The CV4004 is the UK government code for the Commercial M8137 special quality version of the definitive audio double triode ECC83 (CV492). This high impedance amplifier triode was designed specifically for audio use and has a high amplification factor of 100.
This valve is still found in modern 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 with the output valves driven with a phase difference of 180 degrees.
The twin sections occupy a small volume within the envelope.
The valve is also labeled as an M8137 showing the direct equivalence.
The anode plate has small ridges pressed into it for added strength.
The anodes are stitched together at one end.
The thin glass tube envelope is 20 mm in diameter and, excluding the B9A base pins, is 47 mm tall.