The Mullard triode pentode ECL86
The new Mullard triode pentode ECL86 is designed specifically for audio applications, and is particularly suitable for use in stereophonic amplifiers. The sensitivity of the valve is such that, in a typical single-valve amplifier, a signal voltage of 50mV at the grid of the triode section will produce the maximum output power from the pentode section. Thus the output derived from low-sensitivity crystal or ceramic stereophonic pick-up heads (about 70mV) is ample to produce maximum output from the valve and also allow the use of negative feedback. The high overall gain of the valve is achieved partly by a high amplification factor in the triode section and partly by a high value of mutual conductance in the pentode section.
The anode dissipation rating of the pentode section of the ECL86 is 9W. With this rating, a useful output power of 3W is obtainable.
The mutual conductance of the section at an anode current of 36mA is 10mA/V. This high value of slope is obtained by using a small cathode-to-control-grid spacing. The curvature in the tail of the Ia/Vg1 characteristic which can result from a small cathode-to-control-grid spacing is minimised in the ECL86 by the use of grid wire with as small a pitch as is practicable.
The triode section of the ECL86 has an amplification factor of 100, which will enable voltage gains of up to 70 times to be achieved in practical circuits.
The triode structure is much shorter than that of the pentode, and is supported by a balcony mica. This construction results in a very robust triode section with good microphony characteristics. No special precautions against microphony are required provided the sensitivity of the circuit is such that an output of 50mW requires an input voltage of not less than 5mV.
Special care has been taken in the design of the ECL86 to keep hum modulation to a minimum. A U-shaped screen has been fitted below the bottom mica to shield the triode grid lead, and a hum voltage, referred to the grid of the triode, of 4μV is attainable.
Very economical stereophonic amplifiers can be designed using the ECL86, and two new circuits are described. See Ten Watt High Quality Stereophonic Amplifier Economical Three Watt Stereophonic Amplifier
In one, high-quality reproduction is achieved with a complement of two ECL86 and one EF86 in each channel, and one GZ34 rectifier common to both channels. The output from each channel is 10W. One channel of this circuit can, of course, be used for monophonic reproduction, the performance being comparable with that of the Mullard high-quality 10W amplifier. (the well-known Mullard 'Five-Ten'). In the other circuit, a single ECL86 is used in each channel, with one EZ81 rectifier common to both channels, to give an output of 3W per channel and reasonably good reproduction.