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A New 25 W Audio Output Pentode

An advert from Mullard Ltd 1952 for the EL34.
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The EL34 is an indirectly-heated octal-based output pentode which is now being added to the Mullard range of audio valves. It has a rated anode dissipation of 25 Watts and the high mutual conductance of 11 mA/V. This valve covers all applications requiring powers between 11 W (single valve) and 100 W (push pull), and is equally suitable for high quality domestic amplifiers and public address equipment. It has a comparatively small diameter for a 25 W output pentode: the straight-sided envelope rises directly from a foot less than 38 mm in diameter. The maximum overall length is 113 mm and the maximum seated height 98 mm.

Two triode-connected EL34's operated in push-pull for a domestic amplifier give an output of either 14 W at less then 1% total harmonic distortion with a line voltage of 430 V, or 16 W at 3% distortion with a line voltage of 400 V. For public address equipment two EL34's may be operated in pentode push-pull, again using cathode bias, and with a line voltage of 375 V the available output is 35 W at 5% total harmonic distortion. For even higher powers fixed bias may be used with anode voltages of up to 800 V; the power output when the anode voltage is 800 V is 100 W at 5% distortion.

A single EL34 operated in Class A gives an output of 11 W at 10% distortion with a line voltage of 265 V.

A special technique has been devised to enable the EL34 to operate at high anode voltages whilst retaining a single-ended octal-based construction. The valve envelope is made completely of glass, with a conventional pressed glass foot, and clamped into a metal ring which holds together the glass and the plastic material of the octal base. The stiff wire leads projecting from the glass envelope line up exactly with the pinning in the octal base; during manufacture these leads are passed straight inside the octal pins, without crossing over outside the bulb, and so the risk of flash-over is very much reduced. For high voltage operation the valveholder of course must also be able to withstand the high tension.

Such a small valve as the EL34, dissipating a large amount of power at the anode and screen grid as heat, needs reasonable ventilation. It should be mounted vertically, and the air should be able to circulate freely. The distance between two EL34's should be at least 40 mm, and the distance from the cabinet and other components at least 30 mm. Wire-wound resistors, mains transformer, and rectifier should not be in the immediate vicinity.

Since the material on the front page of this leaflet was prepared for the April issue of 'Wireless World', further work has been carried out on the distributed load operation of two EL34's in a push-pull output stage. Hitherto this kind of output stage has not received much attention, though one version of the circuit was discussed by D. T. N. Williamson and P. J. Walker in 'Amplifiers and Superlatives', Wireless World, September 1952. (In the U.S.A. this type of output stage is referred to as ultra-linear'.)

The term 'distributed load' arises because the screen grids of the output stage are connected to taps on the primary of the output transformer, so that part of the primary load is common to the anode and screen grid circuits. The circuit diagram in the top right hand corner of the figure shows the approximate position of the taps for optimum operation of two EL34's. A 20 watt amplifier using this type of output stage has been described by W. A. Ferguson, of the Mullard Valve Measurement and Application Laboratory, in 'Design for a 20 watt High Quality Amplifier' in the May and June issues of 'Wireless World'.

From the circuit diagram it can be seen that distributed load operation lies somewhere between triode and pentode conditions. Connecting the output pentodes as triodes is equivalent to moving the screen grid taps to the anode ends of the windings. On the other hand, for the normal pentode connection the screen grids would effectively be connected to the centre tap. Thus the distributed load conditions provide a compromise between the low distortion of a triode and the high sensitivity of a pentode. The EL34 belongs to the Mullard 'World Series' of audio valves; other important types are the EF86, ECC83 and EL84.

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