If I am using ELECTROLYTIC CAPACITORS in pedals what is the minimum voltage I should use?
Well
if your pedal is running at 9V they should be rated at least 9V, but I think the minimum you can find is 12/15V, which is even better since it leaves you some safety margin in case somebody would plug a 12V adaptor in ...
Buy yourself some 16 or 25v rated caps and get it over with. You'll ver have to worry about caps in 9v circuits again If you have caps already that you aren't sure if they are rated high enough, you can probably feel free to use 6.3v caps in series with the signal path since the signal will rarely exceed a couple of volts in the majority of pedals.
If I am using ELECTROLYTIC CAPACITORS in pedals what is the minimum voltage I should use?
No less than the maximum voltage they will ever be exposed to.
Sometimes that's simple...if you're using a 9v battery, they'll never see more than 10v.
Other times not so simple, adapters marked '9v' can put out much more than that with no-load to low-load conditions. Use the DMM to find the actual voltage.
Nothing wrong with 'overkill' [25v...50v, 100v etc.] except they become physically very large as the voltage ratings go up.
Actually, there is a problem with overkill - capacitor tolerance.
As the caps age, capacitance drifts up and withstanding voltage drifts down. Starting with a high voltage cap means that the capacitance will get bigger with aging.
That's OK for a filter where all you want is MORE CAPACITANCE but if you use it for any frequency sensitive stuff, it's bad.
The moral of the story is - never use an electrolytic cap where the actual value needs to be a tolerance closer than about +/-50%.