Captain Video,
This is a strange way to connect a rectifier diode, unless the design
called for a high current diode and only a bridge was handy.
Is this a "commercial" piece of equipment or is is someone's homebrew
design. .
Take a close look at the circuitry and components and fairly easily
you should be able to determine the approximate voltage and current
needed for the replacement (single) diode if in fact it is used as a
rectifier in a half wave configuration.

If there is something else going on here with the circuitry and it is
a commericial design and it has worked that way for years then why not
just replace it with an "original" 6A, 200V bridge or even a higher
voltage and current rating as these bridges are commonly available,
even from scrapped televisions and other devices.

In either case, you should consider Sofie's textbook argument
regarding parallel and series diode connections, he is absolutely
correct with what he stated.

electricitym