The Colossus Rebuild Project by Tony Sale

Colossus Reborn

The Photocell Amplifiers

Here is the circuit diagram of the amplifier for each photocell.

This is the actual diagram produced by Arnold Lynch in 1942 and it is initialed by him.
He produced it for Heath Robinson which used GM22 gas filled photo cells which were fast enough for the slower tape speed, 1,000 ch/sec, of Heath Robinson.

The modifications for the higher speed hard vacuum cells and higher tape speed of Colossus can be seen starred on the diagram.

Arnold Lynch used SP41 valves in the original circuits. When we discussed the rebuild he suggested that I use EF36 valves instead. These have nearly the same electronic characteristics as the SP41 but take far less heater power. He said that one of the problems with the original chassis, which had 80 valves on it, was the vast amount of heat generated (4 watts per valve) from all the valve heaters, so large that the metallised coating peeled off the valves!
The EF36s which only require 1.2 watts per valve have given no problems.

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.