Colossus Reborn

The Decade Counters

The back of the C Rack, the Counter Rack, holds 10 plates each holding two decade counters. (counting 0 to 9). These panels are connected in pairs to give 5 counters each of 4 decades, counting 0 to 9999.

Each counter is of a bi-quinary design, a divide by two followed by a ring of 5 counter.

This is the divide by two bistable fed by the pulses to be counted from the counter control plate' or from a previous decade if it is not the first decade.


This is the ring-of-five circuit. It derives from the pre war work of Wynn-Williams on electronic counters for particle physics experiments. Because of the feedback resistors, only one valve can be conducting at any one time and it being conducting causes the next valve in the ring to conduct when the next pulse comes out of the divide by two circuit. thus stepping the ring round.

The counter is set to zero by forcing to zero volts all the screens of the valves in the ring and one of the valves in the bistable. This is what the large 807 valve on the counter control panel does.

This page was originally created by the late Tony Sale, the original curator of the Bletchley Park Museum, and Secretary of the Bletchley Park Heritage Society.