Why TWO diodes per side in the rectfier?

[nee]

In case of failure? Better rectification?

[UR12]

In case of failure? Better rectification?

Basically most of the diodes we use are 1n4007s. While others use different diodes this is the most often used. The 4007 has a rating of 1000v 1a. Your power transformer takes 120v and changes it to say 400v so that is about a 1 to 4 ratio. If you were to get a large spike of noise or transients on your power line this would be transformed to a voltage that has the potential to go over the 1000v rating which may blow your diode. By using two diodes in series you have effectively doubled the voltage rating of the diode to 2000v.

[nee]

Thanks Dana. I thought it may be something like that.

While we're on the subject of rectification, is using a bridge rectifier appropriate in the 'wreck? Or is that only for a transformer with no centre tap?

[skyboltone]

Dana is of course correct. In addition, I might add; check the EMR tables on the filter cap of your choice and you'll find that the first couple of cycles they are at very low resistance to ground. This places enormous strain on the diodes at fire up. The repetitive current rating of a 4007 is 8 amps? Something like that. Easily exceeded every time you turn your amp on. Stacking them spreads the switching current between the diodes.

[doctord02]

While we're on the subject of rectification, is using a bridge rectifier appropriate in the 'wreck? Or is that only for a transformer with no centre tap?

Only needed on PT's with no center tap.

[tubeswell]

FWIW (and its probably been covered in a few places) when using diodes in series in a rectifier, placing a high voltage cap in parallel with each diode (i.e. 1 cap per diode) will help distribute the load evenly between the diodes (if that is important).