The 2A3 is an audio power triode that is still used in amplifier designs. Modern manufacture reproduces this directly heated design.
The 2A3 is normally used in a push-pull arrangement and care needs to be taken in the bias settings for each valve as a small change in grid bias will greatly alter the standing anode current and any mismatch will impair the reproduction quality.
This example has two triode assemblies within a common anode. The sections are connected in parallel internally. This arrangement is believed to date to the 1940s.
The base is printed with the RCA Radiotron branding.
One of the triode pair seen face on. The oxide coated filament passes through the grid four times in a double inverted V form. The top mica holds two coiled filament tension springs that each hold a hairpin bend in the filament.
The carbonised anode is formed from two shaped plates that are held in place on stiff supports that form the electrical connection to the anode.
The lower mica showing the parallel connections.
The 1935 circuit diagram of the National Dobro amplifier. Thank to Sam Palmer for sending this image that confirms that the 2A3 existed in 1935.
The classic envelope is 52 mm in diameter, and excluding the UX4 base pins is 117 mm tall.
References: data-sheet. Type 2A3 was first introduced in 1935. See also1935 adverts.