Theremin amp

Concerning the amps, there are numerous and I am very confused.
there are for guitar: acoustic and electric, for bass, than for keyboard.... which one is "normal" to use?

Another question, about watts. What is "normal" sound power for a chamber music playing (small/medium concert hall)?

MANY thanks!

What do you need the amp for?  Personal practice, or playing out as PA, or playing out as personal monitor?

Keyboard amps are supposedly the least "colored" when it comes to frequency response, so I'd maybe go for a small one of those.  You could also use headphones for personal practice, that's what I do.  The "open" variety can allow you to hear external accompaniment.

I need for a concert use.
Chamber music, medium hall.
What power do I need? 

Also, very important, what cable is used connecting to amp, an instrument or audio cable?

Others could probably offer you better advice, but I'd get something with at least one 10" speaker and 50 Watts RMS (real Watts).  The Roland KC-80 or KC-200 might be sufficient?  You also might consider powered PA speakers.  Why don't you take your Theremin to a music store and try some amps out?

Connection is usually via a 1/4" "instrument" cable.

Thanks for your advices. I use to buy online stuff, because they are cheaper, but anyway to test it is a good idea.

And thanks also for suggesting the output power and the cable.