[Arran]

Other then a few books I have on selenium rectifiers, I haven't seen anything specific about how to replace a given type of tube rectifier, like a 117Z6, 35Z5, with a solid state one. In the case of a 5Y3 I wouldn't even bother, it's a common enough tube that there really isn't any advantage in using a solid state substitute, unless you want higher B+ voltage. Generally speaking when someone is talking about substitute rectifiers it usually involves replacing a worn out selenium rectifier with a silicon rectifier, selenium rectifiers have a higher forward drop then silicon but they are still instant on.
To mimic the operation of a tube one would have to use a current limiter or a thermistor, although with a directly heated tube rectifier they are still almost instant on. The last time I substituted a tube with a silicon rectifier was to replace a 117Z3 in a weird post war radio/phono combo unit, since it was 117 volts (120 volts) it ran directly off the power line.
The other time I substituted a tube with a silicon rectifier was in a brand Z three way portable because I did not have a 117Z6. I needed to add a dropper since the battery tube filaments are run in series with the B+ supply, but the instant on aspect did not matter since battery tubes are pretty well instant on as well.
Regards
Arran