This exhibit is a French Dario Resistron triode from 1928.
The Resistron indicates a valve made when transformer coupling between stages was being replaced by resistance/capacitance coupling. Increased gain of the valves made this possible.
The maker, type and 'Made in France' are very etched into the envelope. The English spelling suggests a valve for the UK market.
The filament is a single strand of thoriated tungsten wire passing centrally through the grid spiral. When energised it glows bright orange. The grid wire is held in place by being welded on one side to a support rod.
The horizontal mounting harks back to the first hard valves but the grid is closer to the filament and wound with a small pitch and diameter.
The filament is intact and grid can be seen to be mounted on a single support.
The wide glass tube envelope is 38 mm in diameter, and excluding the B4 base pins is 80 mm tall.
Reference: 1043. Type Resistron Bivolt was first introduced in 1928. See also1928 adverts.
