The Definitive Series 5751 Pair's are Handpicked, Curve traced, Matched and supplied with a graph showing the curves of all triodes being matched throughout their operating range at full working voltages. These are also available as Trio's. The obvious application for Definitive Series tubes is any tube preamp, compressor, or processor and of course any stereo application. You can of course use these tubes in guitar amps if you simply want the best of the best!