Groove Tube GT-5U4 Rectifier Tube Common to many medium powered amps such as the Fender Blackface amps. This is the factory selected rectifier for the Mesa Dual and Triple rectifier series. May be used replaced a 5Y3 for improved dynamics and power. Also some players have replaced their 5AR4 with the 5U4 in their Vox AC30 amps to reduce heat and extend the life of the power tubes, we recommended this as a "stability enhancer", if you can live with the slightly lower power.