Vintage large bulb rectifier tube, used in high-end audio amplifiers but also in modern recto type amplifiers, such as the Mesa Boogie Double/Triple Recitifer. It can replace the 5U4GB. Its tone is softer than that of a solid state rectifier.

The Straight One! The 5U4GB has a straight glass envelope. This type has been widely used in U.S-made guitar amps from the 1950s to the 70s. The 5U4GB has a slow start feature, which protects output tubes from high voltage peaks when you switch the power on.