Artisan 15 valve problem

Hi.
My Artisan has an intermittant fault where volume violently moves up and down. I have been told it is a valve problem. I don't have much of a clue about valve details, but want to learn.
Does anyone know
a) How to tell which valve is faulty?
b) If I find out, can I replace it myself? (I don't have a backup amp so don't want to send it back to Blackstar for repair 'coz I would die if I couldn't play for a while!)
c) Any other info that might be good for me to learn some valve stuff?

Thanks :-)

Simon

The easiest way is to swap out the tubes, one at at time, with a set of good tubes. This way you can narrow the problem down. It is always a good idea to have an extra set of good tubes when you own a valve amp. Hoffman amps website is a good resource for troubleshooting tips.