Disclaimer: The following comments are specifically related to Chinese tubes based on Grant Fidelity’s hands-on experience through factory tours and reselling Chinese vacuum tubes.
1. How are tubes are made?
Vacuum tube making is a highly labor intensive manual process. Like any handcrafts, it’s next to impossible to have 2 vacuum tubes 100% identical. To see the tube making process, visit Shuguang with Steven Rochlin, editor of EnjoytheMusic.com on his Dec 2009 factory tour: http://www.enjoythemusic.com/china_2009/shuguang/
2. What type equipment is used in making vacuum tubes?
Instead of diving into technicality and confuse everyone, I have a short answer here – vacuum tubes are typically made with the same equipment that tube manufacturers used in the 1950′s. This is a boutique industry that has maintained the old time charm. Shuguang has modernized some manufacturing process such as enhanced vacuuming process by much larger scale self-designed new equipment. However, the overall manufacturing process of a vacuum tube doesn’t changed much from decades ago. In 2008, when Shuguang launched their new Treasure series which has 60 extra steps in parts selection and production for making the absolute high end audio tubes.
3. How is quality control done in vacuum tube production?
Please note this question is about QC during production. All parts going into a vacuum tube are pre-selected through testing and visual inspection. The glass enclosure is also screened for proper shape and strength but they are not always 100% identical. This is not considered defects. After each installation step, the work-in-process vacuum tubes are further examined at various quality control stations in the factory.
4. How is quality control done in vacuum tube distribution and retail?
Step 1: QC at factory:
A well-made vacuum tube needs to be burned in and further tested before it is ready for sale from the factory. Shuguang conduct burn-in on all tubes when they are off the production line. After burn-in, the tubes are tested and sometimes re burn-in is required at the factory and then matched according to “plate current” (aka ‘Bias Point’) with industrial equipment which will be calibrated every couple of months, as advised by the production engineer at Shuguang. Factory tube testing results usually have some inherent statistical error due to the time from the most recent equipment calibration. Such an error is normal in most industrial testing procedure as in large scale production. It is impossible to calibrate the testing equipment for every tube.
Test results obtained right after equipment calibration are tend to be more accurate then those obtained close to the next calibration. Chinese tube manufacturers typically only measure plate current at the factory QC process. At Shuguang, microphonics, transconductance, heater-to-cathode leakage and internal gas are not numerically tested in the production or QC process. These parameters are “controlled” by the manufacturing process but are not “measured” with a test equipment. Logically there will be some tubes with inferior or even failing results on these untested parameters will end up into distribution as if they are quality tubes. These are the ‘lemons’ covered by Shuguang manufacturer warranty which typically lasts 6 months from the factory shipping date. Shuguang historically select the tubes with better plate current test results (strong emission) for its export market. These tubes are sold as “Premium Grade or Export Grade” with higher price and tend to work closer to the tube’s designed specification and has a longer life expectancy. Weaker tubes are sold mostly within Greater China (including Hong Kong and Taiwan) with lower price for domestic consumers to match local affordability.
Shuguang DOES NOT make any special mark on their Premium tubes so in most cases it’s not possible for consumers to tell by naked eyes which ones are Premium Grade and which ones are Standard Grade, once the tubes are out of the original packaging. This is something Shuguang has to work on to ensure that Standard Grade tubes are not sold as Premium Grade by unethical resellers.
Step 2: QC at Distribution
Once vacuum tubes are sold to a distributor or reseller, the tubes have to be shipped from factory warehouse to the reseller’s location. Tubes are extremely fragile – excessive shock, drop or mis-handling during this shipping process could result in internal damages to the tubes which are invisible to human eyes but can be detected by sophisticated vacuum tube testing equipment. This detecting process is called ‘QC at distribution’. At old times, tube vendors will have test equipment for re-test the tube before handing it over to a buyer in a store.
This 2nd step QC is now only carried out by some resellers but not all – either due to unavailability of test equipment or due to the internet sale sight-unseen nature . For example, individual tube re-sellers on eBay are very unlikely to have a sophisticated tube testing equipment to confirm the tubes are still in factory condition after the shipping process, so the so-called ‘factory tested and matched’ is not always relevant to consumers at this point of time since you have no idea if any internal damages have happened up to this point of time and how accurate it has been tested. There is certain risk in buying tubes as ‘factory tested’ only so typically the price is lower than a distributor re-tested tube.
Some commercial tube resellers do apply some QC once they receive factory shipment. However, some commercial sellers only have older tube testing equipment that is made in the 1940s-1950s which can only tell a tube is functional or not, but not for exact matching, leakage or excessive internal gas testing.
We highly recommend consumers to purchase high end vacuum tubes from reputable professional resellers to ensure quality and after-sale service.
A bad tube could cause short or burnt components inside an amplifier. Such repair cost could be much higher than the price difference between high quality grade tubes vs. the unknown ones. Typical tube amp repair labor rate is $65.00/hr in today’s market, let alone the shipping cost if you don’t have a qualified technician nearby. Please take these ‘consequential costs’ into consideration when you decide where to buy your tubes.