The MESA 5U4GB rectifier tube is our very best overall 5U4 and our tube of choice in the production of all new Mesa amps featuring this tube. Based upon classic 1950's technology, the 5U4GB is a vastly improved 5U4 design that is extremely reliable and consistent, as well as much more compact. It is basically the size of a 6L6 power tube, so please take special care to install them in the correct tube position(s), especially when replacing 6L6's at the same time. This tube produces a "traditional vintage sag" with the most elastic and compressed feel of the two 5U4's that we offer. Amazing feel and exceptional overall quality make this our factory favorite. Measures 3 3/4" from bottom of base to top of glass.