The GT-5U4 Rectifier tube replaces the tube common to medium-powered amps such as the earlier Fender Super Reverbs and Bandmasters. The GT-5U4 may be used to replace a 5Y3 for improved dynamics and power. You can replace the stock 5AR4 tube with the 5U4 in the Vox AC30 amp to reduce heat and extend the life of the power tubes, though the amp will operate at slightly lower power.