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Roy Simison (froggy)
Username: froggy

Registered: 09-2006
Posted on Sunday, October 01, 2006 - 07:35 am:   

I thought this might be helpful to those interested in cleaning up that rectifier I found this at 4gu-OUOsJ:www.tonequest.com/askour.htm+g uitar+amp+fred+diode&hl=en&gl=us&ct=clnk &cd=7&ie=UTF-8

What are FREDS, Why Do I Need Them and Where Can I Get Them!?!
I recently contributed a short Q & A to the new Tonequest Report (www.tonequest.com) regarding FREDS (fast recovery epitaxial diodes), and inadvertently caused and avalanche of inquiries on how they are to be installed. Never fear Tonemeisters! What follows is a basic explanation on their proper installation.
A FRED is a diode. It performs the same function as the standard 1N4007 diodes most guitar amplifiers employ. Like a 1N4007, it has an anode and a cathode. The cathode on a 1N4007 is designated by a white band on the diode body. The cathode in a schematic drawing is designated by a line in front of an arrow. On Fender schematics, the cathode is distinguished further by a "+"; the anode with a "-". Upon inspection of your amplifier, it should be easy to find and understand which end is which. Please remember that not all amplifiers look the same under the hood. On a Fender amp, it will be quite easy to sniff out the 1N4007's..well, on old Fenders anyway..some others may take a little snooping around. If you cannot locate the silicon diodes in your amp or are unsure at all about which end is which, HIRE A QUALIFIED TECH to perform the mod. In experienced hands this swap should take 10 minutes.

The FRED is different in appearance than the familiar cylindrical shape of silicon diodes. It is square shaped with a metal plate on the back which extends beyond the body. The two leads (cathode/anode) extend from the diode body on the same side; this is different than a silicon diode which has the leads extending from opposite sides of the diode body. The cathode on the FRED is the left lead when looking at the FRED from the top with the leads facing you. There is some printing on the FRED body..this is the top; the metal plate is on the back side. The metal plate is also the cathode. Let me repeat that. THE METAL PLATE IS ALSO A CATHODE CONNECTION. You don't need to know why; you just need to know that it is. (in some applications this plate is used as a heat sink--this is not applicable in vacuum tube amps because the current is so low..) When we install FREDS, we wrap the plate and body with electrical tape to insure that it doesn't inadvertently short against other components.

The actual installation is quite simple. Once the original silicon diodes are identified, remove them and insert the FRED in its place. In Fender amps, one FRED will take the place of the 3 diodes in series on the small eyelet board in the upper left hand side of the chassis. There typically will be six 1N4007's total--3 for each side of the B+ winding (red leads coming out of power transformer). Use one FRED for each side. This will differ depending on model and year, but the concept is basically the same. One exception is later Fenders and most Marshall/Hiwatt style amps. On these, there will be 4 diodes. On the schematic they form a "square" with two of the anodes going to ground. ALL FOUR must be replaced on amplifiers employing this type of rectification. If there is any doubt in your mind, let someone who is more knowledgeable perform the mod. Remember to insulate the metal plate with good electrical tape to avoid any contact with other components. It is also a good idea to use a heatsink (a hemostat will work nicely) when soldering the FRED; the heat of your soldering iron can damage the little bugger. And at risk of sounding redundant, remember that the cathode is the banded end of the 1N4007 and is the LEFT HAND LEAD on the FRED when looking at it from the top with the leads facing you. Pretty straightforward stuff..

As I mentioned in the article in TQR, the installation of FRED's in the power supply of your amplifier will make the amp smoother and more musical. It's a mod that you will be glad you performed.

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

MarkBaier@VictoriaAmp.com or calling us at 630-820-6400/630-369-3527

Please leave a message if you get the machine..when we're testing amplifiers, we can't hear the phone ring!

Hope this helps clear up what's involved when installing these groovin' diodes!!

Keep jammin',

Mark Baier
Edward Solberg (edward_solberg)
Username: edward_solberg

Registered: 05-2006
Posted on Sunday, October 01, 2006 - 03:41 pm:   


i ran across the very same article while i was net searching FRED's this very morning. I've become interested in putting them into my rectifier because of the reputed enhancement in the tone of our amps. (I also plan to "taste test" op amp replacements for the venerable LM307H ICs in our preamps). ed g got me interested in the idea...it seems worthwhile and at a nominal cost...ed says, newark electronics has them significantly cheaper at $2/unit cost and shipping should also be reasonable (this beats Victoria Amps' considerable mark up of $4/unit; and who knows what % shipping). unless one lives on guam or the southwest pacific islands the cost certainly warrants a look/see :>)

....of course, I plan all of these mods and upgrades "after" I find the source of the considerable overcurrent that currently (ouch!) has my amp benched.

ed s
Roy Simison (froggy)
Username: froggy

Registered: 09-2006
Posted on Friday, October 06, 2006 - 03:11 pm:   

Hi Ed I really posted that because I was investigating the change also and have not done it yet. I think I would rather use a short piece of shrink tubing instead of electical tape on the dodes but will soon see how it works out.

Ijust bought a batch of ten Fairchild RHRP 3060's for about $26 they may be a bit of an overkill being rated at 600V 30A replacing the 1N 4003's rated at 200V 1A.

Not being a tech I have no Idea if this upgrade will present any problem and would like anyones input the price was right so I went for it.
here is the data on the fairchilds gaW_HrEJ:www.fairchildsemi.com/ds/RH%252 FRHRP3060.pdf+fairchild+rhrp3060&hl=en&g l=us&ct=clnk&cd=1&ie=UTF-8
Like I said any input on this before I do it would be much appreciated I certainly want my RD50 to sound better and am open to a better solution for the rectifier bridge if there is one.
Ed Goforth (ed_goforth)
Username: ed_goforth

Registered: 06-2006
Posted on Friday, January 12, 2007 - 11:27 pm:   

Hi Roy, I did the FRED mod on my RD-50 and also added .01uf/600vdc brown turd Xicon caps in parallel with the FRED's. It helps warm up the tone and the FRED's do sound much more musical, not harsh at all, but I also did some serious mods to the overdrive circuit that gave it a more usable, articulate overdrive and not mush when maxing out the gain. The overdrive sounds similar to a D*mble overdrive but with the RD-50 flavour. Easier sustain/feedback. Good luck on your RD, its a great amp!
Ed Goforth (ed_goforth)
Username: ed_goforth

Registered: 06-2006
Posted on Thursday, June 30, 2011 - 09:54 pm:   

I only use the 10ETS12 FRED (2) diodes, no Snubber cap. It is not needed. Sounds great!