Sensibly equivalent¶ to:
This exhibit is one of the Mullard 'PM' series valves, dating from c.1927. This is deduced from the fact that the base cap in the exhibit is not of the style used in the earlier PM series.
The marking 'F27' is a factory batch code (possibly a date code). The valve itself might well be a PM3 or PM4 although these are not the only possibilities. The valve would originally have carried a paper band round its base cap marked with its type number and ratings but these bands were only lightly glued on and are often missing. The BBC stamp remains visible, but is not shown on the photograph.
The anode is a square box, positioned horizontally.
The balloon envelope is 43 mm in diameter and, excluding the B4 base pins, is 90 mm tall.
Reference: Observation. Type PM3 was first introduced in 1926.
Absolute Maximum Operating Conditions¶
Updated April 04, 2015. shape:balloon construction:pinch type:triode age:1920.1930 base:b4 pins:4 pin:1.a pin:2.g1 pin:3.f pin:4.f