Solid Tube Audio
Everest C3EQ Etna E1TechnikSQ Tubes
SQ Tubes

The exclusive use of "SQ" tubes inside the Solid Tube Audio products

Detailed explanations of these special quality tubes you will find below:

  • Filament switching-stability
  • High durability
  • Interlayer-free Special Cathode
  • Extremely  shock- and vibration-proof
  • Narrow tolerances
  • High reliability: All the tubes we use surpass these very special requirements and beside that, they all have extremely low noise and distortion figures.




Switching the filament frequently on and off generates a heavy dilation due to thermal cycling, which represents a substantial strain to the tube. SQ-Tubes are highly resistant against these strains. These tubes comply with the very strict military standards.

High durability 

The guaranteed life span of a SQ-Tube is 10.000 hours in general, averaged on 100 tubes (tube types and conditions we use: 40.000 hours). The manufacturer guarantee for each single tube is 12 month, provided that the filament voltage remains within the limits of +/- 5% (The filament voltage in the Vintage "C3" remains within the limits of +/- 0.01% from nominal).

A stronger "underheating" makes the tube susceptible to toxications, while "overheating" causes an evaporation of the active layer. This might cause gridemission and produces intermediate-layers on the surface of the Cathode SQ-Tubes are equipped with special "High Durability Cathodes". Of course, not only the Cathode of a tube, but all other electrodes must be protected against inadmissible temperatures (transgression of the Anode load, insufficient heat discharge), to avoid grid emission, Gas eruption etc. In this respect, the maximum admissible Body temperature, as specified for special tubes by the manufacturer, is to be taken into account.

Interlayer-free Special Cathode 

When tubes are operated, in heated condition, without Cathode current (Pulse-Mode), an Interlayerresistor arises within the Cathode, due to depletion of active layer. 

This Interlayer-Resistor, acts like an unshorted Cathode-Resistor and generates, particularly at low frequencies, an undesirable negative feedback, which is interfering especially in tubes with a high mutual conductance.

Interlayer-free SQ-Tubes are equipped with a special Cathode and they are checked for Interlayers during the production process, whereas mutual conductance measurements at 10Khz and 20MHz provide information about the Interlayerresistance (at high frequencies, the negative feedback is suspended by the Layer capacity)

Extremely shock- and vibration-proof 

Tubes, which are used in industrial or mobile systems, must not be affected in their operation by vibrations. A special system construction of shock- and vibration proof tubes meets this requirement to ensure maximum protection against sound feedback and mechanical damage of the system.

The vibration-proof is being tested on a vibrating table, whereon the tubes are exposed to 2.5G (acceleration due to gravity), at 25 – 50 Hz in three different directions, for a period of 96 hours. To check the shock-proof, the tubes are exposed to 20 shocks of 500G (acceleration due to gravity) in different directions.

Narrow tolerances

When constructing electronic equipment it is very essential that the engineers can rely on very small exemplar deviations and a very high permanence of the electrical figures throughout the whole life span to avoid expansive and long winded adjustments when changing a tube. In the production of SQ-Tubes, this is achieved by high accuracy in dimensions of the single systems, as well as during assembling, and by strict controls and measurements at random samples.

With respect to this, the Span-Grid-Technology plays a very important role because of the very high stability of the control-grid, which is vitally important for the electrical figures, especially of high mutual conductance tubes. SQ-Tubes with narrow tolerances have to pass several special tests and measurements. They are being stored for a long time and then tested again before they will be delivered.

High reliability 

This demand means largest possible security against unexpected and/or premature tubes loss during the service life and is placed especially in units, that contain a large number of tubes or with which a tubes loss has serious down times as a consequence. (The Solid Tube Audio C3 console with the extension step 56 K contains in total 1539 SQ-Tubes.)

The reliability is defined by the factor p, that the procentual probability indicates how large the tubes loss can become during a service-time of 1000 hours maximal. The common p- factor of SQ tubes is 1.5%. This factor applies to a service with nominal data and for the wear free area of the service life (10.000 hour). High reliability is reached in the tubes fabrication by special pre-treatment of all system component (elimination of all injurious contaminations by washing and ultrasound cleaning, anneal in vacuum, installation under overpressure to achieve dust-free conditions, etc.) Also the defendability of the weld, that is subjected to an especially severe inspection, is highly important. After the fabrication, the tubes are aged to stabilize the operating point and subjected a short test-run in order to avoid early losses.

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