ampage Tube Amps / Music Electronics	For current discussions, please visit Music Electronics Forum.

ampage archive

Vintage threads from the first ten years

Right! said FRED's

:

8/24/2000 8:41 PM

Right! said FRED's

I know there was an earlier thread on this subject but I'm curious if anyone here has tried FRED's (Fast Recovery Epitaxial Diodes) in their guitar amp and if they noticed any change in the amp's tone after installation. I have a customer that wants me to install some in his Twin Reverb after reading about them on the Victoria Amp page. http://www.victoriaamp.com/
He seems to be under the impression that these diodes will respond like a tube rectifier. That's not what I'm getting from Mark's article / sales pitch. I must say that I'm curious since Mark is obviously sold on them, that does carry some credibility with me.
Also, anyone have a good source, part #'s etc?
I will post with my impressions after I install these.
I'm too sexy for my diodes, too sexy for my - little rectifiers...

8/24/2000 10:01 PM

There was an article in Vacuum Tube Valley regarding FREDs by John Atwood (One Electron) and paraphrasing the article without having it in front of me, the fast recovery reduces RFI produced in the rectification process, which can get into the audio circuit. It was in the VTV with the 6SN7 on the cover. If you’ve seen the concept of having a cap or a snubber across a diode, those are alternate ways to eliminate this RFI noise.

They will have the voltage drop characteristics of silicon, ie stiff, but the fast recovery keeping the noise out may clean up the sound. Tube rectifiers are inherently fast, so using FREDs provides similar performance for that parameter anyway. The VTV article stated that they eliminate some harshness in the highs as compared to normal silicon.

I recently purchased some of these for a project and they are a little hard to find. Here are two possibilities:

Manufacturer Model Specs

International Rectifier
HFA06TB120
6A, 1200 PIV

IXYS
DSEP8-12A
8A, 1200 PIV

Newark was some 600V ones, Mouser and Allied none. I did find the HFA06TB120s at Arrow Electronics for about $3.50 a piece: 1-800-833-3557.

I haven’t got them yet (ordered them Monday 8/21), so I don’t yet know if they are worth the bucks.

With respect to the blurb at the Victoria site where he recommends using only one FRED per side of the full wave recto is very close to exceeding the PIV rating for a 400-0-400 tranny if I’m recalling the FW PIV formula properly (2.82 x 400).

8/25/2000 5:43 PM
Ken Gilbert	and to add to this... i remember over on RAT some folks experienced INSTANTANEOUS breakdown once the PIV was exceeded... there is NO safety margin with these FREDs. kg

8/25/2000 7:53 PM

Thanks for the info Scott & Ken.
For the record, most Fender amps run around 350-0-350V on their p/t, so a 1200V FRED would probably work here. I would think a pair per side would give one some extra comfort though. In a higher voltage amp a pair per side would certainly be necessary.
I took exception to a couple comments on Mark's page & emailed him about them. Specifically - the use of electrical tape to insulate the metal tab on the TO-220 package. I think heat shrink tubing would be a bit more professional and safer here.
I personally consider electrical tape inside an amp as hack work. (my definition of "hack" is anything that is messy or potentially dangerous) Tying the tape with bundle lacing or pan-ty's (A Panduit product, not the ones on the bathroom floor) is an acceptable solution if you must tape, but heat shrink is always the preferred insulator. The other comment that irked me was the one that the FRED installation should take about 10 minutes in qualified hands. This would likely be at least a 1/2 hr. job in any reputable repair shop with disassembly, removal of the old parts, cleaning & preparing the eyelets, insulating the diodes, installation, testing, reassembly & re-testing of the amp. Don't forget the time it takes to write the tags & invoice the repair.
I haven't heard back from Mark yet, I know he's pretty busy.
RE

Page 1 of 1