Mullard Z302C Dekatron Tube  
Written by AnubisTTP on 2009-06-05  


The Z302C is one of the only dekatron tubes manufactured by Mullard that was not merely a clone of a more popular Ericsson part number. It's internal construction is somewhat unusual, as Z302C tubes are designed to be directly coupled into multi-stage counters with no intermediate active components. Though the tube has 30 cathodes it is not intended to be driven in a 2-guide fashion, as the tube only has one set of guides, located in the first position after each main cathode. The second electrode positions are designated as 'extinguishing electrodes' and are wired in two groups, based on whether the given extinguishing electrode precedes an even numbered cathode position or an odd numbered position. In normal operation the extinguishing electrodes are not tied to a phase shifted version of the input pulse signal, they are driven by the successive charge and discharge of an external capacitor network. The zero position extinguishing electrode , shown in the inset photo to the left, sits outside the count loop and is tied to a -300 volt supply. When the glow transfer reaches the zero position electrode, the sudden increase in current draw produces a pulse at the anode of the tube, which can be directly propagated into the guide of an adjacent counter. Overall, this system of counting is rather clumsy, requires a separate -300 volt supply, and limits the tube's count speed to a mere 1kHz.

The tube can be driven in a normal two-guide fashion by tying all the extinguishing electrodes together, but it should be noted that all commonly available dekatron drive circuits can only drive this tube in a counterclockwise direction. Clockwise counting is impossible because the gap in the counting loop at the zero cathode position prevents the glow from transferring properly.


Mullard Z302C Dekatron Tube
Mullard Z302C dekatron glow transfer counter.

Mullard Z302 Dekatron Tube
Detail showing the separate zero position extinguishing electrode, which resides outside the counting loop.

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