LOOSE STRING FOR DOUBLE BASS Helicore Solo '', Si (B), HS613 3/4 medium tension. The Helicore para strings have been specifically tailored and tuned for the notes: AEBF #, a pitch higher than standard tuning. They are made with a core composed of various braided steel strands, which produce a warm tonal quality with excellent response when playing with the bow. The core design makes the strings very easy to play under the left hand. The cushioning is designed for bow use, making them ideal for solos.