Before shipping, all tubes are tested on one of our calibrated testers :
FOR NOS tubes and NB tubes, failure or operation below manufacturer’s specification of any one of the functions renders the tube unsuitable for sale by dBtubes. For UT tubes, mutual conductance and emission values of 80% or more of original manufacturer’s specification is considered acceptable.
It sometimes happens, especially for audio tubes, the tolerance of some circuits is unforgiving, and even a slight deviations from manufacturer’s specification of other components in the circuit apart from the vacuum tubes, leads to loss of performance. We are aware of this; so please let us know of particularly tight constraints, and we will do our best to accommodate you.