Three stage screening
A well-made vacuum tube needs to go through a long period of burning and testing before sale. After a sufficient time burning, all tubes will be tested. Currently Shuguang Factory only measure Ia values and use it as the quality testing standards, products with Ia values flow below or above the standard will be considered as non-qualified products and be destroyed altogether in the factory.
Usually in order to meet the additional demand for our products, we will plant a premium to place orders to our factory. The additional requirements mainly include 2 aspects: the appearance and Ia values range screening.
1, Glass bulb is clean, transparent and crack-free
2, A vertical stem, no move and abnormal noise during shaking
3, Glass bulb and tube base are combined well, no loose
4, Tube pin is smooth, no loose
5, Diameter and height of paired tube are basically the same
Ia values range screening:
Since the vacuum tube can’t be repaired when production is finished, in order to improve the pass rate, generally factories would set loose Ia values standard. For example, usually the allowed Ia values range of an ordinary KT88-98 is between 25-65mA under testing conditions as Ua = 510V and Ug1 = -63, but we set more strict standard to control the Ia values range between 35-60mA, in order to obtain vacuum tubes with stronger emission capability and longer service life.
Company Internal Screening
For the other important indicators of audio vacuum tubes, such as hot filament – cathode leakage current, internal gas current, transconductance and microphone effect etc., the factory will make the product to meet these targets specification as much as possible during the product production process, by technical control and some other methods. This means that these indicators are the control indicators during factory production process, but not quality-test indicators, so products with these indicators not up to standard may flow into the market, this is not ruled out. Take KT88-98 for example, when we used our testing system to test products in the market which past the factory screening, we found that at least 5% -10% of them are unqualified for gas current or hot filament – cathode leaks etc. reasons. That’s why a lot of domestic and international customers blamed the quality stability of products produced by Shuguang Factory.
We introduced the world’s most advanced vacuum tube test system. It can carry out an all-round test simultaneously to Ia values, hot filament – cathode leakage current, internal gas current, transconductance, microphone effect and other indicators, it can also output voltage-screen flow graph. We are the only company with the ability to test these indicators in China currently.
According to test and paring condition of the vacuum tubes, our products are divided into PQ Level (Premium Grade) and FQ grade (Factory Grade).
- PQ Level: Indicators as internal gas current, the gate current and the hot filament – cathode leakage current etc. past the test; the screen flow la<5% and the transconductance GM <5% between the set of pairing products. This grade products need to be booked in advance.
- FQ grade: This grade products past our company’s second stage screening, repairing results are subject to the factory’s paired test results.