Preamptube testing and SQ classes
All preamp tubes are tested by factory. However the factory testing isnät that strict in terms of quality or parameter tolerances. Because the wide tolecances the preamp tubes are tested by suppliers and others in delivery chain. TAD tubes are tested in Germany by tight tolerances before the delivery to us. Our SQ tubes are tested in house prior delivery. Most of the NOS tubes are tested by us or by our suppliers. We recommned to use SQ tested tubes where possible. These are picked from the lot with tighter tolerances. We also offer better warranty for SQ measured tubes.
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 usen 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, balaced studio gear and HiFi applications.
5. Balanced and matched SQ - Burned in and tested. Selected by matched triode pairs and paired with similar tube with same characteristics. Price per tube, buy by pairs. For most demanding applications.
6. Reverb grade SQ (only for few 12AT7 / ECC81 models) - Quality is suitable for parallel connetcter Fender type tube driver where tube halves may be mismatched. May be used as phase inverter where the unbalanced circuit adds even order distortion components.
Usually balanced version works best in phase splitters and in hifi. However the guitar amp may be considered as part of the tone so unbalanced ciruits sounds better.
Additional info about testing
All preamp tubes are burned in for proper time. The testing is computer aided process. Tube is tested by 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 unlinearities of the tube. We've deleveloped different kinds of measuring methods during the years and the system used now is most reliable compared to time being consumed per tube.