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. 5R4's will drop the B+ voltage producing lower headroom (earlier onset of overdrive/distortion) and compression than the stock 5AR4/GZ34. It will also reduce high frequencies and the idle current and dissipation of the power tubes. Often used in AC30's and similar amps, Tweed Bassmans and other 5AR4 amps.