A tension control device is provided, for example, for locking and unlocking a movable support arm for a keyboard or the like, by engaging or disengaging rotational joints having axially displaceable rotationally lockable couplings traversed by a tension line. A control, preferably a caliper, has levers pivoted on a pin, normally biased by a spring to close. The spring pulls the caliper closed; however, the caliper is arranged by linkages to clamp tension onto the line rather than to couple spring tension to the line. A front linkage isolates the spring bias from the line when closed, by folding inwardly such that tension is transmitted substantially exclusively via compression of two pivoted members of the from linkage that align nearly parallel to the line. The biasing spring is coupled between a rear linkage that operates oppositely, and a fixed point such as the caliper pivot pin. The two linkages are such that the spring has maximum mechanical advantage to hold the caliper closed when in the closed position, tension nevertheless being rigid, clamped and not resilient, for rigidly locking the support arm.