The original tubular envelope of the original design ORA was replaced over time with several other designs. This exhibit has the type one BBC stamp. This stamp was used from 1st November 1922 until September 1924, giving a band of dates for the use of this shape.
The plated nickel base cap and above the ORA Type designation and logo. The Mullard name is above the logo.
A better view of the Mullard name with the BBC stamp on the right.
This image has been enhanced to bring out the detail of the etched images.
The microscope shows the detail of the BBC stamp letters. The letters look to have been printed onto a moving glass tube.
The grid helix is fixed along the length to a stout wire support the is then welded to the lead from the pinch. The filament wire is held in folds in the end supports. Note that this low temperature filament does not have an obvious additional tension spring. It is probable the the bend in the support from the pinch applies sufficient tension if the wire is stretched during assembly.
The microscope shows the filament strand and the grid fixing to the support.
The grid loops are fixed to the backbone support. It is not clear if the fixing is weld or notch.
The filament top support fold. Note that the wire beyond the top of the support has a thicker coating than the active filament. The oxide coating would be decomposed by heat during evacuation of the bulb and the outside wire would not heat as much. The grid loops appear to be just touching the thicker support - probably evidence for spot welding rather than notch and peen assembly.
The lower end of the filament and the weld between the grid support wire and the copper wire from the pinch.
The thin glass tube envelope is 34 mm in diameter, and excluding the B4 base pins, is 88 mm tall.
Reference: 1003. Type ORA was first introduced in 1922. See also1922 adverts.
