The ECC81 high impedance amplifier triode was designed specifically for audio use and has a high amplification factor of 100. Mullard describes the ECC81 as a high μ double triode, having separate cathodes, primarily intended for use as a resistance-coupled amplifier or phase inverter.

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.