We offer some output tubes as UralTone SQ classified. It's possible to choose from Low (current) 1)), Low medium (2), Medium (3), High Medium (4) ja High (5) -classes. As extra we offer HiFi classified tubes with tight tolerances as well as tubes for certain amplifiers (for example Classic 30). These are drop-in selected tubes by right bias value on stock / unmodified amplifier. All SQ tubes are tested and matched by a custom-made UralTone tube tester and burned in for a needed time before measuring.
It's usually best to choose medium rating tubes but if have a cathode biased amp (vox, tweed fender) that runs tubes hot you may want to choose a low or low medium rating.