# Fast-Recovery Diodes in the rectifier circuit



## dbts808 (Sep 5, 2020)

I've done a lot of experimentation on Fender's AB763 circuit.
Has anyone ever tried F.R.E.D.'s, replacing the cheap diodes?
I love the sound, tight bass and sweet highs!
Also, try a 47 pf instead of the 120 pf on those bright switches.
Trem not slow enough? Make those three caps in the tremolo circuit all 0.02's.
And if the tremolo ticks then solder a 0.1 cap across the photocell's two legs closest to the pilot light.
These are the only modifications I can suggest of Leo's design.


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## alwaysflat (Feb 14, 2016)

These (FREDS) are for switched mode power supplies afik, so I wouldn't expect too many responses from folks trying this. If you were to take this on, pay specific attention to PIV specs, and go double, and pay attention to thermal needs ( heatsinks ). I'm scratching my head, thinking the difference as to what you'll improve upon over over a typical rectifier diode, for 50/60 Hz ...like what would increased precision buy us here, that doesn't get filtered by the same old choke and filter caps.
Anyone ?


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## jb welder (Sep 14, 2010)

alwaysflat said:


> These (FREDS) are for switched mode power supplies afik, so I wouldn't expect too many responses from folks trying this.


The only advantage I know of is reduced diode switching noise (if it's even audible).
They eliminate any need for the snubber caps you sometimes see across the rectifier diodes (SF 135W Twin for example).
They (FREDs) should not have any effect on the amps tone. Those other mods mentioned should.


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## dbts808 (Sep 5, 2020)

alwaysflat said:


> These (FREDS) are for switched mode power supplies afik, so I wouldn't expect too many responses from folks trying this. If you were to take this on, pay specific attention to PIV specs, and go double, and pay attention to thermal needs ( heatsinks ). I'm scratching my head, thinking the difference as to what you'll improve upon over over a typical rectifier diode, for 50/60 Hz ...like what would increased precision buy us here, that doesn't get filtered by the same old choke and filter caps.
> Anyone ?





jb welder said:


> The only advantage I know of is reduced diode switching noise (if it's even audible).
> They eliminate any need for the snubber caps you sometimes see across the rectifier diodes (SF 135W Twin for example).
> They (FREDs) should not have any effect on the amps tone. Those other mods mentioned should.





alwaysflat said:


> These (FREDS) are for switched mode power supplies afik, so I wouldn't expect too many responses from folks trying this. If you were to take this on, pay specific attention to PIV specs, and go double, and pay attention to thermal needs ( heatsinks ). I'm scratching my head, thinking the difference as to what you'll improve upon over over a typical rectifier diode, for 50/60 Hz ...like what would increased precision buy us here, that doesn't get filtered by the same old choke and filter caps.
> Anyone ?


- - - - -
“We have 11A 1200V FRED's for sale for $6.00 a piece plus a nominal shipping charge of $5.00. We also are offering a complete diode/negative bias supply board that will drop right in on older hardwired Fenders. It is available for $30.00 plus $5.00 shipping. Please specify model of amp it's going in when ordering.
Parts can be ordered by e-mailing us at
[email protected] or calling us at 630-820-6400/[email protected]


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## dbts808 (Sep 5, 2020)

F.R.E.D. RECTIFIER
“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. FREDs 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.”
- - -
As a player siting in front of my amp with FRED’s, I can tell you that I do not miss the tube rectifier sound or the cheap diode sound.
I know what it sounded like both before and after the FRED’s; the FRED’s are a must-have improvement for me.
The cheap diodes are uninspiring to play. - Dave
P.S. My next experiment is those new, expensive mica caps ($8 each).


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## nonreverb (Sep 19, 2006)

There is no need for FRED's in a typical tube amp.


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## dbts808 (Sep 5, 2020)

Mike Baier at Victoria amps disagrees with that; and I do hear a difference.
I also provided an explanation for why FRED’s do make a difference.
You only gave an opinion, with nothing to back it up.
Maybe you should reread the justification for them and then counter it with facts that you have.
I’m always interested in learning new things. - Dave


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## dbts808 (Sep 5, 2020)

nonreverb said:


> There is no need for FRED's in a typical tube amp.


“Critical listening tests have demonstrated that the recovery time and type (soft or snappy) has an effect on the “subjective quietness” of a high end audio product. This is not “quietness” from the standpoint of signal to noise ratio traditionally measured in decibels. This is a subjective sense of silence between notes in a musical passage. To investigate further, this phenomenon can be measured with a spectrum analyzer. This was demon- strated in an article by Rick Miller with pictures and data in the “Audio Amateur” of Jan ‘94. The higher order harmonics generated by the recovery time of the standard speed rectifier can be clearly viewed on the spectrum analyzer. These upper harmonics are absent (or at least below the noise floor of the instrumentation). Audiophiles spend lots of money on AC line conditioners to eliminate line noise from the power supply inputs to their equipment, which is certainly counter productive to generating this noise internally. This component substitution Schottky rectifiers have switching speeds in the single nanosecond times and below.”
- There is so much about F.R.E.D.’s (fast-recovery epitaxial diodes) and Schottky’s that I shouldn’t post anymore on it here.
(Also components that sound good on paper might not in the amp.)
I have them in a 4-6L6 Fender amp and I love them.
I just wanted to share this with other tone-freaks.
- Dave


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## nonreverb (Sep 19, 2006)

Like jb welder quite rightly points out, the use of snubber caps pretty much solves a problem that for all intents and purposes, doesn't exist in 50/60 cycle rectification. FRED's use in switching power supplies where switch speeds are infinitely faster is relevant...not in this application. Plus, filter caps take care of any transient noise...that's partly what they're there for. Remember....most tube guitar amps don't even have perfectly balanced output transformers and since you mentioned AB763 circuits, I dare anyone to find just one of these amps with carbon comps that are even remotely within spec after 50 odd years. All to say, it's just another sales spin like the cork sniffing preamp tube reviews....all of this originating in the hi end audiophile market. IMHO.


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## alwaysflat (Feb 14, 2016)

Ok, with some more reading on my part, from a subjective standpoint, I recognize that there is a +ve argument "for" faster, smoother switching (recovery). If it gives you piece of mind,  I'd say give it a go. It has a small audience in the audiofile world. How to gather some quantitive evaluation would be a nice part of the exercise. I think looking for this artifact would be a first order of business. There are some nice examples demonstrating switching noise in diodes tho I've not yet seen examples representative in the "DC" filtered side of the supply, so I won't argue "its not there".


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