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Description: | 6L6G (acid etched on the glass envelope). Ken-Rad MADE IN U.S.A. B7 (moulded in the bakelite base). X 24/MA (scratched on the bakelite base). 27 (written in black on the glass element support). LICENSED ONLY TO THE EXTENT INDICATED ON THE CARTON (moulded in the bakelite base). MANU Ken-Rad Tube & Lamp Corporation. circa 1950 Owensboro, Kentucky, The United States of America. DES after ROUND,Henry,Joseph,Captain. 1926 Chelmsford, Essex, England. DES after Marconi's Wireless Telegraph Company Limited. 1933 Chelmsford, Essex, En This is a Ken-Rad 6L6G "kinkless" power beam tetrode with an indirectly heated cathode. It is a varient of the American RCA 6L6. The 6L6G is a larger valve designed as an audio frequency amplifier but could also be used as a modulator for AM transmitters. It has an 8 pin loctal B8B bakelite base with one pin removed.
The valve has a getter suspended from one of the plate (anode) support wires. Unusually this valve has a carbon coating on the inside of the glass envelope but the getter is mounted below it so the carbon does not disguise the splashover.
The beam tetrode was invented by Marconi engineers in 1933 but they thought the valve would be too difficult to construct for normal production purposes so the technology was licensed to RCA in 1936 for their 6L6 valve.
A tetrode has four electrodes and all the electrodes are usually mounted coaxially. In an indirectly heated tetrode 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 a metal cylinder. 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. The control grid is 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 controller "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.
In a beam tetrode there is a fifth element, which is in the form of metal plates connected to the screen grid that becomes electrostatically charged and by repulsion focusses the electrons from the cathode onto a small area of the plate.
Also in a beam tetrode the pitch of the screen grid and the control gridwindings are the same so that if viewed from the side they share the same aspect and the control grid windings disappear.
The getter is suspended from the plate support wires.
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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