You'll find the MESA 5U4GB inside all the new Mesa/Boogie tube amplifiers. As M/B puts it, it's their "factory favorite" and for good reason. The MESA 584GB is based on 1950s tube technology, and is essentially an improvement on the 5U4 design. Not only is the tube more reliable, but it's much more compact. In fact, it's about the size of a 6L6 power tube, so take caution when you're positioning it inside your amplifier. What you'll get is a traditional "vintage sag" with an elastic and compressed feel very cool, MESA 5U84GB. Mesa/Boogie MESA 5U4GB Rectifier Power Tube Features:3-3/4" replacement power amplifier tube - Based on classic 1950s technology - Delivers that traditional vintage sag - Mesa/Boogie's "factory favorite" tube found on all new amplifiers - The Mesa/Boogie MESA 5U4GB sports amazing feel and exceptional quality it's Mesa/Boogie's "factory favorite" power tube!