The ECC83 high impedance amplifier triode was designed specifically for audio use and has a high amplification factor of 100. Mullard describe the ECC83 as a high μ double triode, having separate cathodes, primarily intended for use as a resistance-coupled amplifier or phase inverter.
With this exhibit Mullard make the point that the 12AX7 has an identical specification by adding the American designation to the envelope.
This valve is still found in today's designs of hi-fi audio amplifiers. In the classic Mullard designs of the 1950s and 1960s the valve's two halves were used as second stage amplification and cathode coupled phase splitter.
The phase splitter being required so that the push-pull output stage was driven correctly.
The anodes seen from the side.
In this face view the the grid can be clearly seen as can the fact that the grid is wound on copper supports. No cooling fins are needed on this low power triode.
The thin glass tube envelope is 21 mm in diameter and, excluding the B9A base pins, is 48 mm tall.
References: Data-sheet, 3002 & 1040. Type ECC83 was first introduced in 1954. See also 1954 adverts.