The Colossus Rebuild Project

by Tony Sale

Tony Sale's
Codes and Ciphers

This is Page 3 of 3 pages by Tony Sale about the Lorenz ciphers and the Colossus.

Rebirth of the Colossus

In the 1970s, information about Colossus began to emerge. Professor Brian Randell of Newcastle University started researching. Dr Tommy Flowers and some of the other design engineers gave papers in the 1980s describing Colossus in fairly general terms.

The rebuilding of Colossus

When I and some colleagues started, in 1991, the campaign to save Bletchley Park from demolition by property developers, I was working at the Science Museum in London restoring some early British computers. I believed it would be possible to rebuild Colossus.

Nobody believed me.

In 1993 I gathered together all the information available. This amounted to the eight 1945 wartime photographs taken of Colossus plus some fragments of circuit diagrams which some engineers had kept quite illegally, as engineers always do!

The first stage was to produce accurate machine drawings of the frames for Colossus (all the original machine drawings had been burnt in 1960). This involved three months of eyestrain poring over the photographs and using 3D projections to transfer the details to a CAD system, EasyCad running on a 486 PC.

The next problem was the optical paper tape reader system. The details of this are not shown in any of the photographs. However I managed to locate Dr Arnold Lynch who designed the reader system in 1942.
Although well into his eighties Dr Lynch came to my house and using my CAD system we re-engineered the reader system to his original specifications.

Then I built it
and here it is:

It uses original Colossus hard vacuum photocells (shown here on the left) and a mask on to which the image of the tape is projected by a Colossus lens.

I found that sufficient wartime valves were still available, as were various pieces of Post Office equipment used in the original construction.

All the racks are now in place.
Here are some of the decade counters.
We are also rebuilding the Tunny machine.

In July 1994 His Royal Highness the Duke of Kent opened the Museums in Bletchley Park and inaugurated the Colossus Rebuild Project. At that point I had not managed to obtain any sponsorship for the project but in 1993 my wife Margaret and I had decided to put our own money into it to get it started. We both felt that if the effort was not made immediately there would be nobody still alive to help us with memories of Colossus. Over the next few years various private sponsors came to our aid and some current and ex-Post Office and radio engineers formed the team that helped me in the Rebuild.

The switch-on

Colossus first worked at two-bit level (out of the five-bit channels from the paper tape). HRH the Duke of Kent returned to the Park on 6 June 1996 to switch on the basic working Colossus.
This was a marvellous occasion with Dr Tommy Flowers, on the left, present. So also were a large number of the people who worked at Knockholt, in the Testery and the Newmanry during the war.
One reason for wanting to get Colossus working in 1996 was that for far too long the Americans have got away with the myth that the ENIAC was the first large-scale electronic digital calculator in the world. It was not, but they got away with it because Colossus was kept secret until the 1970s. As 1996 was the 50th anniversary of the switch-on of ENIAC I made sure that Colossus was rebuilt and working in Bletchley Park, just as it was in 1944.
Dr Tommy Flowers
There has been a stunned silence from across the water!

The American information

One ironic twist to the Colossus story is that most of the information about how Colossus was used has come from America. In 1995 the American National Security Agency (the equivalent of GCHQ) was forced by application of the Freedom of Information Act to release about 5,000 World War II documents into the National Archive. The listing of these documents was put on the Internet and I quickly obtained a copy of the list. When I scanned this I was amazed to see titles like The Cryptographic Attack on FISH. I managed to get copies of these documents only to find that they were reports written by American service men seconded to Bletchley Park when America entered the war. The most important one was written by Albert Small and is a complete description of Colossus code breaking. Having this report has enabled us to work out the function of many more of the circuits and programme switches on Colossus. We have now, we think, incorporated nearly all the circuits and although there may still be some parts which cannot be worked out, we think we have about 90% of Colossus correct and working.

The performance of Colossus

Colossus is not a stored-programme computer. It is hard-wired and switch-programmed, just like ENIAC. Because of its parallel nature it is very fast, even by today's standards. The intercepted message, punched on to ordinary teleprinter paper tape, is read at 5,000 characters per second. The sprocket holes down the middle of the tape are read to form the clock for the whole machine. This avoids any synchronisation problems: whatever the speed of the tape, that's the speed of Colossus. Tommy Flowers once wound up the paper tape drive motor to see what happened. At 9,600 characters per second the tape burst and flew all over the room at 60 mph! It was decided that 5,000 cps was a safe speed.

At 5,000 cps the interval between sprocket holes is 200 microsecs. In this time Colossus will do up to 100 Boolean calculations simultaneously on each of the five tape channels and across a five character matrix. The gate delay time is 1.2 microsecs which is quite remarkable for very ordinary valves. It demonstrates the design skills of Tommy Flowers and Allen Coombs who re-engineered most of the Mark 2 Colossus.

Colossus is so fast and parallel that a modern PC programmed to do the same code-breaking task takes as long as Colossus to achieve a result!


The rebuild, which is still ongoing towards a MK 2 Colossus can now be seen in the Museums at Bletchley Park

It is marvellous tribute to Tommy Flowers, Allen Coombs and all the engineers at Dollis Hill and a great tribute to Bill Tutte, Max Newman, Ralph Tester and all the code breakers involved at Bletchley Park...

...not forgetting all the WRNS who operated and supported Colossus and the interceptors at Knockholt without whom there would have been no messages to break.

Lastly I would like to thank my wife Margaret for agreeing to the use of our own money to start up the project and for her continuing support and encouragement.

The financial sponsors:

A E & M D Sale

Mr Frank Morrell

The Mrs L D Rope Third Charitable Settlement

Mr Keith Thrower OBE

Quantel Ltd

Contributions by special low prices:

Charles Head (Blacksmiths)

Billington Exports Ltd

Claude Lyons Ltd


Thanks also to:

The Bletchley Park Trust which has allowed free use of the room in H Block where Colossus has been rebuilt.

The many hundreds of individuals who have searched their garages and lofts and sent valves for Colossus.

The Colossus Team

The regulars:

Cliff Horrocks, David Stanley, Paul Bruton (deceased), John Lloyd, Bob Alexander

Every other weekend:

John Pether, Don Skeggs, Adrian Cole, Ron Clayton


Don Grieg (deceased), Philip Hopkins, Richard Watson, Derek Turton, Mark Hyman

By parcel post to Wales:

Gil Hayward, the original designer of Mark 2 Tunny who turned some 600 pattern plugs and modified over 1,000 octal valve bases.

Lorenz cipher index page

Join the Tour of Bletchley Park to see the Colossus area

You can now see more of the Colossus Reconstruction. Continue to Walk Around Colossus.

You can also use an interactive Virtual Colossus.

These pages may not be suitable for use with a Macintosh and there may be other browser-dependent features which are not yet optimised. Tony Sale will welcome your comments.

You should also see the original 1945 document, The Newmanry History, which was released from secrecy in 2000. It details the methods by which the Lorenz cipher was broken.

Tony Sale has now made available HTML and PDF editions of this crucial document.

Tony Sale's Codes and Ciphers index page

For information about when Bletchley Park and its Museums are open to visitors you must go to the Bletchley Park Trust which is responsible for it. This website has no connection with the Bletchley Park Trust.

This page was originally created by the late Tony Sale the original curator of the Bletchley Park Museum. Funding for the ongoing maintenance of this section of the site is kindly being provided by Fisher Scoggins Waters Health and Safety Lawyers.