Phono section of ARC Sp6b went dead.What is the reason

I have a ARC SP6B tube preamp and the left channel went dead,and the Cd player is not playing good.Only the tuner works.Any suggestion to what the problem is?
Please voice your opinion.
Fuses? Tube gone bad?
I think there is only one tube in there--replace it.
No Sc53, there are six 12AX7 tubes in there, with a SS power supply. The SP-6B is one of ARC's best and it would be wise for you to troubleshoot the problem. Line stage is very good with CD playback. Call Steve Huntley at Great Northern Sound.............Frank