sentences:  Early 1969-70 "blue lines" used 6146B beam power vacuum tubes in the output stage, which proved unstable and was switched to the more robust, reliable and commonly-used 6550 tube around mid-1970.The second version of a vintage SVT is what is called the "black line" SVT, earning its name from the black (rather than blue) faceplate screen printing.Like the later-revision "blue lines" models, the "black line" SVTs utilize 6550 power vacuum tubes instead of 6146Bs.