What does a Hexfred do?

I believe Fig 1 was simplified, and only represents how a transformer’s secondary connects to a diode bridge, as stated. "There are Schottkys operating at higher voltages." I have no idea, how old "QuikNote 108" might be. Actually, my only interest, was the paragraph immediately under Fig 1(which had no expiration date). That Senior Applications Engineer did mention, "With regards to Schottky rectifiers concerning LOW VOLTAGE applications....... almost all solid state preamplifiers, CD players, digital processors, tuners, high end surround sound, low voltage class “A” power amps and tube filament supplies." I have no idea what specific components the author had in mind and- I suppose, "low voltage" could be considered, a relative/subjective term. Contact General Semiconductor, for further information.

Amps using Silicon diodes can sound harsh and
brittle because the diodes are slower in response than a tube rectifier(they're so nice). By replacing the diodes with FREDs or Schottkys your amp
will have a smoother/sweeter top end, more detailed tone definition and be a touch more responsive. FRED’s are
much faster than the cheaper diodes that most amp manufactures use. Although they seem kind of simple, rectifiers are not well understood from an RF-emission standpoint. If you think
of them as really crude Class B (not Class AB!) high-power output stages, the mental picture will get clearer.
The rectifiers commutate the AC waveform just like a fast mechanical switch, and like a mechanical switch, the
zero-crossing region is not dealt with cleanly. Conventional silicon diodes actually knock a hole out of the
waveform thanks to charge-storage effects. Fast/soft recovery diodes(or zero recovery SIC Schottkys) do a much smoother/quieter job of it. It would be nice if the power supply filter caps in your amp could rid the DC output of all of the garbage created, but they don't.

BTW: There are manufacturers that appreciate the diffs that better rectifiers provide. ie: Note the upgrades that are available from Rogue(http://www.rogueaudio.com/The_Magnum_Series.htm)

and Cary Audio(http://www.caryaudio.com/upgrades-and-modifications/)

I used six each of the following, in my Cary SLM-100s. High
voltage SiC Schottkys weren't available at the time.
(http://www.digikey.com/product-detail/en/DSEP12-12A/DSEP12-12A-ND/560364)