Having trouble in thumb position playing lines that switch strings quickly...rightly or wrongly I play thumb position pretty much the same as electric bass...one finger to a "fret" (well...one is a thumb)

For instance...B flat to F to B flat again, I would play using my index finger for every note with the thumb anchored on 12th fret A, same as electric...is there a better way to do this, or a way to practice to get it together better??? (switching strings quickly and smoothly)

Thanks

People usually do one of two things when playing notes directly across on adjacent strings in the same position. It depends on if the notes go by quickly or are sustained. One - use two adjacent fingers. On the G string Bb use 2nd and the F on the D string use 1st. Or you could use 3rd on the G string and 2nd on the D string. You could also try using a flat finger bridge where the lower string is more on the tip and the higher string is closer to the first knuckle of the same finger.