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Replacing Amp Caps

I am currently trying to replace the caps in my Bedini 100/100 amp and I am having quite the time trying to find a replacement that will work. I am hoping that someone with more knowledge can give me direction on which way I can go. The amp recently stopped outputting a signal and after all the usual internal checks, with an amp as old as this, cap replacement should be on the list of things to do.

The values and associated numbers on the OEM caps are as follows:


National Cap

85DX

+22000MFD 65VDC

85C USA 8137

 

Screw Terminals

D=2.5"

H=4.0"

Apparently 65VDC caps are something in the distance pass, of the possible replacement values is either 63V or 70V
is there a rule of thumb of which direction this value can go? Since it is for an amp, is there a brand/style I should be looking at? I can find little to no information on National Cap, do I assume they are no longer in business and move to another brand?

I have been searching for five days and fully understand I am out of my element and looking for all the help I can getand open to all suggestions.


SteveA
128x128stevea11757
gs5556
1,240 posts
 
Always always DC voltage rating greater or equal to original. The screw terminal spacing must be exact. The physical dimensions can be different as long as it fits (don’t neglect top clearance to cover). The farad value can be 24mfd, which should be a common size. You can go bigger but you have to know the current rating of the rectifiers to see if they can handle the resulting higher inrush current, as well as being mindful of the effect on the transformer (higher stresses can do damage).

Warning: accidentally reversing the polarity of filter caps can cause an explosion (literally!) so triple check the original orientation with the new. It is always recommended to use a variac to power on the amplifier with about 10 volts AC and check the dc polarity across the caps to make sure they are correctly installed.
l
erik_squires
16,472 posts
 
In a power supply you should strive to keep the mfd about the same. Exceeding this amount by too much can stress the power transformer by increasing the surge current on turn-on.  Stay between the rating and maybe 10-15% more to be safe.

However, the V and temperature ratings can always be safely exceeded. A cap with a higher temp rating is particularly good as it will have a longer life span.

The V rating refers to the maximum before the capacitor breaks down. So, you can replace a 63V cap with a 300V cap safely. The temp is kind of the same thing, but is related to operating temperature and hours. By keeping the ambient temperature the same, but increasing the cap temp you are essentially increasing the rated lifespan. Kind of. Caps will vary. :)


Best,

E
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