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enter image description hereI am novice to capacitors and need some help.
My amplifier has two large can capacitors rated 9000 uF and 125 V. These capacitors are no longer available. The closest one that can fit in the place/size that I could find is 10000 uF and 100 V.
This is not a question that can be answered without more information. Information like the model of the amplifier means we can search out the schematic and hopefully determine what the actual working voltage is and then we can advise whether a lower voltage part would be a safe replacement.
Generally, it is bad juju to replace with a lower voltage part. The working voltage might be 100V and the 125V rating is for some safety margin. Substitute a 100V part, the safety margin has gone so the new part might blow up (literally). You could tolerate a lower capacitance part.
Why do you think you need to replace the capacitors? Have you tested your capacitors to see if they are still in spec? There's a lot of old wives tales out on the interwebs regarding electrolytic capacitors. Unfortunately, you need some electronics background to make sense of what is claimed.
Here's the service manual
That was not hard to find
On page 12 they say with a 120VAC input, you should have 97V on the DC rail.
Next notice the spec "1000W bridged at 8 ohms". From P = V^2 / R...
V = sqrt ( P * R ) = sqrt (1000 * 8) = 89V
To bridge the outputs, those caps are effectively in series in bridged mode, means they have around HALF that voltage on them (plus some for headroom)
This is also suggested by the spec 375 W / Channel into 8 ohms
V = sqrt ( 375 * 8) = about 50 volts.
Have a DC voltmeter? This is SUPER EASY to check. Those caps are sitting in open air, easily accessed. While it's powered up, very carefully measure the DC voltage on each. I bet they're no more than about 75V, probably less. If so, you are def good to make the substitution IMO
I didn't look up a datasheet for those caps, and probably can't. But note sometimes there is a bleed resistor internal to them. This is there to drain the caps (make them safe) after you power off and to help 'balance' them in certain circuit configurations (like tube amplifiers with high voltage DC rails and high output power). If those caps have resistors and your new ones don't, you'll be missing those functions. Place about 470k 1W resistor across each cap will not harm anything and will ensure those functions (if originally present) are still there.
Some will say if they're not showing signs of bulging or leaking, they're still good. You need special equipment to really know. After almost 40 years I would def replace them even if it still works. At a minimum, with fresh power supply caps, you should find an audible difference - punchier bass notes and maybe less 120Hz hum (if there is any now).
