this is a common topic and search, so it'll be easier to find now
this mod should be done to all tube amps using tube rectifiers, to reduce the chance of damaging the tube rectifier

I would like to make this mod. Can anyone tell me if the 1N4005 diode will work. It is the only one I can get at a local store. (Radio Shack)

The 1N4005 has a reverse breakdown voltage of 600 volts as compared to a 1N4007's 1000 volt rating. It will work, but I'm not sure if the diode's breakdown voltage needs to be higher in order for it not to be damaged.

With it in series with a 5AR4 it probably doesn't matter, but I'd wait for another opinion. My guess is it would work just fine.

Tony

Thanks Tony and Bob for the help. Tim.

I saw this on DiyAudio: "Dynamutt, The ST-70 that should have been". Two MUR4100 are placed in series (total 4). I'm not sure why they did it this way.
http://geek.scorpiorising.ca/contrib/Geek/ST-70-P10CVD_power.gif
http://www.diyaudio.com/forums/blogs/geek/140-dynamutt-st-70-should-have-been.html

Agreed. I was only wondering about the diodes in series and why the Dynamutt guy may have thought it was benificial. I suppose it is a "backup for the backup" redundant design. Thanks again for all of your help. Rich

While the 1N4005 would work as protection devices for the rectifier tube, for safety reasons I'd want to go with a diode having the voltage rating they would require if they were doing the rectification job all by themselves. If the rectifier tube were to fail shorted, the 360AC RMS voltage, recalculated to peak voltage, could go far over the 600PIV of the 1N4005, which could then itself fail shorted and throw a huge current on the power transformer until something blew (hopefully the line fuse).

Silicon rectifiers used in a full-wave, two-phase power supply should have a PIV rating of 2 times 1.4 times the RMS voltage being delivered by the transformer. With the Dynaco Stereo 70 transformer at 360-0-360Vrms the diodes should be rated at 2 x 1.4 x 360 = 1008V PIV. This is very slightly more than the 1000V rating of the 1N4007 but with tolerances on the diodes' PIV generally all on the positive side, the 1N4007 would probably survive.

I appreciate that parts are not as available as they once were, but being conservative in design matters, I would suggest that instead of settling for what Radio Shack has, you look into other vendors that could supply the 1N4007.

I failed to tell you that my amp is the SP-120 if that makes a difference. Well, after making the mod with the 1n4005 diodes it was put to the test with one of those quick power outages when the power goes out and then comes right back on again during a wind storm. The rectifier tube is still working. Yes!!

Hi,

I put some time (too much time) into researching this mod after I lost a nice NOS Sylvania big bottle GZ34. I think the real fix to the lost GZ34 was me not turning my ST120 amp up to full blast... Anyhoo...

I found some conversations about what diode to use for this Mod. I ended up going with a UF4007. the UF4007 is basically the same as a 1N4007 having 1A / 1,000 volt rating but a more modern Ultrafast recovery version diode. One post spoke of the UF4007 potentially limiting the possible “switching” you could hear. This “switching” got shot down by a few who talked about all the capacitance that would buffer this out. I also saw some conversations about the need to go with the full 1,000 volt and I read some detail about why the 1,000 volt diode is needed. I wish I could find it. I'm not as tec savy to have this in my head. I’d go safe and run the 1,000 volt and error on the side of over spec... So go UF4007 or 1N4007

1N4007 is a $.09 each (http://www.mouser.com/ProductDetail/Fairchild-Semiconductor/1N4007/?qs=sGAEpiMZZMuQUXCJI7Y4lvWy%252b1U8RtCq)
UF4007 is $.49 each (http://search.digikey.com/us/en/products/UF4007/UF4007CT-ND/965732)

You may have a harder time finding the 1N4007 as the UF4007 is sort of the current diode for the application I think… Either way the mod should cost 20 cents or you can go Deluxe for the full 1 buck! I thought I’d live it up and go a buck! … Oh, plus shipping.

I use www.digikey.com because they are in Minnesota where I’m at. So I support em.

Don’t use something else.. wait the 3 days to have the UF4007 or 1N4007 come in the mail… any pay the couple bucks to have them come. We all have audio problems and probably pay too much for this hobby… 3 bucks is not worth a cut corner!

Cheers (insert beer drinking smily here)


www.mouser.com
www.digikey.com

Bummer - I want to do this mod, I only show one red lead going to my rectifier socket. I have a van Alstine modded ST - 70, so assume changes may have been made. In the diagram at the beginning, it says to move the red leads to pins 5 and 7. That being the case, what two pins are the power leads originally on? Is it 4 and 6 where the diodes (after the mod) jump to?
I apologize for my ignorance. I am fairly new to this, but trying to learn fast. Any help is much appreciated.
Thanks

Your transformers two red power leads originally went to pins 4 and 6 and since the pins 5 and 7 are not used it makes sense to use them. Pull the wires off pins 4 and 6. Install a diode band side to pin 4 and the other end to pin 5 install one of the power wires to pin 5. Install the second diode band end to pin 6 and other end to pin 7 install other power wire to pin 7. If you cannot find your two red power wires on the 5AR4's socket then trace the wires from the transformer and see where they go.

There is absolutely no difference sonically between a standard 1N4007 and the fast recovery. YOU WILL NOT HEAR A DIFFERENCE!!!

Thanks for the help. I knew there wouldn't be any difference sonically, I just want to preserve my rectifier as long as possible

Here's how I did it. I mounted a 2 tab ungrounded terminal strip on one of V1's mounting screws and moved the red transformer wires to that. Follow the diodes in the picture to see the rest. Note-Try to give the diodes breathing room because they get hot.