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Description: | G.E.C. [MADE IN] ENGLAND (printed in white on a black and grey label which has been dammaged in one corner). W727 6BA6 NF D (Mullard valve code and Marconi Osram equivalent with the date and factory code in a white stencil on the glass envelope). MANU Mullard Radio Valve Company Limited. June 1957 New Road, Mitcham, Surrey, England. DES after TELLEGEN,Bernard,D.,H., and HOLST,Gilles. 1926 Philips Physical Laboratory, Eindhoven, Holland. DES after PORTA,Paolo,della. 1952 Lainate, Milan, Italy. This is a Mullard high slope, variable mu, high frequency (HF), screened grid, miniature, remote cut off, pentode with an independently heated cathode. It has an all glass construction on a 7 pin B7G base and it has a ring getter mounted on the plate (anode). Although this valve was reputedly designed as an HF type it was used in the AM section of stereo amplifiers. The discrepency is accounted for in the history of its development because it is a wide bandwidth variable mu replacement for an earlier type.
The miniature B7G valve base was first introduced in 1940. This 6BA6 type was first registered in America in October 1945 but the company that introduced it is as yet unknown. However, it is the miniature replacement of the RCA 6AB7, a loctal based video IF amplifier valve introduced in 1939. It can also replace a similar loctal based valve the 6SH7 a wide band width version of the 6AB7 and the 6SG7, which has the added variable mu characteristic.
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. 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". If the pitch of the tirns of the helix of wire forming the control grid changes then the gain of the valve can be altered and so the valve becomes a variable mu pentode.
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 |
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