Miscellaneous varieties of valves 2

The numbers are still growing!


This is a rare Cosmos AC/G mains-operated triode dating from 1927

 Because this sort of thing was so new and revolutionary, scant thought was given to valve numbering. Perhaps, as often happens, it was imagined that as this was the pinacle of valve development, nothing would ever usurp it.

The letters "AC" followed by a single letter were quite adequate. A little later, when more complex valves were introduced, "PEN" and all sorts of combinations of letters like AC/4Pen were used. Unfortunately the control of numbering was poor and the "/" sign was sometimes dropped, such as AC/HL or ACHL, which must have led to some confusion as the types were often different.

A second difficulty arose, no doubt because of accelerating popularity leading to a plethora of new valve types. Two manufacturers would each bring out a valve having exactly the same characteristics but one would have a 5-pin base and the other a 7-pin base.

Further difficuties arose when screening between valves was accomplished, in 1931, by coating a valve with metalisation. How then did one know whether you were ordering one with bare glass or with metal coating?

If one wanted a 5-pin metal coated valve and a 7-pin shiny glass one arrived, it could be a very expensive mistake as not many could be bought from a typical wage packet. It took many years before universal numbering systems were commonplace.

This "Cosmos" valve was made by Metropolitan Vickers. This name was such a mouthful that it was truncated to "Metro Vick" then "Met-Vick". The valve number "AC/G" stood for "AC-powered filament, Shortpath, Green Spot". There was an identical looking valve, the Red Spot. The former was a high gain type and the latter a low gain power output valve. Unfortunately, despite peering at the faded letters on my example I cannot determine whether it is the "G" or the "R" version. Both types cost a princely 22/6d plus tax in 1927. The earliest types had a top pip and one could convert an existing battery powered set by first interposing a 6d adaptor plate between the 4-pin holder and the AC/G or AC/R valve, then connecting the heater wires, that came wired to the adaptor plate, to a low voltage transformer winding.

 These valves have three longer-than-usual pins arranged non-symmetrically and two short pins and I'd puzzled over this for a time before the penny dropped. It was clearly designed to replace a battery valve. The anode and grid are connected as per usual, but the indirectly heated cathode is connected to one side of the filament circuit. The pins are extra-long because room had to be left between the valve base and the socket for connection of the heater supply to the two short pins. Very clever! I wonder if any other manufacturer had the same idea?

Since collecting the first example I picked up two more. The left one is a "pip top", indicating an earlier date, possibly around 1925?

The one on the right has its pip inside the base, making it more robust. The two later additions are "Short Path" and designated a "Red" and a "Green" spot respectively.


BU100/14 Barretter

 This valve was used to dispose of unwanted voltage.

The coding "100/14" represents a nominal drop of 14 volts at 1 Amp although the voltage rating has a tolerance of some +/- 50%, a far cry from the tolerance of a modern semiconductor devices. It probably had a number of uses. The filament resistance changes as it heats up so it can provide surge limiting and can be used in a series chain of valve heaters preventing excess current flow when applying a voltage across them from cold. TV sets used to use a thermistor device made from a sort of carbon to do a similar job many years later. CRT TV sets use another device called a posistor, not for controlling valve heater current, because they don't have any, but for controlling a burst of AC power to the cathode ray tube degaussing coil at switch on. As the posistor heats up its resistance rises very rapidly, fast enough to prevent the mains fuse from blowing up when the very low resistance degaussing coil is connected across the mains supply. This is one reason why a TV set uses a mains fuse with a Time Lag. Use a fast action fuse and it'll go pop.


Cadmium Sulphide Detector

 This valve with a standard 4 pin British base is a Cadmium Sulphide light sensitive detector. I can't remember where it came from but this example looks fairly old. The valve contains a piece of metal reflector shaped as a paraboloid focussed on a detector wire. When light falls on the surface of the reflector which is coated in cadmium sulphide the material emits electrons which are picked up by the detector wire. Rather than emitting electrons by heating a thoriated filament this technique is cold and requires no heater. One application for such a device is in a camera exposure meter. The type used in these are much smaller and benefit from being able to directly drive a small meter without the need for batteries.

I've no idea for how long the material lasts but I imagine it would need to be protected from light to preserve its properties if its end use relies on an absolute reading otherwise photographs would get more overexposed as the years went by.

Another application may be associated with turning on lights when it gets dark. Street lights used to be operated either from little control boxes carrying a clockwork device that was rewound by electricity, an electronic device using a light sensor such as this driving a relay or in some cases by control signals sent through the mains.

I've heard from John, an ex-RAF radio operator who told me that these cells preceded photo-transistors in 16mm optical sound projectors of 1940/50 vintage, such as the Bell & Howell series. The "valve" converted light pulses projected from the optical soundtrack with the help of the "exciter lamp" and a special lens unit which put a hair-line beam of pure d.c.light on to the track.


Early high power lamp

 This lamp is of French manufacture "Buisson Ardent", which translates literally as "Burning Bush" and is rated at 1000 watts at 110 volts. Said to be made for the French Military by the chap I bought it from... what for exactly? With its pip-top it looks 1920s.

The lamp is 10 inches long and the Edison screw connector is an inch and a half diameter

Who knows what it was for?


Two Old Triodes

 Each of these old valves carries the code "C.T.10" and on the opposite side "SELFRIDGES".

What has a London department store got to do with valves?

Selfridges was very well known to radio enthusiasts in the early 20s as it was one of the birthplaces of broadcasting. Towering over Regent Street in London, on the roof of Selfridges store was one of the first broadcasting aerials, that of "2LO" on 365 metres, and no doubt, as one of the perks of leading the way was to market their own branded radio goods.

Obviously, if they had a big radio transmitter and a huge aerial on their roof, they must be experts in the field and a reliable place from which to buy expensive investments like radio valves. The transmitter operated there from 1925 to 1929.

Even in the early 50s, if one were prudent, one could spend under a pound in the local grocery shop for a weeks provisions, so paying over a pound for a single radio valve 30 years earlier must have been a tremendous outlay for the typical family man.

In 1925 Selfridges had also demonstrated television to the general public, although this aspect of broadcasting was slow to come to fruition as its inventors had to wait for developments in components (around 10 years) before the art was of sufficient quality to attract a large audience.

Unfortunately, some years later in 1931, the head of Selfridges started importing Philco radio sets from the US, and blotted his copy book with British Industry, who were incredibly protectionist.

They would have done better, perhaps, to commission Plessey, for example, to make sets with the Selfridges name., after all this was the sort of thing their competitors were doing. Take the Co-Op for example, selling "Defiant" sets... now why "Defiant"? Well...It wasn't just a name... but that's a different story.

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