A friend of mine builds tube audio amplifiers for a hobby. Being an electrical engineer, who works exclusively with micro-electronics, I thought I might mess around in spice a bit to better understand some of the standard circuit topologies they use. My friend is not an engineer so he doesn't really know the history or justification of many of the components so they typical answer to "why is this here?" is "because everybody does it that way." Here is an example circuit very similar to what he is currently building: http://www.electra-print.com/300bdrd.php The second schematic on this page shows how to make a full bridge rectifier One thing that looked odd to me was the two diodes in series on the rectifier. I understand what the filtering caps and resistors in parallel to the diodes are for, but why put two diodes in series? Maybe it's a safety thing? I've seen redundancies like this in some battery circuits when there is concern of shorts. I also notice the 5V supply does not have them.