I've had a few units that have been upgraded in this fashion.It always seemed to help the amplifiers disappear better,with no drawbacks.Never done it myself,usually done by the manufacturer.
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IME in a high-voltage custom full-wave bridge, Cree SiC Schottky surpasses HEXFRED. For low-voltage/high-current filament supply, a ladder of 50V Schottkys sounds better than FRED. In a manufactured 4-pin bridge, IXYS 600V/6.6A SiC Schottky is reportedly good, although expensive.
You will hear across-the-board improvements, but particularly improved dynamics and faster bass.
Transparency and dynamics are what I'm hoping for. I've got a pair of Cary SLM-100s that I'm thinking of tweaking slightly. The B+ right after the rectifier is 488V, and I was thinking of going with an IXYS 1200V bridge to handle turn on surge. I'd love to go with the Schottkys, but- yeah, SALTY! I can get the 600V/6.6A Schottky for about $65.00 ea, which isn't bad. I'm wondering if it would handle the surge though.
Generally speaking, the safe thing to do is to select replacement diodes that equal or exceed the rated capacity of the stock bridge. Chances are good that this can be done without using heat sinks, unless the original rectifier uses them.
It's definately worth using Schottkys if you can figure it out-- both for B+ and for the low voltage filament supplies.
Those Crees do really look interesting. A bridge of four TO-247's (1.2kV/10A) would take up a bit of space though. The 5A package(TO-220) is only slightly smaller, and I'd really like to have heatsinks on them. This is obviouly going to have to wait until after the holidays($$). I'll be doing the three bridges(600V) in my Hafler TransNova 9505 (woofer amp) too. That's twenty SiC Schottky diodes, and eight of those need to be 25A. HMMM- I wonder if the kids have bought my presents yet?