This rectifier has a rise time from 300-400 volts that is about 1/2 that of a 5U4 so they are very nice in amps such as the Mesa Rectifier series which use the 5U4 if you want a feel and response that is tighter than the stock rectifiers yet retain the feel and compression character of a vacuum rectifier over the solid state setting.
Groove Tubes GT-5AR4 (code 55079)
Guitar Amplifier Tube, Most Common Rectifier Used in Higher Powered Amps
£19.56 (inc vat)