Sunbeds tubes vary in length, type and power. Sunbeds tubes produce ultra violate rays in varying amounts of A and B rays (UVA & UVB).
Different types of tubes (with different A and B mixtures) are designed for different types of tanning. Some produce extra quick tanning results, some produce slower but deeper longer lasting results. All tubes have an average life span e.g. 400 to 600 hours after which the tubes will still light up, BUT the amount of UVA and UVB being output will have diminished to the extent that you will no longer tan. A good tube cooling system will help maintain the tube life.