The 5AR4 is Sovtek's version of a common rectifier tube used in rectifier-equipped high powered amps such as Fender 40W and Vox AC-30 amplifiers. Common to higher powered tube amps, like the later Fender Super Reverb amps and the Vox AC30 (or Matchless copies). May be used to replace a 5U4 or 5Y3 tube and will give a faster more dynamic tonal result. The Sovtek 5AR4 rectifier has a rise time from 300-400 volts that is about 1/2 that of a 5U4 tube. Sovtek 5AR4s work nicely in amps such as the Mesa Rectifier series which use the 5U4. Great if you want a feel and response that is tighter than stock rectifiers yet you want to retain the feel and compression character of a vacuum rectifier over the solid-state setting.
Order tubes BEFORE they need to be replaced, and save the gig.