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This is a picture of the rack John used for the 2002 World Tourbulence tour. Among the various units there's Korg DTR-1 rackmount tuner (first from top to bottom). Confirmation of this being John's rack can be found in [this diagram from *Guitar Geek*](http://www.guitargeek.com/john-petrucci-dream-theater-guitar-rig-and-gear-setup-2002/) and [at :15s in this video](http://youtu.be/4yUUCRJGdko).more
It might be excessive now to dedicate a a full rack space to a tuner, but in the days before clip on tuners this was the way to do it (if you had a rack). As well as the oversize tuner display with 3 tuning modes (Hz, cent and strobe) this unit allows you to switch between two inputs and has a convenient cable tester built in to help diagnose problems on the fly.
The only issue I've had over the years is that if you run it first in the signal path then engage a gain effect it gets a bit noisey, hopefully this is something Korg have sorted in the years since I got mine, so these days I run it in a rack between the gain effects and time based stuff where it appears to have zero effect on tone but does make the switchable inputs a little redundant. so 4 stars for this one.