Korg makes a clamp-on pickup on a long wire leading to a ¼ inch phone plug which can then be plugged into any tuner with a `1/4 inch jack. Here's a picture. I use this at home plugged into my old through-the-air chromatic Korg tuner. It works great at all frequencies, and is easily moved from one kind or instrument to another. The only downside is the long wire is a hassle in a jam so I rarely use other than at home. $10 American on Amazon.
- Phil