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PT25

 
    
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This valve is clearly an early directly heated mains output pentode and the style places it to the early 1930s. The box anode is not carburised to increase radiation efficiency and the output is probably only a few Watts. The closest we can place this is the PT25.
The balloon envelope is 60 mm in diameter and, excluding the B5 base pins, is 138 mm tall.
Reference: Observation. Type PT25 was first introduced in 1933.

 

Updated January 16, 2013.
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