[1] Tubes tester: the first selection
All of the tubes used in our products are meticulously selected with a custom made emission tubes tester, that is able to manage with digital precision the measurements of the mutual conductance (Gm) and amplification factor (Mu), rejecting the tubes not matching the typical characteristics declared in the manufacturer's datasheet.
Proceding to the internal triodes matching for every twin channel tube type (12AX7, 6CG7, 6BC8 etc.)

[2] Characteristic curves: tube's working point
With a particular curve tracer, it is possible to visualize the characteristic curves of the devices under test, and to further refine the selection process, permitting to obtain pairs (or quartets, or sextets) virtually identical, perfect for use in differential circuit topologies (balanced signals).
Tubes used in current designs are also individually tested for internal noise, microphonics, grid current and are measured for gain. Again, inferior tubes are disposed of.
The curves permit to establish with extreme precision the ideal working point for each device (triode, pentode, etc.), to further assure reliable, sonically acceptable performance.

[3] Matched pairs
Only after a tube successfully passes these exacting challenges it is labeled and put into stock to be used in a new product or sold to the aftermarket. Then graphic plotted results reports are made, (just a few resellers actually do that), allowing to recall the same matching criteria over the years.
Our tested, selected tubes will often cost slightly more than generic types offered by aftermarket vacuum-tube sellers, who may not have the technical or financial ability to conduct exacting quality-control measures. These vendors also are not likely to eliminate and dispose of inferior, poor-quality tubes. For best performance and reliability in our products, QES Labs quality-tested tubes are strongly recommended.

[3] Download the product brochures
in PDF format.