Brian Goldsmith wrote:

snip 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.


The 5R4 has a maximum piv voltage of 2800 volts,
so you would still need 3 1N4007's in series
in each leg for equivalent safety factor.

The rated max supply voltage of 750V rms per plate means that
you should not use a transformer higher than
1500V rms center tapped in a full wave single phase
rectifier circuit using a 5R4.

The designers expected switching transients
when the current is interrupted in the HV transformer
and HV choke, and therefore included a max
supply voltage for guidance.

73,
Ed K4PF