Valve of the Month

This month, lovely viewer, we have the EF86. An audio pentode.
Mullard introduced this miniature low noise, low hum device to supersede the EF37A and EF40 (octal and loctal valves).
When new they are really excellent valves with an equivalent noise voltage of 2uV and hum voltage of 1.5uV when correctly wired to a balanced 6.3 volt heater supply.
Their lifespan, however, is reduced by electron poisoning of the screen grid, leading to secondary emission and increased noise.
Careful design reduced microphony to a very low level especially if the valve was mounted on a sound insulating valve base, enabling its use in tape recorders with internal loudspeakers and motors sharing the same chassis.
Sensibly equivalent to CV2901 6267 6BK8 6F22 & Z729

---