This beautiful valve is a very early double grid valve. Low voltage space charge grid valves originated in Germany with the SS1. The design operated so that the slightly positive first grid suppressed the space charge to improve sensitivity.
Other two-grid valves were designed in the 1930's so that a signal and a local oscillator could be mixed together as in DG2 or 41MDG but these are not bi-grid valves in the same sense. Also the screened-grid valve S625 introduced in 1927, was not a bi-grid.
The construction still follows the coaxial pattern within a horizontal cylindrical anode, but the fixing supports have become more formalised, and thus the parameters more consistent. The second grid is also a wire helix, but with far fewer turns than the first grid.
The balloon envelope is 51 mm in diameter and, excluding the B4 base pins, is 88 mm tall.
References: Observation & above article. Type Q(Bi-Grid) was first introduced in 1922. See also 1922 adverts.
Updated April 26, 2013.