The MESA/Boogie 5U4GB rectifier tube is among their very best overall 5U4 and their tube of choice in the production of all MESA/Boogie amps that feature this tube. Based upon classic 1950s technology, the 5U4GB is a vastly improved 5U4 design that is extremely reliable and consistent, as well as much more compact. This tube produces a "traditional vintage sag" with the most elastic and compressed feel of the two 5U4's that MESA/Boogie offers. The 5U4GB short version has an amazing feel and exceptional overall quality that make it MESA/Boogie's factory favorite.