Valves in the attic

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A chance to play a part in conserving British and world computing history

The National Museum of Computing needs a supply of valves to ensure that future generations can marvel at and be inspired by working computers from the dawn of the computing age.

Two of the Museum's most treasured working displays -- the rebuild of Colossus, the first semi-programmable electronic computer of the 1940s, and the Harwell Dekatron / WITCH, the oldest working digital computer from the 1950s -- rely on valves that are no longer made. But those valves may be in attics and workshops around the country.

Kevin Murrell, a trustee of TNMOC said: " The Museum has supplies of valves for the Colossus rebuild and the WITCH, and with careful handling they are expected to last for quite some time yet. However, we do need to build up a reserve to ensure that these machines can be kept running for as long as possible to inspire future generations of computer scientists and engineers.

"Although valves are still manufactured for audio and hi-fi enthusiasts, the type of valves we are looking for are no longer made. When our reserves run out, we will have to operate these classic computers in other ways and that would not be as authentic as we would like.

"So, while these specific valves still exist, we are asking people to search their attics to see if they have any that we are looking for. They are most likely to be found in the collections of radio enthusiasts. By donating these sought-after valves, the public can play their part in conserving an outstanding period of British and global computing history."

First generation computers were based around wired circuits that used vacuum valves to control the electric current. Valves or tubes resemble and behave like light bulbs: they can generate a lot of heat and can therefore burn out and need to be replaced. Computer engineers have found ways to prolong the valve's life, but they are always vulnerable to burn-out and malfunction. In computers, valves were replaced in the mid-1950s by transistors, and these were superseded in the 1960s by integrated circuits on the silicon chips we know today.

In building Colossus , Tommy Flowers used 2,500 valves and, unlike many of his contemporaries, he had the conviction that they would be reliable enough for the world's-first electronic computer.

In the Harwell Dekatron computer designed by Ted Cooke-Yarborough, 828 valves (technically "tubes") are used and act as an early form of RAM for storage.

The valves that TNMOC is seeking are listed below. If you can help, please email

The Colossus Rebuild
Top priority: 807
Also needed: EF36, EF37, EF37A, 6J5, 6V6, 6K8, 807, GT1Cs

The Harwell Dekatron / WITCH computer
Normal valves:
EL33 (CV2938)
EL360 (CV5830)
85A2 (CV449)
EF91(6AM6, 8D3, CV138)
12AT7 (ECC81, CV455)
ECC91 (6J6, CV858)
EB91 (6AL5, CV140)
EL91 (6AM5, CV136)
E90CC (CV5214)
VLS631 (CV342)
GC10A (CV2199)
GC10B (CV2271, Z303C)
GS10C (CV2325, Z502S)
Trigger tubes:
G1/371K (CV2224)

Also required: passive components (resistors, capacitors), to allow enable authentic repairs

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