Based upon classic 1950's technology and currently used as "Factory Standard Equipment". The 5U4GB is a new and vastly improved 5U4 design that is extremely reliable and consistent, as well as much more compact. Approximately the size of a 6L6 power tube, it produces a flexible and resilient feel that is a bit more controlled and slightly less "saggy" than the "Coke Bottle" 5U4G. Amazing feel and exceptional overall quality make this our factory favorite. Measures 3 3/4" from the bottom of base to the top of glass.