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See also:
The Round Valves - Some Important British Valves
    
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It is not too easy to identify the round valves. This fine sample has 2V-PIF on the envelope and this would be associated with the type CA or the type N. The CA does not have the GES base so this would be a type N. The Serial number starts with N also. The N Marconi triode dates to 1914-15 and PIF indicates a Platinum Iridium filament. The optimum anode voltage of 35 can be seen in the sticker on the envelope.
The Round valves were all soft vacuum types with the performance controlled by the amount of gas in the envelope. If performance was dropping off then the top bulb would be heated to release more gas. To prevent an excessive current flow through too much ionised gas a series resistance of about 2,000 Ohms would have been used in the anode circuit. These valves outperformed the early high vacuum types but were difficult to both make and use. The demands of WW1 for large volume communications saw the development of the high vacuum valve and the end of the era of the Round valves. See also R Type.
The angled picture shows more clearly the arrangement of the electrodes.
The close-up of the grid makes the type of mesh used clear.
The type N was a receiver valve and used as a high frequency amplifier. The Marconi Type 27 receiver used the Round N as an HF amplifier. This receiver covered 60 - 500 kHz.
The asbestos pellet is held in a 10 mm diameter tube and from the dome of the envelope this tube is 38 mm in length.
The wide glass tube envelope is 48 mm in diameter and, excluding the GES base is 160 mm tall.
References: 1003. Type N was first introduced in 1915.

 

Updated April 30, 2013.
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