The raw materials required for the production of valves comprise mainly glass , for the envelopes, wire for grids, heaters and electrode supports, metals for cathodes and anodes, emissive materials for the cathode coating, cathode-to-heater insulating material, mica for electrode supports and getter material. Certain of these are produced by Mullard from their constituent parts; glass and wire are manufactured in separate Mullard factories close to the Mullard Blackburn valve factory; the emissive materials for cathodes are carefully prepared from barium, calcium and strontium compounds; alundum for cathode-heater insulation is prepared to a high degree of purity from the basic aluminium oxide.
Other incoming materials are subjected to the closest scrutiny to ensure that they are of the highest possible quality. Cathode nickel, for instance, is analysed spectroscopically to determine the concentration of impurities present. A concentration of one part in a hundred thousand can be detected by this method. Such a high degree of accuracy is essential to ensure adequate freedom from unwanted impurities, and also to control precisely the concentration of certain "activators" that are added in minute quantities to the nickel.
The wire used for frame grids in Mullard valves is only some ten micrometres (microns) in diameter; human hair is about seven times thicker. Furthermore, because of the close spacings vital to the frame-grid technology, a tolerance of ±0.2um on the wire diameter must be maintained. To meet this exacting specification, Mullard manufacture the wire themselves, as they do also the wire used for screen and suppressor grids, heaters or filaments, electrode supports and base pins.
Two types of wire are needed—tungsten and molybdenum wire. Ores of both materials are subjected to intensive heat treatment to extract the metals, which are produced as fine powders. Very careful control is exercised at all stages of extraction to ensure an even distribution of particle size in the powdered metal, since this is a decisive factor in the subsequent processing.
The metallic powders are compressed at high pressure into rectangular bars which at this stage are, of course, porous and fragile. These bars are sintered by passing an electric current through them which heats them to a temperature above 2000°C. This sintering gives the metal a crystalline structure.
The bars now undergo several stages of swaging: they are heated to redness and hammered out into stout wire. This wire is drawn through circular dies, the diameters of which get progressively smaller until the final thickness is achieved. For the finest wire, as many as three hundred dies are used. The dies used in the earlier stages are tungsten carbide, while those of the !ater stages are diamond, fashioned by Mullard from expertly selected diamonds. By this process, a fourteen-inch ingot of sintered metal can be drawn into some two hundred miles of wire.
Alundum is used in the manufacture of valves to coat the heater wire and provide insulation between the cath- ode and heater. The incidence of cathode-heater breakdown and the susceptibility of a valve to hum modu- lation depend to a large extent on the purity and texture of this alundum.
Innovations made within the last few years in the Mullard valve factory have enabled Mullard to produce alundum with extremely low concentrations of certain impurities (known to be troublesome) and research has led to modifications to the texture of the purified powder. Improved hum performance and better heater-cathode insulation have resulted.
Frame grids undergo 100% inspection before being incorporated in Mullard valves. An optical projection unit used for this inspection is shown. (Because of the fineness of the grid wires, they are not visible in this photograph).
Frame grids and locked-seam cathodes are two examples of the many notable advances that have been made in valve construction by Mullard. All Mullard television frame-grid valves now incorporate locked-seam cathodes, and all Mullard valves intended for television r.f. and i.f. stages use frame grids. Other constructional advances made by Mullard include stranded, equalised heaters and finger micas.
The location of the getter is important since the interelectrode capacitances are affected by the metallic film on the valve envelope. Thus, getters with directional properties are used, so that the getter film evaporates onto part of the envelope where its effect on the inter-electrode capacitances is negligible. Intrusion of getter material into the electrode structure is prevented by the use of metal or mica shields; therefore the formation of conductive paths across the spacing micas is avoided.
Mullard first incorporated frame grids in a mass-produced television valve in 1958, when the PCC89 was introduced. The latest Mullard valve—the PFL200 double pentode—has a frame grid in the video output section.
The advantages provided by frame-grid valves can be summarised as follows: more gain per stage, better signal-handling ability, improved noise factor, and reduced cross-modulation.
In a frame grid, the backbones are rigid molybdenum rods, and they are held a fixed distance apart by cross-pieces. The backbones are drawn to size within close limits. On this rigid and precisely dimensioned frame, grid wire of any desired thinness can be wound, since the strength of the grid is in the frame and not in the winding.
The grid-cathode separation is determined by the diameter of the backbone wire and the thickness of the precision cathode. (To prevent deformation the tungsten grid wire is wound under tension.) The excellent dimensional control provided by a frame grid is enhanced by the close tolerances which are used. The molybdenum backbone rods are drawn to a tolerance of ±5 micrometres (microns), while the tungsten grid wire is drawn to within ±0.2um.
All Mullard television frame-grid valves use locked-seam cathodes in place
of drawn and pressed cathodes. This type of cathode is made on an automatic
four-slide machine which cuts and folds nickel strip into tubes having a rectangular
cross-section with a very flat surface to complement the flat frame grid.
Great precision is achieved in this operation, tolerances of ±4um being common.
The locked-seam technique reduces contamination of the cathode nickel, and has made possible adjustments to the amount of trace elements used in the preparation of the nickel strip. Certain impurities in the nickel (including some of thetrace elements used as activators) are volatile at the cathode operating temperature, and can thus form leakage paths on the spacing micas in the electrode structure. The adjustments made possible by the locked-seam technique therefore give improved insulation between the electrodes while maintaining the level of emission and length of emission life. In fact, over the periods of life expected in a television receiver, valves using locked-seam cathodes, when operated under normal conditions, show a negligible rate of failure from emission faults. Because of the great benefits deriving from the use of locked-seam cathodes, the technique has been applied to other Mullard valves wherever possible.
The benefits of the flat locked-seam cathode and the extremely small spacing between the cathode and a frame grid would be lost if the emissive surface on the cathode were rough or uneven. Large particles could cause short-circuits or at least impair the insulation between the cathode and grid. Great care is therefore taken in the production of Mullard valves to ensure uniform particle size in the emissive materials, and even deposition of the coating on the cathode surface.
Conventional valve heaters have to fit within the cathode and must therefore
be as compact as possible. However, the heater wire must be thick enough to
carry the current to heat the cathode adequately, and thick wire is subject
to large stresses when wound in a tight spiral. In valve types in which the
dimensions are such that the heater could prove to be a source of trouble,
Mullard have replaced the single strand of wire by two or three wires wound
together. In this way, the stresses on the heater wire are reduced without.the
current carrying capacity of the heater being lessened. The emission properties
of the valves are thus unaffected, but a marked improvement in heater reliability
during life is ensured.
Mullard have also developed methods of manufacture which produce equalised rates of heater temperature rise in all their television valves. This property prevents damage to the valve heaters during the warm-up period, and eliminates the need for any protective device such as a thermistor.
Mica discs which support the various electrodes are stamped out of high-grade
mica sheet. Leakage between electrodes can occur across the surface of these
discs. The problem—and therefore the solution—is different for valves required
to operate at e.h.t. voltages than for those that operate at lower voltages.
In the pro- duction of the lower-voltage valves the well-established practice
is to coat the micas with insulating oxide. This roughens the surface of the
micas, and hence lengthens any leakage path that may be formed on the surfaces
and increases its resistance.
In e.h.t. valves, internal flashing is caused by the charging and discharging of areas on the surface of the mica supports, and is noticeable as ragged edges on the raster. In the production of Mullard e.h.t. valves, the micas are sprayed with a compound having a low chloride content. This prevents the areas of the mica supports from becoming charged.
Microphony is the unwanted modulation of the signal due to the vibration of electrodes, and occurs when the electrode structure is insufficiently rigid. A major cause of non-rigidity is the incorrectly sized locating holes in the micas which allow movement of the electrodes. Close control over the dimensions of the holes in the micas of Mullard valves minimised the risk of microphony.
Further protection is provided in valves designed for operation at very low signal levels and high gain levels by the use of finger micas. These are extra micas, normally mounted on top of the main micas, in which is cut a shaped slot. The resulting finger in the mica is used as a sprung stop to exert a slight pressure on the electrode, thereby damping any movement. The combination of finger mica and closely toleranced locating holes gives the lowest attainable level of microphony.
Good valve design and strict control of raw materials and component parts
make possible the production of high quality valves, but they alone do not
ensure it. Equally strict control must be applied to valve assembly if the
required quality is to be achieved.
In the assembly of Mullard valves, the main objectives of control are the exclusion of dust and other contaminants, and the avoidance of mechanical damage and distortion.
Extensive use is made of covers on the assembly benches, and dust is excluded from the assembly room as far as possible. The operatives wear nylon overalls and use latex finger stalls where necessary. The assembly rooms are cleaned by suction cleaners, and only outside working hours, so that there is a minimum of floating dust while assembly is in progress.
Mechanical distortion and damage during assembly, already guarded against by careful design, are mini- mised by the extensive use of jigs and mechanical assembly methods. These methods ensure the precise positioning of each component as it is added, and obviate skew insertion of cathodes, grid backbones and anode lugs into the micas, thus avoiding enlargement of the holes and slots. The complete assembly therefore reaches the high level of accuracy and consistency that are required.
Completely automatic assembly is used for some types of Mullard valve. The various component parts of the valve are fed, after cleansing, into hoppers at appropriate points in the assembly chain, and the structure is built up automatically from these parts without being touched by hand. The highest standards of cleanliness and precision are thus ensured throughout the assembly.
Special features, not described in the preceding sections, are used to fulfil certain functions in specific valves. For instance, a single-sided electrode structure has been used in the PC88 to give the special qualities required for satisfactory operation at ultra-high frequencies. Again a "cavitrap" anode is embodied in the PL500 to ensure that the performance of the line timebase in dual-standard receivers does not deteriorate from one line standard to the other.
In the Mullard PC88 u.h.f. triode, a frame grid and locked-seam cathode are used to give the requisite high value of mutual conductance. Grid lead inductance, which is troublesome at u.h.f., is minimised by con- necting the frame grid to five base pins. Valve capacitances are minimised by using an anode that presents a surface to one face only of the cathode. Stability is thus ensured at the ultra-high frequencies for which the valve was designed.
Screen-grid current can seriously limit the power to be obtained from line output pen- todes. In the Mullard PL500, a specially designed anode—the cavitrap anode—is used to limit secondary emission, which is a primary cause of screen-grid current. The cavitrap anode contains deep partitions which recapture any secondary electrons emitted from the anode. A good ratio of anode current to screen-grid current is thus achieved, so that adequate output power can be delivered to prevent significant changes in performance between 405-line operation and the more exacting 625-line operation.
For many years, Mullard have pioneered the manufacture of multiple valves
such as double triodes and triode pentodes. The latest of these—the PFL200—is
the first double pentode to have been mass-produced. One section, the 'L'
section, is designed for use as a video amplifier on dual-standard receivers;
the 'F' section is designed as a voltage amplifying pentode.
The requirement for a video output pentode with very high slope together with high-peak current capability calls for the use, firstly, of a frame grid and, secondly, of an increased length of the cathode. The usual construction of a frame grid in small receiving valves has been modified to make the required increase in length possible. A cross-member has been introduced at the centre of the frame to give additional support to the longer side rods. The power pentode section of the PFL200 is the first mass-produced television power pentode with a frame grid.
A conventional stretched grid and locked-seam cathode are used in the 'F' section of the PFL200.
Special measures have been taken in the spatial arrangement of the two valve sections in a 7/8in diameter glass envelope. The two systems are at right angles to each other to minimise mutual heating by direct heat radiation. This arrangement also allows wide flanges to be fitted to the anode of the 'L' section to assist in the radiation of heat outside the valve. For this reason also, there is no circular screen surrounding the two systems, the intersection screen being sufficient to prevent interaction. Because of its position, the'F'section assumes a peculiar shape which follows the curvature of the bulb. The screen-grid wire of the power pentode is carbonised to permit higher dissipation, especially under no-signal conditions in video amplifiers.
Measures have been taken in the con- struction of the PFL200 to ensure the minimum interaction between the two sections of the valve. Use of the new 10-pin decal base has enabled the two sections to be completely separated. The important cross-capacitance between the control grid of the 'L' section and the anode of the 'F' section has been reduced to below 0.005pF by the use of an intersection screen, a screening box on top of the 'L' section structure, and screening between the leads from the electrodes to the base pins. A finger mica is used to damp vibrations of the 'L' section cathode, thus reducing the possibility of microphony.
From design, through development to the final execution of a mass-produced valve, Mullard expertise is abundantly evident. This expert knowledge and experience enables those qualities and characteristics to be built into the valve that are essential for the optimum operation of the television set, radiogram or record player for which the valve is designed. It is the embodiment of the Mullard philosophy of product design and service.
|CLICK HERE TO RETURN TO
THE VALVE TECHIE PAGE
2nd February 2002