The most curious thing about the A201 is pin 9, whose function translates as 'screen' or 'shield'. It's not shown in use on any circuit diagram I've seen for this tube. The only other counting tube I'm aware of with such a structure is the G10/241E Nomotron (and of course the 11TU7 lookalike). The datasheet for that also does not discuss the function, but specifies that it be set at a +90V bias. J B Dance (Electronic Counting Circuits) suggests that the purpose of the shield is to confine the glow to the required part of the tube. It could well be a similar function for the A201. In the polyatron configuration, unlike the dekatron, the discharge glow is associated with the common electrode rather than the electrode which is spatially specific for each counting position. This may mean that additional measures like the shield are required to stop the glow spreading across the common cathode and degrading the counting performance. It's interesting that Marcin's datasheet says that the glow is purple, whereas every A201 I've ever seen is neon-orange... Jon.
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