The Mullard DF.ORA was introduced in 1923/4 as a dull emitter replacement for the ORA. The 60 mA filament was a thin thoriated tungsten wire that vibrated in use and thus made the valve microphonic and was also fragile and prone to breaking.
During the mid 1920s valve development was a rapid process and a few months later in 1924 the D06 replaced the DF.ORA. The advert from December 1924 shows the DF.ORA as reduced in price.
This exhibit has the third version of the BBC stamp and thus dates to between 1924 and 1927.
The valve has etched identification and a paper band around the plated base shell. Although obscured by the gettering the electrode assembly of the DF.ORA was inclined at 30°.
The Mullard trade mark.
The balloon envelope is 35 mm in diameter, and excluding the B4 base pins is 90 mm tall.
Reference: 1003. Type DF.ORA was first introduced in 1924.