How to add tension to one of these springs? Clip off one or two coils.
It is a great advantage, of course, to have adjustable spring tension, as is the case with The Clavietta and some early Pianicas, which have compression springs. It would seem that ideally we want the same degree of key tension up and down the entire keyboard. Greater tension is an advantage in relation to control of the individual key, for example for bending notes, but there will be corresponding disadvantages. Just now checking several different brand and model melodicas, I find that with several, tension seems to increase as we move up the keyboard, so that the highest key has noticeable more tension than the lowest key. Possibly it’s because the lower end has been played more than the higher end.