GT-5AR4/ GZ34 The most common rectifier used in higher powered amps which use a rectifier such as Fender 40 watt amps and Vox AC-30 amps.                 GT-5U4R Common to many medium powered amps such as the Fender Blackface amps. This R version is the physically smaller straight bottle version                 GT-5Y3 Common to lower powered Fender amps such as the Champ, Princeton and Deluxe amps. May be used in many 5AR4 and 5U4 amps for lower levels                 GT-6CA4 A common 9 pin rectifier for smaller amps like the Silverstone guitar/amp case combos of the 50's.