was only one of the ways they could be built. Another was to have
the "slot" between two of the elements angled.

All charged particles are affected by electric fields and moving charged
particles are also affected by magnetic fields. It doesn't matter if
they are electrons or ions. In the case of an electric field, the force
is proportional only to the field strength. So an electron or a singly
charged ion would experience the same force. However, since an ion is
much heavier, the same force produces much less acceleration. But since
both have been accelerated by the same electric field in the first place,
an ion would be moving much slower than an electron, thus it would
experience the field for a much longer time. The end result is that in
an electric field, an ion would follow exactly the same path as an
electron. This is why electrostatically deflected tubes don't have ion
problems and don't need an ion trap.

On the other hand, a charged particle moving in a magnetic field
experiences a force that depends on both the strength of the field and
how fast the particle is moving. Since a massive ion would be moving
much slower than an electron, it would experience much less force and
even though it is in the field longer, it would not be deflected nearly
as much. Thus, ions in a magnetically deflected tube will hit mostly
near the center of the screen, causing a burn. The gun of such a tube
will have an assymetrical electric field causing the electrons and ions
to hit the side of the tube or part of the electron gun. The magnetic
field of the ion trap bends the fast moving beam of electrons so that it
goes down the center of the tube, while the slow moving ions are bent
much less and continue to hit near where they would have hit without the
magnet.

--
Jim Mueller

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