I installed a matched quad of these tubes in a restored/rebuilt Conn organ stereo tube amp. This Tung Sol quad replaced four vintage, unmatched and tired tubes that were in this amp. I have been using the new Tung Sol tubes for nearly a year and a half, and they have performed flawlessly since installed. I can't vouch for their use in guitar amps, but clearly they're one of the best new production 6L6 types for hi-fi use. Very happy with their cost and performance.