As an interesting example, we show a sectional illustration of a typical battery-operated Cossor Valve. First of all, a word about the filament. The efficiency of a valve depends very largely upon the electronic emission from its filament. The Cossor filament consists of a very tough metallic core on which is deposited a coating capable of emitting a very prolific stream of electrons at an exceptionally low temperature.

The fact that the Cossor filament functions practically without visible glow ensures consistent service from the valve. For obviously if it were necessary to heat up the filament to incandescence to drive off the electrons, such excessive heat would set up crystallization in the metal and ultimately cause a premature fracture.

So much, therefore, for the strong, economical and efficient Cossor filament. Examining the illustration more closely it will be noticed that the grid and the anode are mounted on very stout vertical supports, the ends of which project slightly through a mica bridge piece secured to the anode.


There are very important advantages to be obtained from this construction. First of all, it is enormously strong. Even the hardest blow cannot disturb any individual electrode. All are firmly locked together in absolute alignment. Again, it ensures extreme accuracy in assembly. No deviation is possible. The holes in the mica bridge piece are accurate to a thousandth part of an inch. The distances between filament, grid and anode, therefore, remain consistent in all valves of the same class - thus ensuring a remarkable degree of uniformity.


It will be noticed that the mica bridge and four insulated hooks welded to the grid supports provide a very precise anchorage for the filament. In this manner a multiple filament suspension system has been evolved which completely eliminates micro-phonic noises. It has been proved that microphonic noises are almost always caused by the filament vibrating at its natural frequency. Impulses from the loud speaker carried either through the air or through the valve pins are inevitably sufficient to initiate the vibration which rapidly builds up. The Cossor system cures this nuisance by damping out filament vibration at its source.


With the evolution of more elaborate types of valves possessing several grids, a short description of the way in which Cossor Grids are made will be of interest. Cossor Grids are manufactured automatically in a very ingenious machine. On each of the two grid supports are cut the requisite number of slots at carefully calculated intervals. The actual cutting of the slot raises a small ridge. The grid wire is wound into these slots with great accuracy. Finally, the ridges are turned down and each turn of the wire is firmly secured in its slot. This is a tremendous improvement over electric welding - the method previously used. When electric welding is used it may happen that one turn, not being properly welded, comes adrift. The result is a loose wire, with a consequent risk of microphonic noises or altered characteristics. Every Cossor Grid is slot wound with a very high degree of accuracy. This is one reason why Cossor Valves function with such an absence of mechanical noise.


Cossor was one of the first manufacturers to introduce a Screened Grid Valve, and the long lead that they had has enabled them to continually improve the design of this valve. It is not possible within the space available to go very deeply into the technicalities of Screened Grid Valve design. It is sufficient to mention that the one controlling factor in the efficiency of a Screened Grid Valve lies in its control grid-anode capacity. The lower the capacity the greater the effective amplification available In the Cossor Screened Grid Valve this inter-electrode capacity has been reduced to the order of 0.001 micro-microfarads, a figure which may be better appreciated when expressed as 0.000,000,001 mfd. This self-capacity is substantially lower than that of any other battery-operated S.G. valves on the market. Therefore the Cossor S.G. Valve definitely permits a much greater effective amplification to be obtained.

As will be observed from the sectional illustration, the construction of Cossor S.G. Valves is remarkably robust. By the use of an ingenious system of mica bridge pieces, the various elements in the valve are secured in permanent alignment. Even in the event of the valve receiving a blow, not one of the elements could be displaced from its correct relative position.

This system of Mica Bridge construction, evolved and perfected by Cossor, is utilised throughout the whole range of Cossor Valves. Naturally, with the development of the latest and even more elaborate types of valve, such as, for example, the variable-mu H.F. Pentode, the utmost accuracy in assembly is essential. The Mica Bridge method is invaluable in making possible very small tolerances.

From a constructional point of view there is very little difference between the three ranges of Cossor Mains Valves. In the case of the 4-volt series the heater consumes 1 amp. and the valves are usually used for A.C. mains working. The range of Cossor Mains Valves consuming 0.25 amp. at 16 volts, may be used for series running on D.C. mains and are valuable for replacements in those receivers that take the standard 16-volt mains valve. The 0.2 amp. series includes an indirectly heated rectifier and is therefore eminently suitable for A.C./D.C. sets.

The construction of Cossor Mains Triode Valves follows along the lines which have proved so successful with battery-operated valves. The mica bridge system has been retained in its entirety - and has even been strengthened by the addition of a second mica bridge below the assembly. As will be observed from the illustration, the cathode - which is heated by means of an internal heater wire throughout its whole length - is secured to two mica bridges. Around it is assembled the grid, mounted on two stout supports. And, finally, surrounding the whole assembly is mounted the anode, securely attached to the two mica bridges. The anode itself is of gauze construction, in order that the heat generated within the cathode shall be more readily dissipated.

Obviously, such a construction is immensely strong - even the hardest blow cannot affect its working characteristics or cause any material damage. As has already been seen in the description of Cossor battery-operated valves, the mica bridge system ensures a very remarkable degree of accuracy in manufacture being attained. And this means, therefore, that Cossor Mains Valves are exceptionally efficient in operation.

Cossor Mains Screened Grid Valves and Mains Pentode Valves are similar in design to battery-operated types, with the exception, of course, that cathodes replace the directly heated filaments.

© A.C. Cossor Ltd., Cossor Works, Highbury Grove, LONDON N.5

© Andy Cowley, 2005    Home