The Colossus Rebuild Project by Tony Sale

Colossus Reborn

The Thyratron Rack

The large rack of Thyratrons, the W rack, (possibly W for wide?) contains 501 valves, one for each of the mechanical lugs on the wheels of the Lorenz cipher machine.

The code breakers had worked out the patterns of left or right setting by the German operator of these small lugs and here is a set of patterns.

The brilliant idea by Tommy Flowers, the designer of Colossus, was to generate these patterns in synchronism with the sprocket hole pulses from the paper tape reader, by precessing round rings of Thyratrons. Each ring to have in it the same number of valves as the number of mechanical lugs on the corresponding Lorenz wheel.

Flowers chose to use Thyratron valves because they could hold one bit in one valve, but the problem then was how to quench the arc struck in a valve before striking the next valve in the sequence. This Flowers achieved by a bi-phase design for the ring.

The very complicated Thyratron ring control circuitry (which is explained elsewhere) is arranged so that when the next valve is struck, the anode of the preceding valve is driven negative which extinguishes it.

Unfortunately the Germans had been very inconsiderate in making most of the wheel contain an odd number of lugs. To allow for this an extra, odd, Thyratron had to be included.

This required another large chunk of circuitry on the Thyratron control panel just to get the ring back into bi-phase operation.

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.