The D.3 design from Mullard dates from 1925 and was specifically aimed at detector use, it is a thoriated tungsten dull emitter triode and the two white rings denote the detector variant. Other variants included one with two red rings for HF use also the LF with two green rings both introduced in 1924.
The D.3 Det had a μ of 7 the same as the Green ringed LF. It is probable that the LF and Det had identical characteristics.
The D.3 range differs from the D.06 range in that the latter was designed with a 3.0 V 60 mA oxide coated filament whereas the D.3's have thoriated tungsten filaments.
The reverse with the feint Mullard logo etched into the glass.
An attempt to reveal the complete image of the face information. The third type of BBC stamp is just visible above the base cap.
The reverse lettering with the word DETECTOR clearly seen.
The angled anode folded into a cylinder and welded to the support. The helical grid is fixed to a sturdy support rod.
With a dark-field the lettering becomes clearer including the elusive BBC stamp.
Same illumination for the reverse side.
The fine serifs of the font are almost visible.
μ-Tracer plot of the characteristics with 100[k[Volts on the anode. The shape of the curve matches the LF for grid voltages of -20 to -10. The saturation as the grid gets close to zero is possibly just low emission from this sample. Is it possible that rejects from the LF manufacture were sold as detectors to avoid the wastage?
Special thanks to John Barber for allowing the museum to borrow his D.3 set.
The balloon envelope is 35 mm in diameter and, excluding the B4 base pins, is 85 mm tall.
Reference: 1003. Type D.3 was first introduced in 1924.