Is it OK to put a 12V battery instead of a 9V one... or not?

Some effects will work fine with 12 volts, but many commercial effects use 10 volt capacitors so 12 volts is not a good idea. If your pedals all have 16 volt capactors (or higher), then the only problems you might see are the extra brightness of the check LED, and some LFO circuits will operate faster. That could be good or bad.

Take care,
-Peter

I once put a 12V ps over some boss pedals.  :x Had to replace all the caps over the power supplies...

My advice is never use 12V for effects. Stick to 9V to keep things easy and tidy.

When fully charged and used for many of my boxes for testing and stuff..charge this 'brick' and I don't have to worry about saggin V's for months and months.
 I chuck any caps rated at less than 16v...

There was a time when higher voltage-rating caps were bulky and pricey, and less likely to result in fitting a complex circuit in a little box, but no more.  In fact, not for a long time.  I've used a regulated 12v supply with pedals for a long time with no problems.  I can't remember the last time I saw an electrolytic or tantalum cap in a pedal that was less than a 16v rating.

As Pete wisely advises, one may see some issues with aspects of pedals that depend on specific voltages, such as LED brightness and LFO speed, but these are aspects which are easily compensated for by a parts change here or there.  A large proportion of commercial pedals will work fine, and sometimes better, with 12v.

NEVER use a supply for pedals that delivers anything more than 14v or so, though, unless you know EXACTLY what you're doing.  CMOS chips, commonly used for solid-state switching, and part of the audio circuit in some pedals, tend to fail permanently if supplied over 15v.

i'll keep all that in mind. If i recall correct, then all my parts are at least 20v or something... so it shouldn't be a problem....

anyway, it was just a thought