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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

Showing 4 responses by gs5556

gs5556
1,218 posts
 
I was just ordering something on Mouser...

Looked at 22mfd 2.5 inch diameter caps. The United Chemi-Cons are 105mm in length and should fit nicely. Also, they use 10/32 threads so your original screws may work.

https://www.mouser.com/Passive-Components/Capacitors/Aluminum-Electrolytic-Capacitors/Aluminum-Elect...
gs5556
1,218 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
gs5556
1,218 posts
 
Those model numbers do not have a mounting stud. Going higher in capacitance by 50% should not be a problem and, as a plus, the higher voltage plus capacitance will result in a lower ESR. 
gs5556
1,218 posts
 
Should have looked at a picture of the amp under the hood... the ground bar between the negative/positive can possibly fit if the new caps are adjusted by rotating slightly. I would make paper templates of the top of the new cap using the dimensions from the spec sheets and superimpose on the existing caps. This way you would know which brand will work.

Also, if there is a cap clamp on the bottom plate, a different diameter won’t fit. You may have to get the correct size and drill new screw holes to accommodate.
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