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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 5 responses by kijanki

kijanki's avatar
kijanki
4,410 posts
 
Steve, Rated voltage has to be equal or higher, but as you can guess if it is too high something has to give.  It is usually size or ESR.  There are many factors that would be helpful, like ESR and max ripple current.  There are tons of capacitors listed in Digi-Key or Mouser, but only few of them are in stock.  I found one that might fit and will be available in the mid November (order now).  It has 2.5" dia but is only 3.125" tall.  It can be more compact because technology always improves.  You have to check mounting (spacing and screw dia)  

https://www.mouser.com/ProductDetail/KEMET/ALS36H223L3C075?qs=b8LFrSqGEF8HOXdlWVz1Bw%3D%3D

Arrow list only the same capacitor, but no stock.  There was nothing in Newark.
kijanki's avatar
kijanki
4,410 posts
 
dannad, I would assume, that larger capacitor has larger ripple current.  The thing that worries me is that, for instance, rectifier was chosen with certain peak currents in mind*.  If they are deigned with certain margin, then by using lower ESR cap we effectively reduce these margins.  Unless I can be sure that designer picked unnecessarily large rectifier I would be afraid of big changes.  If we're talking 10% or even 20% lower ESR that should be OK, but people often put largest capacitance that fits. 

*larger, but narrower charging pulses might have the same average value, but RMS value will be way higher, increasing rectifier's temperature.  It will also increase transformer's temperature - not only because of higher rms (copper losses) but also higher frequency component (narrower pulses) and related core losses.
kijanki's avatar
kijanki
4,410 posts
 
Dannad, I cannot see you simulation, but load and not the source should be a constant current.  Because capacitor voltage is pretty constant you can just use resistor as the load.
kijanki's avatar
kijanki
4,410 posts
 
Be careful with high capacitance values that you might not need.  By getting much more capacitance you are also adding much more inductance, making sluggish power supply.  Yes, you can always bypass it with lower inductance smaller caps, but then you create parallel resonance circuit (L+C), that might ring.  Inductance can be reduced by placing many smaller caps in parallel or by using more expensive low inductance caps (like slit foil), but then you have to modify existing circuit.
kijanki's avatar
kijanki
4,410 posts
 
dannad, Thank you - very interesting.  I wasn't expecting that.  So my reservation about inductance is unfounded.  I would still be a little afraid of lower ESR.  If we reduce voltage ripple using larger capacitor (and lower ESR), we have to be sure that this larger capacitor also has higher max ripple current, since shorter charging time requires larger charging current to deliver the same amount of energy (at the same load).
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