depend on lighting at concert ... sometimes may be enough even stock lens (iso @ 400/800), but preferrable will be some lens with aperture F2.8 (or better), then iso may be set to 200/400 for same brightness, and will be less noise
I've been using a very inexpensive Asahi 55mm f2 lens (Pentax m42 mount with an adapter) and I've been quite happy. It's pretty long so a few meters back is pretty much required.
I also have a "Formula 5" 28mm f2.8 lens (Pentax m42) that is quite a bit more usable up close though lacking in sharpness. But it's still pretty fast.
I paid $30 for each of them at my local used camera store here in Victoria (camera-traders.com - no, I don't work there) with an inexpensive m42 adapter from eBay.
I blew my budget on the camera, but I'm quite happy to spend $30 to experiment on a new lens
Remember, if possible, not to film the cables on the stage floor itself, otherwise you'll have flickering /moire effects going on like fireworks.
I have some footage I shot some weeks ago and on that dark stage floor the snaking cables seemed alive
Probably the fact that I shot handheld, and therefore there was some minimal hand motion obviously compounded the problem :-/