The MESA 5U4G rectifier tube is a classic 1930's "Coke Bottle" design - It produces a flexible, elastic feel that is a bit more controlled and slightly less "saggy" than our new 5U4GB - It has a nice spongy feel but has moderate life expectancy and reliability compared to our new 5U4GB - Due to its tall "Coke Bottle" design, this tube may not fit in all amplifiers - Please confirm the height between the amp chassis and the cabinet before choosing this tube (The Rectifier is part of the Power Supply and its purpose is to convert AC into DC).