"Edward Knobloch" wrote

A 5R4GYB is rated at 3100V PRV. You would need 3 1N4007's in series
in each leg (total of 6) to match the peak reverse voltage rating.

*** You are correct,however the original poster stated he wanted a
replacement for a 5R4.This tube has a maximum plate Voltage of 750 RMS
per plate for which a 1N4007 will suffice.

Brian Goldsmith.