|
Date: |
|
Description: | Mullard EF9 A BVA A BRITISH MADE (black stencil on the red coating on the glass envelope). 438 4H2 (white stencil on the bakelite base). MANU Mullard Radio Valve Company Limited. circa 1939 Mitcham, Surrrey, England. DES after TELLEGEN,Bernard,D.,H., and HOLST,Gilles. 1926 Philips Physical Laboratory, Eindhoven, Holland This is a Mullard EF9 red series variable mu pentode with an indirectly heated cathode. It was designed for use in in the automtic gain control (AGC) stages of superhet receivers. It could aslso reputedly be used for intermediate frequency (IF) and for high frequency (HF) receivers also radio frequency (RF) amplifiers. But it seems to have been used originally for 130 kHz sets. It has 8 Ct8 base pins and a top cap instead of a IO base. The valve is screened with a red metalised coating, which was added largely to disguise the splashover from the getter.
The EF series of valves were launched with the EF1 in 1934 the red series were launched in 1936 in France and appeared in Britain in 1937. The EF9 was launched in 1938.
A pentode has five electrodes and all the electrodes are usually mounted coaxially. In an indirectly heated pentode the cathode heater is usually in the form of a coil surrounding the cathode or "filament" at the centre of the assembly. The anode, or "plate" is the outermost electrode and in this case it is in the form of two wings. When the valve is switched on current flows through the "filament" generating electrons, which pass from the hot filament to the plate at a higher voltage determined by the geometry and the temperature to which the filament is heated. This element is a voltage amplifier and diode or half wave rectifier.
Around the filament, usually made of an oxide coated or thoriated tungsten wire or rod, and the heater coil is the control grid made of another coil of wire. Depending on the voltage flowing through it the control grid it alters the flow of electrons, from the cathode to the the plate effectively switching the flow on and off. This element is the triode or voltage amplifier and contoller "valve".
Around the triode is another coil of wire called the screen grid. A fixed potential across this electrode, or auxilliary grid, helps to isolate the control grid from any voltage fluctuations that may occur on the plate. This element of the valve is in effect the tetrode.
The fifth electrode, the supressor grid, is another coil of wire mounted between the screen grid and the plate. When the voltage in this coil is equal to or at about the same voltage in the filament it helps prevent electrons, reflected from the plate, causing interference. This electrode completes the pentode valve.
Sir John Ambrose Fleming (1849-1945) investigated into the cause of the blackening of the insides of lamps due to the evaporation of the filament. He also looked at the shadows in the deposits on the lamp envelope cast by the filament supports. These effects were caused by the fact that at about 2,500 C a tungsten filament in a vacuum begins to slowly evaporate. When used as a hot cathode in a thermionic valve it limits the life of 'bright' emitter filament. However, lower temperatures dramatically lower the thermionic emissions of the cathode greatly reducing its efficiency.
Malleable tungsten is produced by a powder metallurgy method invented by William David Coolidge (1873-1975) in 1903. Coolidge found that the addition of a little thorium oxide made the tungsten even more ductile. Irving Langmuir (1881-1957) found that filaments made in this way also produced greater electron emissions than pure tungsten and could work at much lower temperatures. The original "dull" emitters exploited this phenomenon and were also known as thoriated tungsten filaments. They operated at about 1,800 C.
In 1903 Arthur Rudolph Berthold Wehnelt (1871-1944) had found that grease contaminating a platinium filament produced enhanced emission and that oxides of alkaline earth metals such as calcium and barium etc. produced emission equal to pure tungsten but at much lower temperatures.
The oxide-coating process is often confused with the azide process, which was developed at Eindhoven by Philips in about 1924 and introduced to Britain when Philips acquired a half share in Mullard's in 1925. Among the Philips-Mullard; PM series of azide valves were the PM3 and PM4.
One feature of the Philips-Mullard azide process was the internal blackening of the glass envelope. For a more cosmetic effect magnesium was often deposited before the azide was decomposed in order to make the glass look silvery. Another more serious problem was that for high efficiency at low temperatures the cathodes required an evenly deposited thin layer of oxide on the filament. The azide process was fairly short lived because the barium was unevenly scattered all over the inside of the valve causing a variety of problems.
The process "chemical gettering" was first used by Sir William Crookes (1832-1919) in 1876 for regenerating the hardness, or chemically pumping down, the vacuum in discharge tubes. | License: | http://www.hmag.gla.ac.uk/spirit/rights/ | Publisher: | Hunterian Museum and Art Gallery, University of Glasgow | Rights holder: | Hunterian Museum and Art Gallery, University of Glasgow | Subjects: | SCIENTIFIC COLLECTION : | Source: | Hunterian Museum | Creator: | Hunterian Museum and Art Gallery, University of Glasgow | Identifier: | http://www.huntsearch.gla.ac.uk/cgi-bin/... | Language: | en-GB | Go to resource |
|
|