Common to medium powered amps like the earlier Fender Super Reverbs and Bandmasters. May be used as a replacement for the 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.