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In Reply to: RE: "The 1st filter cap. can't exceed 20 mF" posted by 1973shovel on May 21, 2012 at 15:08:39
You didn't dig hard enough. ;>) Mouser stock # 598-81LX330M450H012 is a 33 μF./450 WVDC snap in part that happens to be 105o C. rated too. :>D Two of those in series will be just fine for the 1st cap. position, when fed by a "potato masher" 5R4.BTW, if you add a NTC inrush current limiter between the rectifier and the 1st filter cap. position, you rate to be safe with 2X 47 μF. parts in series. Start up arcing has to be prevented. Once that's been accomplished, a few extra 1st position μF. are just fine.
edit: added NTC remark
Eli D.
Edits: 05/21/12Follow Ups:
I had a feeling you might find caps I could use, Eli, thanks! Technically they are too tall to use with the SDS board, but I doubt 0.4mm will be an issue. I'll know more when I assemble the boards.
I did some 5R4 reading last night. The first question is the first filter cap. The data sheet and many posters say 4 µF is the limit. I linked to one of your posts where you said 20 µF with "real" potato mashers. Jim McShane lists more than a few 5R4's. Which one's "real" and safe to run with the extra 16 µF?
I've already got CL-90 's ready to go on the primary. I'll look at adding one to the secondary, if I go the 5R4 route. I did that to the secondary of my SCA-35 (as well as one on the primary) per your recommendation, and it worked great.
The RCA 5R4GYB data sheet shows 20 μF. as the max. sized cap. allowed. So, that variant is definitely safe. IIRC, AA sponsor Jim McShane has some RCA 5R4GYBs.
The "potato masher" gets its moniker because it slightly resembles a WW2 German hand grenade. The actual types are 5R4WGA and 5R4WGB made by Chatham and Cetron. The variant was developed for use in (sic) B52 bomber avionics systems. :> D Oddly enough, the data sheets I looked at show 4 μF. for the "masher". However, I've never heard of the "masher" arcing into 20 uF., at the voltages we routinely work at. It's a very tough hombre.
You can look at a photo of the "potato masher" by using the link that follows immediately.
http://tctubes.com/images/products/detail/36_5R4WGB_800.1.jpg
Eli D.
I did some more homework last night. I read that you ran them with 20µF, and at least one guy used 30µF, for five years with no issues.
I will get these MkIII amps up and running, and then decide exactly which route to take.
Thanks for all the food for thought!
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