Can you replace a selenium rectifier with a silicon rectifier?

The short answer is yes, usually. If you are working with an ultra high voltage stack all bets are off. For old stereos, etc. you need to estimate the peak reverse voltage and the current requirements. If you have documentation, use it, otherwise: rule 1. PIV is 30 volts per plate, divided by the formfactor: rule 1a. Formfactors: diode=1, half bridge (center tap)=2, bridge=4. rule 1b. Use the maximum of the above and the input electrolitic working voltage multiplied by 1.4, then round up to the next standard voltage within reason (silicon is cheap). rule 2. Assume 500mA per square inch of a single plate's area. rule 3. You may need to heatsink the new rectifier, especially if the current is over a few hundred mA. rule 4. Replace the electrolitics!!!! If the unit is old enough to have a selenium, they are well past the warentee. rule 5. If the voltage is low (under 10VDC) you may need to put a small valued resistor between the rectifier and capacitor. example: a 12 plate center tapped rectifier 1.5 inches square; 12*30=360V divide by 2 =180PIV area=2.25 square inches which gives a current requirement of a little over one amp so use a 1.5A 200PIV unit.