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This valve is marked as M25 and is believed to be an experimental prototype. It is clearly marked Madrid and December 1929. It is believed that the design was based on work that originated with Western Electric in the USA.
The filament is made of tape that is folded into a double inverted V or M shape.
Further research suggests that the construction of a glass pillar rising from the pinch tube and from which rigid supports emerge to hold two flat plate anodes and grids either side of an M shaped filament and all mounted in a spherical envelope matches the description of the Western Electric PA4 from 1925.
The anode is formed from a pair of bright thin plates. The grid is also formed on either side of the filament. The electrodes are supported on a substantial glass rod. Interestingly this rod is split into two. Possibly a fault.
The grid is formed by horizontal wires lightly tensioned between stout supports.
Seen face on to the anode, the filament can be seen at the bottom as a small 'v'. The wires at the top connected to the glass support form the top filament supports and tension spring.
The tape nature of the filament is clearly seen above.
The balloon envelope is 60 mm in diameter, and overall it is 90 mm tall.
Reference: Observation. Type M25 was first introduced in 1929. See also 1929 adverts.


Updated April 30, 2013.
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