The Colossus Rebuild Project by Tony Sale

Colossus Reborn

The Logic Circuits

Here is the circuit of the logic gates on the top panel of the J rack.

It is the same as the XOR logic used to get the delta on the R rack panels.

The EF36 pentodes chosen by Tommy Flowers, allow this to be achieved because they have an external connection to their suppresser grids. This means that current only flows in the valve when both grid AND suppresser voltage is zero or slightly positive. A negative voltage on either grid or suppresser stops current flow to the anode.

There are five independent XOR logic gates on this panel and another 5 on the panel below the Jack field.

I located the positions on the racks of the various panels by realising that the Dollis Hill draughtsmen who had drawn up the circuits had used a standard numbering system for valves which also related to their position on the panels. For instance V1 on the circuit was always the top left hand valve on a chassis and the valves then went numerically from left to right across a chassis, with the next row starting at 21, 31 or 41 depending on how many valves there were in a row.

Realising this allowed me to construct a visual image of a plate because I knew from the circuit diagram which types of valve where used where. I then searched on the photographs for panels that had this visual signature, and I found them. Slowly we pieced together all the rack layouts.

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.