a short length pirect~view picture.tube
• JUNE 1953
by J. L. H. JONKER.
621.385.832 :621.397.62
The design of cathode-ray tube$ for television reception is in a .continuous state of
development. Progress is usually directed along conventional lines, but occasionally an
unorthodox approach yields fruitful results. This article describes a novel, approach to the
problem of shortening the length of picture tubes; although this development has not been
accepted for production. purposes, it appears' to uiork surprisingly well.
, The 'original cathode-ray
tubes employed in
television receivers were no different from those
which had for some time previously been used for
oscilloscopes. It was not long, however, before
these were no longer able to meet the increasingly
high standards
set for television reception, for
which - a separate
class of cathode-ray
came to be developed, known as t.e le vi sio n
(in America: "kinescopes").
. As far as these requirements are concerned, the
public demand has been (and still is) for the
p o.ssih l e picture
compatible with a
receiver of reasonable dimensions and pricevAs to
. the dimensions, designers of television sets request
that the tube shall be as small (particularly as
short) as possible; furthermore, to reduce costs,
they prefer to use the lowest possible
of power for focusing and deflection of the beam.
Also, it must be possible to mass-produce
tube in such a way as to ensure uniform charac- '.
In recent years there has been no lack of evidence
of the general trend in the development of picturetubes. Although the tubes remainbasically cathoderay tubes, the dimensions and form have undergone
such modification that the requirements referred
to above can now be more closely satisfied. Under
pressure of public demand, larger and larger screens
have been made 1,2); in the United States, for
example, screens have now reached a size of 75 cm
diagonal. If it were not for the fact that the ratio
of tube length to screen width has also undergóne
a radical decrease in the meantime, it .would have
been necessary - even with medium-sized pictures
- to make TV cabinets so deep that they would
have represented ,a real obstacle in the average
1) Round screens are steadily giving way to rectangular
screens, thus saving the space occupied by those segments
of the round screens whieh the rectangular picture does
not utilize.
2) Another method of securing a large image is by pr oj ection
(see Philips tech. Rev. 10, 69-78, 1948), but this is not
within the scope of the present article.
living-room. Thus reduCtion
in the length of
picture-tubes has been one of the main objects of
!he designer in the last few years.
Means of shortening the tube
'I'he length of a tube can be divided roughly into
three parts (jig. 1), viz. the lengths l1 of the neck
and l2 of the cone, and the depth la of the screen,
which has to be curved to withstand the pressure
of the atmosphere.
Fig. 1. The length of a television picture-tube is the sum
of the neck length (11), the cone length (12) and the depth of
the screen (13)' D is the diameter or diagonal of round or
rectangular screens respectively.
Every effort has been made to reduce the length
of each of these parts in relation to the diagonal D;
by achieving the maximum compactness of the
electron gun and the focusing and deflection coils, it
has become possible to "effect some reduction in the
length of the neck; we shall refer to this again presently. The depth of the screen (la) has been
reduced in those tubes of which the cone is made
of metal instead of glass, as a smaller curvature
is then practicable a). The most important reduction,
however, concerns the cone. Any reduction in the
length of the cone will of course be accompanied
by an increase in the angle of deflection a, which is
3) J. de Gier, Th. Hagenberg,
H. J. Meerkamp v an
Embden, J.A.M. Smelt and O.L.van Steenis,A steel
picture-tube for telévision reception, Philips tech. Rev. 14,
281-291, 1953 (No. 10).
VOL. 14, No. 12 •
the angle between the extreme limits of the deflected
'beam (fig. 2). Step by step, this angle has been
increased 4) from 50° to 70° or even 90°, and this has
necessitated a greater number of ampere-turns for
the deflection; more, in fact, than would be proportional to the increase in the angle a. This results
Fig. 2. If the cone be shortened, D being constant, this
involves an increase in the deflection angle a; In older tubes
this angle was about 500,but in modern tubes a is 70° to 90°.
from the fact that the point about which the beam
pivots when deflected lies in the centre of the
deflection coils; if the deflection angle a is increased,
the deflection coils must be shorter, as will be seen
from jig. 3. This means that the electrons are
subjected to the deflecting field over a shorter
distance, i.e. that an increase in the strength of
this field in proportion to a is not sufficient.
1) So-called economy circuits have been devised
for producing the deflection current, whereby
a large part of the magnetic energy that accumulates in the deflection field can be recovered
and fed back into the supply5).,
2) Special amplifying tubes (e.g. PL 81 and PL 82)
ensuring higher efficiency, have been designed.
for these economy circuits.
3) Losses in the deflection coils have been reduced,
e.g, by the use of· Ferroxcube.
4) Economy in the power required for deflection
can be achieved in the first instance by reducing
the neck diameter of the tube, as far as is
compatible with mechanical strength. As will
be seen from jig. 4, however, a reduction in the
thickness of the neck leads to shorter deflection
coils. As pointed out, a short coil requires more
ampere-turns, and the saving is therefore less
than anticipated; but this effect can be largely
counteracted by making the cone and neck
merge into each other gradually in a certain
manner and adapting the deflection coils to the
resultant contour 6).
Fig. 4. A reduction in the neck diameter from hl to h2 means
that the deflection coils (dl' d2), for the same angle a, must
be shorter.
Fig. 3. If the deflection angle be increased from al to a2, the
deflection coils (dl and d2) must he shorter, sinee the
pivoting-point of the beam (AI' A2) must always be in the
centre of the coils,
In general, a large number of ampere-~urns will
demand a greater amount of power. However,
means have been found to reduce the amount of
power required, the more important of these being
as follows.
4) L. E. Swedlund and H. P. Steiner, Short 16-in. metalcone kineseope development, Tele-tech 9, 40-43 and
59-60, Aug. 1950. H. W. Grossbohlin, The design of 900
deflection pieture tubes, Tele-teeh 10, 42-44, Aug. 1951.
However, all these devices do not alter the fact
that the amount of power needed for deflection
purposes constitutes a limitation on the size of the
angle a. And this is not the only limitation. As the
angle a is made larger, two further adverse effects
become apparent.
As the cone is 'shortened, i.e. as the pivoting-point
of the beam (A, fig. 2) is displaced in the direction
of the. screen, the surface defined by the focus on
deflection will differ more from the surfacé' of the
screen, this being accompanied by greater fluctua5) See J. Haantj es and F. Kerkhof, Philips tech. Rev.10,
307-317, 1949. An article on new economy circuits will
later be published in this Review.
6) C. V. Boeciarelli,
Low-power deflection for wide-angle
C-R tubes, Electronies 25, 109-111,Sept. 1952.
tion in the SIze of the light spot. In order to limit
this defocusing effect as much as possible, the
greatest attainable depth of focus of the beam must
be aimed at, i.e. the beam should be as narrow as
possible 7).
The other effect experienced with increasing a is
related to "pin cushion" distortion. This occurs when
the surface defined by the focus does not coincide
with the surface of the screen. It is corrected by
making the deflection field non-homogeneous - the
field being weaker at points away from the axis.
However, when the beam passes through an inhomogeneous field, a certain amount of aberration
is introduced (which increases with the deflection)
as a result of the unequal deflection of different parts
of the beam. The narrower the beam, therefore, the
less the aberration.
Both these effects can be counteracted only by
using a narrower beam. Apart from refinements in
the focusing, this can be done only by reducing
the beam current (and hence the brightness of the
picture), or by increasing the anode potential, which
entails higher costs. A practical limit on beam
and thus also on the angle a, is
accordingly soon reached.
The conclusion, therefore, is that any increase in
the deflection angle may be accompanied by a
useful reduction in the length of the tube, but also
involves less desirable effects which in turn have
to be remedied. It seems doubtful whether the angle
a will be made much larger than 90°, which is
already quite large.
Let us now consider the neck of the tube once
more. It has been found possible to shorten this to
some extent by reducing the length of the deflection
coils and, again, by using a shorter electron gun.
As regards the latter, two of the components have
undergone some modification in the last few years,
viz. the focusing system and the ion trap; we shall
now take these in turn.
The focusing
field is generated by a coil, a stronger lens means
more energizing power and this is not readily
Different ways out of the difficulty have therefore
been sought, and one of these consists in combining
the magnetic lens with electrostatic
pref 0 c u sin g: a less powerful lens is then necessary.
This has the disadvantage,
however, that the
stray field of the magnetic lens, at the point
where the electrostatic lens 111 situated, must
be negligibly weak; otherwise, if the centering is
not perfect, astigmatism occurs. This necessitates
a certain neck length, which partially nullifies the
reduction in the length already obtained.
A similar
sta tic
is encountered
to which
in wholly
much attention
e l e c t r o-
has been given
in recent years with a view to economy
in materials B). Here
it is the stray
coils that
field of the
A special
has to be
is therefore
necessary which, in turn, once more
increases the length of the neck.
Perhaps the best method is one that makes use
of permanent
no energizing is then
necessary, and a considerable quantity of copper is
saved. Furthermore expensive magnet steels can
now be replaced by the non-metallic material
which contains 110 scarce materials
such as cobalt or nickel "].
The electron beam can be focused by means of a
magnetic lens, an electrostatic lens, or a combination
of both. In picture-tubes the first-mentioned method
is the more usual.
The reduction in the cone length referred to above
has brought the focus nearer to the lens; in other
words, the image distance has been reduced and the
lens had to be made more powerful. In any shortening
of the neck, i.e. reduction in the object distance,
the lens must also be stronger. If the magnetic
Fig. 5. Picture-tube in which the electron beam is focused by
two ring-shaped magnets
(1, 2) of Ferroxdure.
These rings
are magnetized in the axial direction of the tube and are
mounted with like poles facing each other. 3 electron gun
with ion trap comprising a small steel magnet (4) and two
pole pieces (5). 6 deflection coils.
7) The photograph
of checking
on p. 368 of this issue illustrates
the deflection defocussing.
a method
L. E. Swedlund
and R. Saunders,
picture tube, Electronics
24, 118-120, April 1951.
C. S. Szegho, Cathode-ray picture tube with low focusing
voltage, Proc. Inst. Rad. Engrs., 40,937-939,1952
(No. 8).
C. T. Allison
and F. G. Blackler,
A univoltage electrostatic lens for television
tubes, Conv. Brit.
Contrib. Telev., 1952, art. R8-1333.
J. J. Went, G. W. Rathenau,
E. W. Gorter and G. W.
van Oosterhout,
Philips tech. Rev. 13,194-208,
(No. 7).
VOL. 14, No. 12
Fig. 5 illustrates
the method of focusing by
means of Ferroxdure magnets; two flat rings of
Ferroxdure are used, these being magnetized in
the axial direction and fitted to the neck of the
tube with like poles facing each other, The desired
configuration of the field is obtained by varying
the space between the magnets. The high coercive
force of Ferroxdure renders this material very
suitable for magnetizing in the direction of the
thickness when made in the form of flat rings 10) .
and, as we wish to keep this thickness as small as
possible, the 'ring-shaped magnet serves the purpose
have been mounted on the tube; in the absence of
such centring the oblique passage of the beam
through the deflection coils produces distortion of
the image. The ion trap magnet thus permits
correction of residual errors in the eentering.
The ion trap
When the picture-tube is operating, negative
ions are produced which originate from residual
gases, or from -the cathode. Now, since the mass'
of an ion is very much greater than that of an
electron, the field of the deflection coils has very
little .effect on the ions and, if nothing is done to
check them, they all strike the centre of the screen;
where chemical action takes place and reduces the
luminescence. After a time, a dark spot becomes
visible in the centre of the picture.
To prevent this, an "ion trap" is incorporated in
the electron gun, and this functions by reason of
the circumstance just mentioned, that a magnetic
field deflects ions much less than electrons; in' an
electric field, however, the degree of deflection is
the same. Fig. 6a illustrates the principle of the ion
trap. An electrostatic lens is employed to deflect
the paths of the electrons and ions from' the axis.
along, successive deflections by two
permanent magnets bring the electron beam back
to the axis, 'whilst the ions strike the second
acceleration electrode (the anode), where they are
rendered harmless.
A' reduction in the length of the neck can he
achieved by mounting the first part of the electron
gun - viz. the cathode, the control grid and the
first acceleration electrode - at an angle with the
main axis (fig.6b, fig. 5); this dispenses with the
need for one of the deflections, and one of the
magnets thus becomes superfluous.
The Induction which the îon-trap magnet has to
provide is 3.5 X 10-3 to 6 X 10-3 Wbjm2 (35 to 60
gauss), and a small steel magnet is generally used
(4, fig. 5).
One function of the ion trap; not implied in the
name but just as important as the neutralizing of
ions, is the means of accurately centering the beam
when once the deflection coils and focusing system
See article referred to in footnote 9), p. 196.
Fig. 6. Old (a) and new (b) form of ion trap. k cathode;
gl control grid (Wchnelt cylinder); g2 first acceleration elcctrode (low positive potential); a second acceleration electrode
(anode, at high positive potential). Small dots: electrons.
Large dots: negative ions.
Action in (a): The oblique gap bctwcen g., and the anode a
imparts to the field a component perpendicular to the axis of
the tube, which deflects the paths of electrons and ions an
equal extent from the axis. A magnetic field 0 perpendicular
to the axis returns the electrons to the axis, but has practically
no effect on the direction of the ions, which strike the anode
and are thus rendered harmless. A second magnetic field ®
opposed to the first, deflects the electrons, so that they move
parallel to the axis.
Action in (b): k, gl' g2 and the first section of the anode a
are here mounted with their axis at a certain angle to the
axis of the tube. Only one magnetic field (®) is required
to bring the path of the electrons parallel to the axis. The
ions are hardly affected by this fieldand again fall on the anode.
Tube with hent neck
In the foregoing we have outlined some of the
obstacles encountered when efforts are made to
reduce ,the length of picture tubes in their present
form. Some ·of these obstacles will doubtless 'he
surmounted in due course, but we will not speculate
on such possibilities here. Instead, we shall describe
a tube which is considerably
those of the conventional type and which was
as. an experiment in the Philips
laboratories at Eindhoven.
Arising from the effect of the ion trap; in which the
beam is given a permanent deflection, the idea was
conc~ived of bending
the neck of the tube
an angle of 90° or more and making
the beam follow
the curve by m e a n s of a
magnet. In this' way the length of the tube can be
appreciably reduced (fig. 7), the bend serving
simultaneously as an ion trap 11).
11) Proc. Inst. Rad. Engrs. 36, 1485, 1948 (fig. 4.) contains
an illustration of a tube whose neck is bent slightly,
solely to produce the effect of an ion trap.
Fig. 7. Left: Conventional
(type MW 36-22).
with bent neck to reduce the over-all Iength of the tube.
The most suitable point for the bend is between
the focusing system and the deflection coils, the
latter being then so constructed that they can pass
over the bend; this has, in fact, proved quite
For curving the beam a small permanent magnet
was employed (figs 8 and 9) having an induction
of roughly 7 X 10-3 Wb/m2 (= 70 gauss). Adjustment of the position and strength of the field (the
strength by means of a magnetic shunt) centres
the beam exactlyon the axis of the deflection coils in
the same way as does the ion trap. To avoid astigmatism, the deflection field must be symmetrical
about the plane bisecting the deflection angle, and
this is not difficult to achieve. A narrow
beam is
again an advantage.
In the construction of the experimental tube
depicted in fig. 7 we used as many as possible of
Fig. 8. Tube with bent neck mounted in a television receiver. 1 focusing coil; 2 deflection
magnet to make the beam follow the curve in the neck; 3 deflection coils.
9. Close-np of the tube neck and deflection
the standard components of tube MW 36-22, VIZ.
the screen (25 cm X 32.5 cm), the cone, the electron
gUll (without ion trap) and the deflection coils.
The deflection angle (a) is the one usually employed,
viz. 65°; the normal deflection power is accordingly
sufficient. Only the focusing coils and the permanent
magnet were specially made.
As the length of the ben t part of the neck in
no way affects the depth of the television cabinet,
it can, if desired, be longer than that of a tube
with straight neck; the neck of the experimental
tube is, in fact, longer as this ensures better focusing:
the distance from object to lens is greater, the
magnification correspondingly less and the light spot
smaller . At the same time, a long focusing coil can be
used, the diameter of which will then be smaller.
By bending the neck through an angle larger than
90° we ensure that the focusing coillies wi thin the
over-all length of the tube, so that only this length
determines the depth of the receiver cabinet, not
the tube length plus the thickness of the coil.
The direction in which the neck is bent is immaterial; in this tube the direction lies in the plane of
the diagonal of the screen, for reasons of available
space in the cabinet (fig. 8).
A neck bent in the horizontal
plane also has certain advantages. Such an arrangement
would facilitate supporting the
focusing coil and the permanent magnet on the chassis. A second
advantage is related to the fact that, when a permanent magnet
is used for bending the beam, the picture is displaced slightly
on the screen when the high tension varies. With the neck
of the tube bent horizontally
this displacement
would also
14, No. 12
be horizontal, and this would not be so noticeable as diagonal
displacement, as is IlOW the case. It may be added that such
be entirely
by employing
high tension supply,
the output of which is
constant in spite of variations in the mains voltage or in the
load 12).
Using this tube a television receiver was constructed (fig. la), the over-all dimensions of which
were as follows:
Le. no larger than a medium-sized radio receiver.
Fig. 10. Front view of the receiver
The loudspeaker
is at the side.
12) See for example,
1952 (No.
J. J.
P. Valeton,
in figs 8 and 9.
Philips teeh. Rev. 14,
JUNE 1953
The height has been kept down by mounting the
loudspeaker at the side of the cabinet instead of
below the tube, a method that is quite often employed (see also fig. 8).
It cannot he pretended that the bent-neck tube
is more than an experimental idea, and it should
not be expected that such tubes will be put into.
for the present. Though practical
application may be delayed, however, it was
considered that publication of a description of this
laboratory model was not inopportune.
Acknowledgements for their co-operation in the
design of both tube and receiver are due to Messrs.
,D,ammers, Diemer, Neeteson
and De Weyer.
Summary. Arising from the demand for larger television
pictures, the development of direct-vision tubes has been
directed towards larger and larger screens. This has of course
resulted in greater tube lengths and, as this length determines the depth of the receiver cabinet, television sets have
shown a tendency to become rather unwieldy. In recent
years, therefore, every effort has been made to reduce the
length of the tube in relation to the picture dimensions.
Some of the methods employed are outlined, viz. shortening
the cone (accompanied hywider deflection angle), reducing
screen curvature (as in the metal-coned tube), focusing by
Ferroxdure permanent magnets, and the use of simplified
ion traps.
A description follows of an experimental tube whose
neck is bent through 90° or more, resulting in an appreciable
reduction in the over-all tube length. A permanent magnet
is used to make the beam follow the curve in the neck. A
television set was built for this tube, the depth of which
was only 13!" (less than, the diagonal of the screen); ,thè
width and height are 20" and 14i" respectively. The deflection
angle is the conventional 65 degrees.
VOL. 14, No. 12
The quality
of a television
the sharpness
of the luminous
line by
on the screen
picture is Iargel y dependent
spot which builds
of the
thus, on the accuracy
is focused to a point on the screen.
the inspection
of picture
tube -
with which the electron beam
The above photograph
of spot size and shape during the
tubes. The tube
up the image,
is set up under
conditions and fed via the grid with
in such a way that
a raster of spots is produced
The voltage pulse magnitude
of the pulsed electron
e.g. 100 [LA.After
a microscope
on the screen.
is so chosen that the amplitude
beam current
is used
are small enough
a series of voltage
to determine
and sufficiently
a partieular
of the focusing current,
or not the
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