The ORA from Mullard was designed in 1922 as a development of the K valve and the redesign was prompted by the commencement of broadcasting. Like the K valve the ORA was made with two different base designs. The ORA-A used the commercial standard B4 base, and the ORA-B used the IS/1 used by the military and demanding applications.
The ORA stands for Oscillator, Rectifier, and Amplifier - its planned usages. It is a bright emitter valve and had a μ (amplification factor) of 8.5.
This image has been enhanced to show the ORA letters and associated logo.
The BBC stamp shows both BBC and Post Master General identification. This stamp was used from 1st November 1922 until September 1924. See The Dawn of Broadcasting in the UK.
The metal strip seen at the bottom of the picture connects to the top cap and is the second filament connection.
The base cap is 32 mm in diameter and 37 mm long. The filament connects to the point on the right of the picture. The anode and grid connect to the side plates. The moulded base clearly indicates which connection is the anode and grid respectively.
The external second filament connection. The remains of the filament can also be seen. With the BBC stamp at the bottom of the image.
The electrodes in close-up together with their fixing arrangements. The ORA and logo is also clearly seen.
Another image of the etched ORA logo. Above can be seen the Mullard identification. The grid is a wire helix spot welded to the support rod and extends outside of the anode cylinder. Of the ORA-A valves in the museum only ORA has the same grid style.
The thin glass tube envelope is 25 mm in diameter, and including the I/S1 base pins, is 84 mm tall.
Reference: 1003. Type ORA-B was first introduced in 1922.