The following is taken from a leaflet entitled 'Mullard world series valves for audio equipment' it probably dates from the early 1960s.
A high gain pentode of special design, the EF86 is particularly suitable for preamplifier and input stages, in which hum, noise and microphony must be kept to a minimum. The low frequency noise generated by the valve is equivalent to a voltage of 2 μV on the control grid for a bandwidth from 25 to 10,000 Hz.
The electrode structure has been made particularly rigid to keep the microphony of the valve to a very low level. There are no appreciable internal resonances below 1000 Hz, the vibration at higher frequencies being effectively damped out by the chassis and the valveholder.
Hum is kept to a minimum by winding the heater as a bifilar twisted pair of wires, with the magnetic field of the one wire opposed to that of the other. Effective internal screening reduces the internal valve capacitances through which hum can be transferred to the signal electrodes. The screening also shields the electrode structure from the alternating fields set up by transformers, etc., which otherwise would induce mains frequency AC in them.
See Also High Quality Audio Amplification - Made for Music Advert - Audiophiles Advert - EF86 Pre-Amplifier Valve Advert & EF86 Advert