The EF86 B9A based audio pentode used in many classic valve and guitar amplifiers. On offer we have EF86 from a range of manufacturers and in configurations of single, twins (x2) and quads (x4) - remember, for those of you with amplifiers utilising paraphrase phase splitters ala Hafler & Keros and Quad II it is ESSENTIAL to equip these with closely matched valves.
The EF86 is the standard early stage audio pre-amplifier valve. Its intrinsic design is to be low noise and anti-microphonic. The landmark amplifier designs from Mullard's application laboratories all specified the EF86, and today it is still the valve of preference for similar duties. Before the introduction of the EF86 the Mullard valve of choice for pre-amplifer stages was the EF37A - equally preceded by the EF37 and earlier the EF36.
Mullard described the EF86 as 'a low noise pentode intended for use as a RC coupled AF voltage amplifier, particularly suited for use in the early stages of high gain audio amplifiers, microphone pre-amplifiers and magnetic tape recorders.'
When used as a normal voltage amplifier at a line voltage of 250V with an anode load of 100 k Ohms and a grid resistor of 470 k, the maximum hum level of the valve alone is 5 uV, the average value being only 3 uV when operated with one side of the heater earthed. A further reduction could be gained by centre tapping the heater to earth which results in a maximum hum level of 1.5 uV. Any greater perceived hum levels than those quoted may be attributed to the use of a valve-holder, in which excessive leakage and capacitive inter-pin coupling pins will introduce considerable hum - good quality amphenol,PTFE or ceramic B9A holders are a must!!.
Any LF noise generated by a valve is a function of an equivalent voltage on the control grid per unit bandwith. In the case of the EF86, for a line voltage of 250V and an anode load of 100 k, the equivalent noise voltage is approximately 2 uV for the frequency range 25-10,000 Hz.
Care in the design of the valve to ensure that the electrode structure and its mounting are as rigid as possible can reduce the microphony of the EF86 to an insignificant level. The structural design of the EF86 ensures no discernable internal resonances at frequencies below 1 kHz and similarly, at frequencies greater than 1kHz, the effect of vibration is usually negligible on account of the damping provided by the chassis and the valve holder.
Physical dimensions for the EF86 are as stated: - envelope of 21 mm diameter and of thin glass wall construction. Device length exclusive of base pins is 48 mm.
Some tasty exotica, past and present used the EF86 and we provide a limited list of these here:-