What does a Hexfred do?

Hexfred? Poor Fred, he didn't understand why the bad witch cast that awful spell on him.

Is this why my ARC CL-60 always had a nasty "buzzsaw" ghost riding the waveform of vocals? It has tube output stage but solid-state rectifier.

It was either this, a Classe DAC (solid-state) or digital jitter (maybe all three?)
that just drove me up the wall.

I hear it from time-to-time with my current system but to a much lesser degree.

Can't the demons just leave me alone???

Rectifier conducts until amplitude of AC voltage is lower
than capacitor voltage (just after peak of the sinewave). At
this point diode is reverse polarized and diode current drops
to zero. Unfortunately it will conduct for a moment in
opposite direction and then it will recover back to zero.
Ideal diode should be very fast to minimize current spike in
opposite direction and very slow to recover to zero. Ratio
of time (on negative side) after the peak to time before the
peak is called "Softness Factor" (RRSF - Reverse
Recovery Softness Factor). You want fast diode to minimize
negative current spike, but not the one that snaps back fast
(you don't want narrow spikes).

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)

HEXFRED rectifiers are about the fastest rectifiers and the softest recovery- just about the closest you can get to a tube rectifier, without the voltage drop of a tube rectifier.

We've been using them for about 20 years.

Sonically they sound smoother. This is because they make less commutation (also known as 'switching') noise for the reasons mentioned above.