The JJ 12AX7S is considered to be one of the best for guitar and hi fi applications. They simply sound better in most guitar amps than the other new 12AX7s. Most players prefer them for their warm sparkly sound. These tubes are immediately discernable from other 12AX7s. They have a construction that is similar to a frame-grid tube - similar to the Telefunken ECC803s. This construction makes them very rugged and reliable and the spiral filament makes tube hum non-existent.. This is one of the best new 12AX7s for any guitar amp. It is very musical - rich with great harmonics. An excellent tube in all Marshall amps in the three preamp positions. The JJ's break up much more smoothly, giving you a huge “sweet spot” and a  smoother, singing tone. The ECC83S is great tube with a tight low end, a very  natural and harmonically complex mid with VERY good definition and a sweet high end that is not brittle. These tubes have very low microphonics and are great in the gain stages of high gain amps like Marshalls, Mesa's and Peaveys.