GE/ KAUFFMAN
These NOS USA made rectifiers are super tough and top quality mil spec. Much more durable than any GZ34/5AR4 being made today. Nothe that they are direct heated types which means they give near instant switch on unlike GZ34s which have a slow warm up. Also the take higher voltage than any GZ34 types at 750-0-750 rms.
This higher performance/durability comes at a price though - They are physically larger than GZ34 types and may not fit in all amps.