Selenium diode

We live in a historical period in which the amount of information is enormous; unfortunately, in some areas, this abundance leads to confusion and only serves to render the information itself useless. For me, this also applies to selenium diodes. This component, which we often find in devices from the 1950s, 1960s, and so on, is now obsolete, but—and this is particularly important—it appears to be carcinogenic. On its own, the component is harmless; however, when it burns, it generates toxic smoke that is harmful to our health. The average lifespan of this electronic component isn’t particularly long, and those that have survived to the present day are unlikely to be in good working condition.  Electronically, it is very inefficient and dissipates a large amount of energy as heat… heat that increases progressively as it degrades until the temperature causes it to emit a lot of smoke and burn.  As for selenium, I looked for more information but, as I mentioned earlier, I found conflicting reports, and I even saw that it’s used in some therapies targeting cancer cells. It’s clear that oversimplifying information can lead us to view elements as belonging to a single category, and as a result, we mistakenly attribute both negative and positive characteristics to elements that are actually very different from one another.  In the diodes in question, I believe selenium is present as an ’oxide‘ and is therefore chemically different from pure selenium. I will therefore not delve into something I have not yet thoroughly researched, and—given the low cost and negligible impact of the component itself—I prefer to replace it with silicon components, which are undoubtedly safer and more efficient. Selenium was discovered in 1817 by Jakob Berzelius, and in 1920, German scientists discovered that by layering sheets of selenium with sheets of another metal, current could flow through the various layers in only one direction. In the past, therefore, diodes were produced by coating aluminum or steel foils with a thin layer of selenium. In this way, at that time, it was possible to rectify alternating current, converting it into direct current without the use of a rectifier valve… and thus achieving significant cost savings. 

Cost savings, less space occupied, the absence of a base and therefore the saving of another component, voltage drop that went from 25-30 volts of a tube to 5-10 volts of the selenium diode... and so this diode was mounted everywhere: radios, TVs, amplifiers, switchboards, etc., etc.
In 1960, the first silicon diodes became available, and selenium was classified as toxic and carcinogenic (see EPA link). Furthermore, it became apparent that as these diodes aged, their operating parameters worsened, significantly lowering their output voltage. This anomaly made devices incapable of operating at the bias voltages calculated by the designer. 
For all these reasons, even if the equipment in question is still working properly, it is advisable to replace these diodes; all the more so if you are restoring old equipment that has been out of use for a long time, it is imperative to proceed with the replacement.