The ORA from Mullard was designed in 1922 as a development of the K valve and the balloon envelope was adopted in 1923. The design was prompted by the commencement of broadcasting.
The picture above shows the anode on this sample is an open cylinder. The grid is clearly visible but it is not known why the anode was made open. We have found no reference to such a variant and it could simply have been a mistake or a sample to show the new grid. Originally the grid was shaped flat and then curved to an open cylinder. See ORA and ORA. The grid in this exhibit is a wide spaced helix and the filament is a single strand, tensioned by the support arm.
The ORA stands for Oscillator, Rectifier, and Amplifier - its planed usages. For the time, a reasonable number were made.
The balloon envelope is 32 mm in diameter and, excluding the B4 base pins, is 80 mm tall.
Reference: 1003. Type ORA was first introduced in 1922.
Absolute Maximum Operating Conditions¶
Updated May 20, 2015.