I thought it was fine and unambiguous, and was surprised to hear you ask. You used plural tubes to describe the aparatus and a singular tube to describe the concept. I think that's a normal way to approach it. It would also be okay to stick with plural, but I think the switch is natural. - A.
I agree with Avangi. The switch from plural to singular actually escaped my notice. It seemed fine. I don't see any need to rephrase (except as suggested earlier).