Dallas Rangemaster

Treble booster, which cuts some of the low end of an already-overdrive tube amp to create a more distorted yet focused sound, emphasizing the treble or midrange. Used by such musicians as Brian May of Queen, Ritchie Blackmore of Deep Purple, and Tony Iommi of Black Sabbath.
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Mentioned in a tweet by Billy on 2011/06/25 while experimenting with various guitars & amp setups to track the guitars for Oceania.