Recapping amps

I have never heard of this practice.Is it just class A type or all amps.I use twenty year old B&K ST-140's (three) in my HT rig.Ten year old Adcoms on the subs????
Most older gear used filter caps that are under 20,000 uF's each in rated value. As such, you can pull the caps and check them using an inexpensive capacitance meter. A simple but reliable device that i've used many, many times can be found at Parts Express as part number 390-735. This device will measure the caps and tell you if they are within a reasonable range of their rated value.

My experience is that most filter caps used within the last 20 years or so hold up VERY well. That is, unless they were used in gear that runs phenomenally hot, they sat for a very long period of time with no use or they were stored in very hot climates ( an attic is a BIG no-no ). I pulled some caps from an amp made in 1991 that were rated at 15,000 uF's and they tested at almost 18,000 uF's, which is obviously quite good. Obviously, some brands hold up better than others.

If you want to measure caps that are above 20,000 uF's with this device, you can hook up two larger caps yet identical caps in series. Two 35,000 uF caps in SERIES ( caps work the opposite of resistors in series and parallel ) will give you appr half the value i.e. appr 17,500 uF's. So long as the meter comes up in that range, you are good. If the meter goes up off scale, the caps are still easily meeting spec and actually have more storage capacity than what they were origally rated for.

I would only caution that caps can store enough of a charge to really "whack" you. In extreme cases, there might be enough there to stop your heart. Don't under-estimate voltage and current potential of a cap, especially if you are a newcomer to tinkering with electronics. If in doubt, unplug the unit from the wall and turn it on. Let it sit for a while. If there are lights, led's, etc... wait until they are off via fully discharging the caps before messing with the unit internally.

On top of that, ALWAYS pay attention to proper polarity and label your wiring as to what goes where. Hooking up a cap with reverse polarity will make one helluva loud explosion and scare the piss out of you. If you think i'm joking, guess again. Big caps hooked up backwards with high rail voltages and high current power supplies can literally be louder than a firecracker and more like an M-80. Needless to say, you don't want to hear this go off in your house let alone inside your equipment, etc... It makes one helluva mess. Believe me, i've had to clean the mess out of more than a few pieces of gear after folks have done this.

While you are doing this, a good and simple trick is to install "bypass caps" across very large filter caps. Due to the higher series resistance of a BIG cap, there can be a small delay in the charge / discharge rate. By installing a smaller value cap ( 470 uF's - 1000 uF's ) and a small assortment of very low value caps ( or even just a single .0001 uF, etc...) across each of the "big cans", you'll probably end up with a much better sounding amp. By doing this, you'll clean up the sound quite a bit and pick up added "punch". This is done by having a blacker background and increasing both the speed and amplitude of "micro-dynamic transient bursts". Just make sure that you use caps rated for the voltage that they will be seeing.

As an example, in the preamps that i'm currently working on i've got four 10,000 uF caps. These are bypassed with twelve 100 uF caps. These are in turn bypassed with four .001 uF caps. While it is best to stagger the values of the caps and NOT use evenly divisible figures like i did here, the benefits of doing something like this are VERY noticeable to say the least. In case you're wondering, these are not "magic values" or anything like that. These are values that would physically fit in the space that i had to work with, were within the price range that i wanted to spend for the specific electrical characteristics that i wanted for this device.

As a side note, this equates to well over 40,000 uF's of filter capacitance for these preamps, which is more than some BIG power amps run. This is obviously WAY overkill, but then again, did you expect anything less from me ??? : ) Sean
Sean's advice is good...Two additional points from my experience.
1. After waiting for the power supply capacitors to discharge, before you touch anything short the capacitors with your screwdriver. It there is still juice there it's better to put it through the screwdriver than through your finger.
2. With a regulated power supply, adding extra capacitance can screw things up.

Also, Sean has focused on power supply capacitors. In the case of tube amps, where the signal passes from one amplification stage to the next by "coupling" capacitors, these will degrade. Since they are directly in the signal path, replacing them was a fairly routine procedure with tube amps.
Here is a good article on capacitors, measuring them, discharging them etc. - with a good suggestion for a more graceful method of discharging them than the possibly quite startling "screwdriver arcwelding method."
I know there are several types of capacitors, in terms of the materials used to make them. Are some types more stable than others? Are some types not a problem at all, regarding time, disuse, heat, etc (i.e. they are very stable over many decades)? I am asking because I will be ressurrecting a late 1970's Quatre preamp, and the friend who will be putting it together for me says that some of the caps will be good and stable despite no use for several decades (depending on the type of cap).