Miscellaneous varieties of valves 1

This is a rat bag of valves which considering the scarcity of ordinary types will all be now pretty rare

DLS19 Thermal Relay

 This is quite a rare thing (meaning that I've only ever seen this one example).

With valved equipment it's advisable to delay application of HT until valve heaters have reached normal operating temperatures. Most equipments used rectifier valves which of course need to warm up before they can pass current and this delay ensures the rest of the valves would have time to warm up also.

Once semiconductor diodes had been commercially released and used in valved equipments there was a problem. Turning on the mains switch would instantly result in HT being applied to the valves. There had been devices prior to this encapsulated thermal relay which had been used in military equipment. These were thermal relays using a heating coil wrapped around a bi-metal assembly carrying contacts. They were about 3 or 4 inches tall and about 2 inches wide and an inch thick enclosed in a perforated metal case. Their delay time was partly dependent on ambient temperature and they needed to be adjusted with a screwdriver.

The DLS19 and similar devices made the thermal relay easier to embody in an equipment and provide a reasonably toleranced delay without the need for careful adjustment and testing. This device operates on the equipment's valve heater voltage and can be fitted alongside the valves. When the thing expired or became unreliable it could be swapped without the need for dismantling, soldering and adjusting like the previous variety.

This example was designed to switch up to an amp at DC and a couple of amps at AC

See the specification

24T12 Thermal Relay

Having found one thermal relay in my collection of valves, I found a second shortly after.

This type has a faster operating time than the DLS19

See the specification


ZM1175 Numeric indicator tube, commonly called a "Nixie" tube.

 This example, kindly donated by Donald Thomson is the Mullard ZM1175, which uses a wire ended base. The valve has a set of numbers arranged in front of one another and uses the neon glow effect to illuminate one of the digits. They use an HT voltage to strike the neon and this is applied to the particular digit to be displayed. Usually the colour is red or orange.

The tubes were used extensively in electronic test equipment and industrial equipments from the 50s until LED's took over. The latter are still occasionally used, but in turn have given way to low power LCD displays. Another type of display is a high-vacuum type that uses a filament and was found in most VCRs.

Coupled with the development of displays are the decoders needed to drive them. In particular LED types normally employ segments, which are lit in a particular pattern to display specific numbers. Special decoder chips that convert either binary codes or one of ten address lines to illuminate the required pattern of segments were used for many years, later to be made redundant by including the necessary decoding into processor chips that integrated many other functions. These make use of a data bus that can route codes and commands around the equipment.

NSP1 Strobe Lamp

 Robert Holmes who used to be kept busy at the Gem Mill works of Ferranti tells me that the NSP1 was still made there at the tail end of valve production in 1969 and is a "Strobe lamp" used in things such as stroboscopes. I guess the modern equivalent of this type, which must have been used into the 70's, is a flash tube filled with Xenon. High power lasers are a distant relative of these devices. I recently read an article in a very old electrical engineering book about the different effects obtained when applying a high voltage across a glass tube carrying a soft vacuum of either plain air, traces of various metallic elements or rare gases. From that science, which developed through experimentation and trial and error, sprang fluorescent tubes, voltage stabilisers and presumably this device. Until the discovery of the electron, near the end of Queen Victoria's reign, scientists were hard pressed to explain what they saw. One old chap who was a famous physicist at the turn of the century hadn't got to grips with new fangled electrons and was convinced that current flowed through the gutta percha insulation of undersea telephone cables rather than the metallic conductor. In terms of exactly where the current flowed he wasn't too far off the mark. The majority actually flowed in the outer skin of the conductor and hardly any in the main body of the wire.

VME4 Tuning Indicator

 Anyone that used to have a good quality wireless will recognise this type of valve.

This one is on the rare side as its got an old B7 base rather than the more common Octal style.

The top circular area lit up in a rather nice shade of green, something like a newish snooker table baize. With no signal the lit area was about 270 degrees and as a station was tuned in the lower unlit section, which was about 90 degrees started to get smaller until, if the signal was very strong, closed right up.

There probably are very few around now that light up with a decent colour because as the valve gets older the original rich green gets weaker.

I read somewhere that decent illumination can be restored. Was it baking in an oven or popping in the microwave for 10 seconds? Maybe someone has tried this and could let me know?

Cold Cathode Indicator Valve

 I have quite a few of these things. The end of the valve has an array of little neon elements which light up in a circular pattern. Its use was in things like counters where the neons would be externally numbered 0 to 9.

The valve does not have a heater, hence the description "cold".

RG1-240A Mercury Vapour Rectifier



 This type of valve was used to provide a high current at a high voltage.

This example is an RG1-240A which has an anode rating of 2.2kvolts and can supply a maximum rectified current of quarter of an amp. It's big brother the RG3-1250 is rated at 8000 volts 1.25 amp.

The envelope contains mercury vapour and when operating glows in the typical eerie greenish blue colour of mercury vapour lamps used to light some streets.

Because mercury is considered to be pretty dangerous the valve was rolled up inside it's box in several yards of soft wrapping paper.

Judging from the note which accompanied this example, this type of valve was not an easy thing to use. The circuitry should have a delayed action relay to prevent the HT supply turning on too early and the equipment operator must have had to be pretty intimate with its pattern of use!

A big problem was the heater supply whose insulation needed to be up to the mark.


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