Selenium Rectifier replacement

You'll need to determine what type of rectification is used if you intend to use the unit as a battery charger. Charging lead-acid batteries with too
high of a voltage will shorten their life. The transformer output voltage (before rectifiers) could possibly be 18VAC or more.
The added resistance that Stan and the others have mentioned is what typically complicates replacing selenium rectifiers with diodes. If the transformer's secondary windings are on the outer layers, it might be possible to carefully remove winding turns to achieve the proper/desired output voltage, when utilizing diodes.
If the transformer isn't a constant voltage-output type design, output regulation would be a good idea, but maybe more involved than you'd care to get into. Disregarding regulation, I'd try to obtain a 13.5V to 14V maximum output voltage for charging 12V batteries. I personally wouldn't want to charge 12V L/A batteries at more than 14V.
Changing the rectifier configuration will also change the output capabilities. 1 rectifier is half-wave rectification 2 is full-wave 4 are the equivalent of a full-wave bridge rectifier There are also other configurations for voltage doublers. Battery charging doesn't necessarily require full-wave bridge rectification (4 each or 2 pairs).
If you want to *not* use it as a battery charger, you can choose any diode configuration you want to.
If you want a bench-use power supply, a jeweled moving-coil type meter would be suitable, but for use as a portable battery charger, a ruggedized moving vane type would be long lasting (typically less accurate, more of a monitoring indicator).
Stud-mounted diodes are relatively easy to use in a variety of heatsink designs. There are both case-anode and case-cathode for many stud-mount diode types/ratings, so that using them on a single heat sink doesn't require insulating washers/bushings, etc.
Diodes are destroyed in the blink of an eye, well really fast, and normal fuses usually aren't fast enough to protect them (common circuit breakers certainly won't). Fast acting rectifier protection diodes are what's used to protect the output semiconductors in DC motor drives. Many battery charger owners have learned not to lend them to anyone, as it often results in getting back a dead charger.
Choosing diodes with a high peak surge current rating (exceeding maximum x3 or more) will often add to their longevity. I wouldn't even consider using under-rated diodes (connected in any manner, or marginally useable diodes) to save money. The common failure mode for PN junctions is shorted, so you can end up with an AC output.
Meters, rectifiers and lots of other great stuff can also be found at Fair Radio in OH. /
WB .............

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