This should be in sci.electronics.basics
What happened to selenium rectifiers was silicon. Around the era of seleniu m, it seems that they were used for higher voltages and copper oxide was us ed for lower voltages. strictly from memory here, the selenium jobs had som e leakage and alot of forward drop. That made them more suitable for higher voltages, like B+. (a summary of that term later) the copper oxide ones, I IRC had alot of reverse leakage but less forward drop, making them more sui table for lower voltages.
If you take apart any of these old rectifiers and examine them very closely you will see that they are insulated from the bolt, and that is why it doe s not constitute a short circuit. Sometimes it is just a little eentsy ween tsy insulator that you almost don't think would do it. but then the dielect ric strength of air is something like 600 volts per thousandth of an inch a t STP. Or something like that. High moisture or anything in the way of a co ntaminant reduces this insulative quality.
Selenium rectifiers did save a filament source, and that was a fairly highl y isolated source because usually that filament supply has to be at B+ pote ntial. Tubes like I think 6X4 or whatever with the indirectly heated cathos de no, but they had their ow limitations when it came to heater to cathode voltage. so for the major voltages, like for a power amp they stuck with li ke %u4s n shit and just dealt with the insulation in the power transformer. Note that most selenium rectifiers could not put out enough current for a high powered amplifier.
Actually, to this day, audiophile tube amps are built and they usually go w ith tube rectification. The tube rectification has some resistance in it an d reduces ground currents and therefore hum. Selenium is almost the same ex cept for its failure mode. When a 5U4 fails, you go online and buy a 5U4, w hen a selenium rectifier fails you evacuate the house for a month and buy a ll new furniture. at least these days. Like these assholes with second hand smoke causes more death than to the smoker, nothing could be more ridiculo us. but people cannot stand smell. we went from taking a bath in the same w ater as our brothers and sisters on Saturday once a week to taking three sh owers a day. these people, one selenium rectifier failure and they would pr obably have the house condemned.
It has been described as rotten eggs but there is more to it, something lik e a tinge of boiling battery acid mixed in.
So basically they were dropped like a hot potato ASAP. People could stand i t back then, and as an advantage when they called the TV or radio repairman and said "It smells like rotten eggs" the guy knew what parts to bring.
The B+ thing is about old battery operated tube equipment. the A battery fe d the filaments and was maybe 3 volts or whatever, while the B battery supp lied the plates and other higher voltage needful sources, many of them were 90 volts. I read somewhere that some units had a C battery but have not fo und any evidence of this and I can't think of why. Grid bias maybe ?
Anyway, if you ever have to replace selenium rectifiers, you need some resi stance in series. First of all, stuff that old is designed to run on 110 vo lts and nominal US voltage is now 125. What's more the better efficiency ta kes it up even higher. You could easily find your tubes replating all over the place if you don't cut it down, and this takes power resistors.