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.This exhibit was made by the Marconi subsidiary in Italy and is clearly marked as such.
The printed Marconi Italia SPA name.
This sticker shows the Italian logo.
The thin glass tube envelope is 20 mm in diameter and, excluding the B9A base pins, is 48 mm tall.
References: Data-sheet, 3002 & 1040. Type 12AX7 was first introduced in 1947.