The 12AX7 high impedance amplifier triode was designed specifically for audio use and has a high amplification factor of 100. This is an american design that was copied by other companies. Mullard describe the ECC83 (=12AX7) as a high μ double triode, having separate cathodes, primarily intended for use as a resistance-coupled amplifier or phase inverter.
In this Brimar exhibit the BVA logo takes centre stage. In 12AX7 the BVA are relegated to the back - and just letters.
The twin triose sections with side flanges at the ends of the grid winding. In a power valve the flanges would be on the working face to assist with heat dispersal.
Made in England.
The thin glass tube envelope is 20 mm in diameter and, excluding the B9A base pins, is 47 mm tall.
References: Datasheet, 3002 & 1040. Type 12AX7 was first introduced in 1947.