Hello,
With respect to your price range, I would highly recommend Belden 1313A speaker cable. This is a 10awg cable with oxygen-free copper and low capacitance and resistance, you should be able to go the whole 50'. I must admit though, that I am not familiar with Tube Amps. That said, for the distance you are trying to cover, this stuff should do the trick!
Good luck!
With respect to your price range, I would highly recommend Belden 1313A speaker cable. This is a 10awg cable with oxygen-free copper and low capacitance and resistance, you should be able to go the whole 50'. I must admit though, that I am not familiar with Tube Amps. That said, for the distance you are trying to cover, this stuff should do the trick!
Good luck!