Traditional ribbon microphones will be damaged if you position them too close to an extremely loud guitar cabinet, but the R-121 was designed to take high SPL's and can handle reasonably close miking duties on a loud cabinet. Ribbons have a strong proximity effect, so placing a ribbon mic any closer than 4 inches from the cabinet grillcloth will result in a bass-heavy sound. While that's usually too thick a sound for a guitar track, it can still be a useful effect when blending mics. Some engineers put an R-121 close on a cabinet for an exaggerated low end response, then blend an SM-57 for the aggressive top end. You can get a massive rock tone like this. R-121's also record bass guitar well, but if the bass is loud, best results will be achieved with the mic placed at least two feet from the cabinet. Miking a loud bass guitar cabinet closer than 6"is not advisable, as the low frequencies at high volume can exceed the R-121's handling capability of 135 dB SPL and possibly damage the ribbon. R-122's are often preferred on bass guitar cabs. Their larger transformers bring more focus and tightness to low frequency sound sources.