[Arran]

Stacking the silicon diodes roughly duplicates the drop across the selenium rectifier, which is basically a stack of diodes in itself. I like the idea of using at least two diodes in series if you are using 1 amp diodes for a safety measure against shorts, this is probably overkill with the 3 amp ones but even those are cheap enough. Some people like using a Zener diode across the filament string rather then adding the dropping resistor in series, I haven't tried this but can see how it would work. With regular silicon diodes connected this way there is less to figure out as the forward drop is fairly consistent, I did this with one of those Electro battery eliminators after the dry metal rectifier failed in the A+ supply, shunt regulation is what it's called I think. One other thing, by using silicon diodes you will need a terminal strip to mount them, which means that you may as well get rid of the hot chassis hookup whilst you are at it.
Regards
Arran