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Professionally Built Mullard 3-3 Amplifier

The Classic 3-3 amplifier design from Mullard Ltd. 1045.
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In mid 2015 the museum acquired this professionally built example of the Mullard 3-3 three Watt amplifier. The build is much later than the 1950s but still uses the originally specified Parmeko transformers for both mains and output. Unlike the prototype the wiring below the chassis is point to point rather than on tag boards.

Under chassis view.

The construction uses an earth bus-bar and each component has been secured mechanically to its anchor point before soldering. This became clear when the main reservoir and smoothing capacitor was changed for a modern version. The heater wires are run together but are not twisted together, even so the hum level is very low.

The output transformer has a 3-4Ω secondary and the amplifier has been tested with a several speakers including a Leak 250 sandwich speaker that dates to 1974 and a Danish Jamo satellite. The hum and noise can be heard when close to the speaker but not when music is playing.

Sound quality is excellent but lacks deep base. At the treble end an audio generator shows a tailing off in response above around 12 KHz, this is higher than my ears now respond to.

The chassis measures 165 mm wide and is 150 mm front to back.

Mullard 3-3 design with Parmeko transformers.
Looking down on the amplifier.

The performance is given below and the THD at 3 Watts was measured as 8%. See Measuring THD with FFT for the method.

Amplifier voltage response at 8% THD into 4 Ω.
Amplifier power response in mW at 8% THD into 4 Ω with log Y axis.

The final graph shows the response curve on a log-log graph. This presentation matches the shape of the graphs used originally by Mullard.

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