After a good ten years use this valve pre amp started developing a little crackling and rustling noise on some of the inputs. The cause is oxidisation on the rotary selectors. As you can see from the dismantled switch in the picture the contacts on the left are dull with oxidisation whereas the ones on the right are shiny and would provide a good contact. Unfortunately cleaning switches is not possible they must be replaced.
Once repaired the owner was pleased to report that the preamp sounded better than ever. Why? Well this is because of how important a good contact is in delivering the whole signal. Degradation of contacts over time is difficult to notice as our ears get used to it.