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The standard format of the Mullard type number as printed on Mullard and equivalent valves comprises two or more letters followed by a group of up to three figures, for example:
E CC 8 3
The type number is organised as follows:
explore ··· 
E C C 8 3
| | | | |
| | | | +-- Second and subsequent digits refer to a
| | | | particular design or development.
| | | |
| | | +----- First digit indicates type of plug base:
| | | 2 B10B (10-pin)
| | | 3 Octal (bonded-on 8-pin plastic base)
| | | 4 B8A base
| | | 5 B9D (magnoval, wire-ended, etc.)
| | | 8 B9A (noval, 9-pin integral with glass envelope)
| | | 9 B7G miniature 7-pin, glass
| | |
| | +-------- Class of second valve sharing envelope,
| | if any (as below)
| |
| +---------- Class of primary valve:
| A single diode
| B double diode
| C triode
| D power output triode
| E tetrode
| F pentode
| L power output tetrode or pentode
| H hexode or heptode (hexode type)
| K octode or heptode (octode type)
| M tuning or level indicator ('magic eye')
| Y half-wave rectifier
| Z full-wave rectifier
|
+------------- Heater or filament voltage or current:
A 4.0V
C 200mA
D 0.5 to 1.5V filament
E 6.3V heater
G 5.0V heater
K 2.0V
P 300mA heater
U 100mA heater
Therefore the ECC83 is a double triode with 6.3V heater and a B9A base, and the design is taken to 'version 3'. Other types can be deduced in this way, e.g., EC90, a single triode with B7G base and 6.3V heater; PCF806, a triode pentode with B9A base and a heater designed for a 300mA series heater chain, and was designed for use in mains powered TVs.
This data has mainly concentrated on the electrical characteristics of valves, since these are important for the circuit designer. However, the manufacture of valves, though of no concern to the designer or user, is equally fascinating and as a subject warrants inclusion in an industrial museum!
The Envelope
It has become obvious that glass was chosen for the envelope because of its ease of manufacture, material consistency, strength under compression to contain a vacuum and its tolerance to wide temperature variations, and was, logically enough, developed from the light bulb. It is also obvious that the plug-in type of valve is almost universal for easy replacement in the event of failure or ageing (but also because it's not a good idea to solder directly to glass embedded metal connections), hence the base pin connections and the use of chassis sockets.
Assembly
In those types where the insides can be easily viewed through the glass, the construction will be clearly seen. The valve components are spot-welded together into a support structure which typically comprises vertical rods carried in horizontal sections of mica or similar. However, getting this assembly into the glass envelope is NOT the same as putting a model ship in a bottle!
In fact the envelope begins its life in two parts – the envelope proper, and a glass base. In B9A valves, this base also carries the plugin connection pins which need an involved glass-to-metal seal and bonding process to prevent leaks (in octal valves these are solid core lead-out wires because the plug-in base is separate). These bases are usually supplied to the valve assembler prefabricated. The valve assembly is spot-welded to the base connections, then the whole slid into the envelope whereupon the two glass parts are joined by gas flame.
Evacuation
The envelope is then pumped to evacuate it. B9A valves have the small diameter outlet tube on top of the envelope, octal valves have it in the centre of the base at the bottom. Initially mechanical pumping is used, but this is unable to achieve a sufficient vacuum on its own, so at the point when no more can be removed by this method the operation is switched over to a mercury vapour pump. Ideally the air content should be reduced to an order of 10–7 atmospheres.
When the number of remaining gas molecules has been reduced to the necessary scarcity, the outlet tube can be closed by melting it with a gas flame. This action leaves behind the ever present pip on the top of B9A and similar small valves. In the case of octal valves, the plastic base carrying the plug pins would be glued on and the lead-out wires soldered into the hollow pins.
'Getter' Rings
But the device is not ready to be used yet. On examining a valve's innards you will also find metal rings or other curiously redundant structures, with what looks like patches of chromium plate adhering to the inside of the envelope, none of which apparently has anything to do with the way the valve works when in use, and that is because it doesn't. These are the 'getter rings', and the silvery stuff is the getter material. The getter ring form is most common, they can be found at the top of many B9A envelopes, but variations are not unusual. For example the GZ34 rectifier has its getter ring encircling its base, whereas the EL34 power output pentode has two small rectangular platforms supported on single rods at the top.
On assembly this structure carries a compound embedded in channels or depressions. After evacuation, the final operation is to insert the finished valve into an RF heater which is aligned to the getter ring. The ring is heated to boil off the compound, which deposits an silver metallic barium layer on the inside of the glass in the vicinity of the ring. The compound is only stable in a vacuum, as its property is one of quickly absorbing air molecules. As such then, the material not only depletes the last remaining air further, but also traps any gases that may be given off by electrodes while in use. A case where the glass has cracked and let all the air in is easy to spot – the getter material will have turned completely chalky white (as barium oxide).
NB: there are now a few films on youtube etc. about valve manufacturing. Particularly look for RCA and Osram.
