My peavy vk 2x12 have a dirty distortioned sound in the clean chanel, i'd took a look at the back side and i saw the problem could be the 12ax7, one looks ok, one is dead, have no light while is turn on, and the third is almost dead, the four 6L6 looks ok. may I replace booth sets or only the 12ax7? I'll apreciate your help, God bless you guys.

* ALWAYS UNPLUG YOUR AMPLIFIER BEFORE REMOVING TUBES *

If you can see that a tube is definitely not lighting up then it may be blown. If this amp has seen regular use then it may be time for a new set of tubes. Try replacing the 12AX7s first before committing to a new set of 6L6s. Though, if you have the cash available, a new set of power tubes isn't a bad idea either. I would advise you to do the preamp tubes first and play through the amp for a few days first before changing the power tubes.

Keep in mind... if a tube (or tubes) has blown, there could be another issue in the circuit. Swapping in a new tubes is a quick way to determine that. Then it would be time to take it to your local repair shop.

Good luck and keep rockin'!