Preamptube testing and SQ classes

All preamp tubes are tested by the factory. However the factory testing isn´t that strict in terms of quality or parameter tolerances. Because of the wide tolerances, the preamp tubes are usually tested by suppliers and others in the delivery chain. TAD tubes are tested in Germany by tight tolerances before delivering to us. Our SQ tubes are tested in-house prior to delivery. Most of the NOS tubes are tested by us or by our suppliers. We recommend using SQ tested tubes where possible. These are picked from the lot with tighter tolerances. We also offer a better warranty for SQ measured tubes.

SQ classes:
1. Standard SQ - Burned in and tested to meet the datasheet specs. Suitable for 90% of the amplifiers.
2. High gain SQ - Burned in and tested. Selected by normally higher gain. Not recommended to use in high gain amplifiers.
3. Low gain SQ - Burned in and tested. Selected by lower gain than normally.
4. Balanced SQ - Burned in and tested. Selected by matched triode halves. Suitable for phase inverters, balanced studio gear, and HiFi applications.
5. Balanced and matched SQ - Burned in and tested. Selected by matched triode pairs and paired with a similar tubes with the same characteristics. Price per tube, buy by pairs. For most demanding applications.


Usually, balanced version works best in phase splitters and in hifi amps.

Additional info about testing

All preamp tubes are burned in for a proper time. The testing is a computerized process. The tube is tested by the audio signal and the output from the anode is measured by level, balance, distortion, and signal to noise ratio. Output is monitored by oscilloscope view which reveals possible nonlinearities of the tube. We've developed different kinds of measuring methods during the years and the system used now is most reliable compared to the time being consumed per tube.