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A Low-Hum, Low-Microphony AF Pentode on the Noval Base

The Radio Constructor, July, 1953
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A valve that should prove of considerable value for use in high-grade amplifier applications in communications and industrial electronic equipment, has recently been made commercially available by the Communications and Industrial Valve Department of Mullard Ltd. It is the EF86 low-hum, low-microphony AF voltage amplifying pentode on the B9A (Noval) Services Preferred base.

This valve has been designed for use in high-grade resistance-coupled, audio-frequency voltage amplifier circuits. In these circuits it is essential that the hum and microphony introduced by the amplifying valve be kept to an absolute minimum. The hum level in the EF86 has been kept to 5 micro-volts, referred to control grid. This extremely low figure has been achieved by careful internal screening and by the use of a bifilar heater. Moreover, the control grid pin is placed equidistant from the two heater pins, so that, providing the heater winding has a centre-tapped earth, any hum induced from the heater pins is virtually balanced out. The rigidity of the electrode structure contributes, to a large degree, to the extremely low microphony inherent in this valve. Other features of the EF86 are its small size, high gain and single ended construction.

The main characteristics of the EF86 are:- Heater Voltage 6.3V, Heater Current 0.2A, Anode Voltage 250V, Anode Current 3mA, Hum level 3μV & Mutual Conductance 1.85mA/V.

Further details of the EF86 may be obtained on request to the Communications and Industrial Valve Department.

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