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 copies 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.
This exhibit is a genuine English made 12AX7. The anode is formed from thin sheet and has the typical notch and lug fixing to close the anode.
The electrode system is flattened and seen end on the anode has a hole in the side. The grid wire is visible.
The thin glass tube envelope is 20 mm in diameter and, excluding the B9A base pins, is 45 mm tall.
References: Datasheet, 3002 & 1040. Type 12AX7 was first introduced in 1947.