This valve looks like a Mullard Type ORA DF/Red which was a version of the Mullard ORA with a thoriated tungsten filament, probably made around mid-1925. The red ring indicates an HF version of the valve which would have a higher Ra value than an LF version (with black? ring). However, Mullard made several valve types of similar appearance at about this time.

This valve is silvered on the inside of the envelope and the electrode structure is obscured.

The balloon envelope is 38 mm in diameter and, excluding the B4 base pins, is 85 mm tall.

This exhibit was last updated on 07 August 2006