[sdiy] outlet as variac? tube rectifiers

[Martin Czech]

SNIP

(yes, tube rectifier guitar amps sound different, at least when
cranked to 11)

:::2) Vacuum rects are very FAST. They pass the AC peak current
:::pulses to the filter cap very easily. Whereas cheap 1N400x 
:::diodes are very slow, and can produce sharp current pulses 
:::which can end up in the output circuit as RFI or hash. 
:::Avoiding this either means considerable shielding and
:::filtering, or the use of special high-speed diodes such
:::as "Hexfred" diodes (not Schottkys--they are for low voltage
:::service only). It's a common upgrade to old tube amps that
:::used silicon diodes, to replace them with Hexfreds. It can
:::make a surprisingly big difference, esp. with the many old
:::receivers and integrated amps that used voltage-doubler supplies.

Interesting, could you please give more information on this subject?
I don't understand why diodes are slow and noisy at the same time.
I thought that due to their low impedance the cap charging current 
is larger and has sharper edges, thus more noise. Therefore
sometimes additional hf caps seem to be used, to short these nnoise
voltages. OTOH I would have assumed that tube diodes with their
softer, slower characteristic lead to not so agressive chraging
transients...

??

m.c.