5R4 rectifiers (any suffix, GY, GA, GB, GYB, etc) are pin compatible with 5Y3, 5U4, 5V4 and 5AR4/GZ34 rectifiers. They produce a similar voltage drop to 5U4 rectifiers but require only 2 amps of heater/filament current from the power transformer as opposed to 3 amps for a 5U4. This makes them good substitutes for the other rectifier types when different output voltage is desired. These are particularly useful in guitar amps that use a 5AR4/GZ34 rectifier as a stock part.

Coke bottle ST shape, brown base.  Labeled Lewis and Kaufman. Received in bulk packaging, sold in white boxes.

Various labels and boxes including Motorola and Sylvania. Slightly more sag than a 5U4GB. Great option for amps that call for 5AR4/GZ34s where lower plate voltage, more sag/compression, and earlier breakup are desired. Great in Vox AC 30s to prolong the life of your EL84s.

Built like a tank for the military. We call this one the "potato masher".