The D.3 design from Mullard dates from 1924 and was a replacement for the bright emitter HF. The D.3 is a thoriated tungsten dull emitter triode and the two red rings denote the HF variant. Other variants included one with two green rings for LF use also introduced in 1924 and the detector with two white rings introduced in 1925.
The D.3 HF had a μ of 17.
The D.3 differs from the D.06 in that the latter was designed with a 3.0 V 60 mA oxide coated filament whereas the D.3 range have thoriated tungsten filaments. Electrically they have the same characteristics.
The cylindrical anode is mounted on the pinch at 45 °. The helical grid is fixed to a single side support. The filament is a single strand.
Distorted by the glass the filament can be seen held in a fold of the support.
The balloon envelope is 35 mm in diameter and, excluding the B4 base pins, is 82 mm tall.
References: Data & 1003. Type D.3 was first introduced in 1924. See also1924 adverts.
