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Trade News: Mullard Output Pentode for Medium Power Audio Amplifiers

The Radio Constructor, July, 1955.
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External and X-ray views of the new Mullard EL34

The Mullard EL34 is an outstanding new audio output pentode with a rated anode dissipation of 25 watts. In view of its high sensitivity, high power output, low distortion and modern construction, it should find wide application in all classes of audio work. It can be used in amplifiers with output powers ranging from 11 Watts (single valve) to 100 Watts (push-pull) and is equally suitable for domestic amplifiers and public address equipment.

Performance: The EL34 has a mutual conductance of 11 mA/V, which gives it high sensitivity. It is a compact octal- based valve of cylindrical shape (seated height 98 mm, diameter 38 mm). It has a heater rating of 6.3V, 1.5A. The high maximum anode voltage of 800V permits operation in push-pull fixed-bias circuits with an output power of 100 watts at 5 per cent total harmonic distortion. For domestic amplifiers, a pair of triode-connected EL34s in push-pull give an output of 14 watts at less than 1 per cent distortion with an HT of 430V, or 16 watts at 3 per cent distortion with an HT of 400V.

For PA purposes, two EL34s connected as pentodes with cathode bias give 35 Watts output with a line voltage of 375V.

Special Construction: The EL34 is of single-ended construction. In order to enable high anode voltages to be used without flashover, a special manufacturing technique is employed. The valve envelope is completely of glass, with a pressed-glass foot, clamped into a metal ring which unites the glass and the plastic material of the octal base. Stiff wire leads projecting from the pressed glass foot line up exactly with the pins in the octal base, without crossing over. This reduces the risk of flashover, and provided that the valve-holder itself can withstand the high tension, anode voltages up to 800V can be used.

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