5R4 Rectifier Swapping
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.