I just have a very basic question. The Fisher TA-500 mono receiver has three 12ax7 tubes. If I am facing it, and looking down at it, there are two 12ax7's right behind the front grill of the receiver, and then one in the back with the three power tubes. I'm thinking that out of the two in the front, one of the 12ax7's is for the phono pre-amp, and another one for the tone amp. But I don't know which one is for which (or if I am even correct about that). And then there is the third tube in the back. What would that one be for? Thanks in advance!