Tube testing methods and equipment
With so much variety in the type and condition of vintage vacuum tubes, our goal is to offer as much detail and transparency as possible to help you choose the right tube for your application.
All tubes posted on our website are tested for the following:
Our standard test results are measured using laboratory grade equipment in fixed bias mode* at typical operating voltages as specified in tube manufacturer data books. We can also provide custom test results based on the actual operating voltages found in audio equipment (up to 1000V with regulated power supply; 2000V unregulated).
- Section balance (where applicable)
- Voltage drop (rectifiers)
- Current draw (mA)
- Transconductance (Gm)
Each tube is shipped with test results for current draw (mA) and transconductance (Gm). Rectifiers are an exception to this, because they are tested for voltage drop over each diode so mA and Gm are not applicable.
* When evaluating test results from an AT1000, is it critical to know what bias mode was used to perform the test. For more information on this topic, see Interpreting Amplitrex AT1000 tube tester results.
Test Result Classifications for Vintage Tubes
In the descriptions on our website, tubes are classified by their test results. These classifications are reflected in the price range. We use the following classifications for vintage tubes:
- Tests as New: These tubes test at or above 85% of the industry standard value for a new tube. For dual triode tubes, we will also note if the sections are balanced.
- Tests Good: These tubes test between the minimum good and 84% of the industry standard value for a new tube.
- Bargain: Occasionally, we offer tubes that work just fine, but don’t pass the minimum standard for “good” tubes. These would be in the “questionable” zone in many vintage tube testers. This is only done when the tube is highly desirable and we don’t have the heart to throw it away. We sell these at deep discounts.
We also offer photos of most of the tubes on our website. Click “more info” to see the product description and find out if the photo is of the actual tube or of a sample tube from the lot. The photo will often be of one of the better looking tubes in the lot so it’s clear what tube type you are getting. For this reason, if cosmetics are important to you, please let us know so we can select tubes with that in mind.
A Note about NOS
You may have noticed that “NOS” is not one of the tube classifications we use. We have chosen to avoid describing tubes as New Old Stock, since it is rarely possible to verify that a vintage tube has never been used. However, it is possible to verify the tube’s strength and performance. Many tubes that bare signs of use on the pins, base, or flashing can still test as strong as a new tube would test straight from the factory. Conversely, many tubes that we pull out of their original boxes sealed from the factory may still test poorly. On our website, we provide images to give you a good idea of the condition of the tube’s label, flashing, base, pins, etc. You can decide for yourself how important the tube’s appearance is. We also provide information on the tube’s test results. Generally—the more valuable the tube—the more detail we offer.
Matched Tube Sets
Tubes are matched according to test results for both current draw (mA) and transconductance (Gm) at current and voltage levels that reproduce typical operating conditions found in audio equipment. This helps ensure that tubes that are matched according to our tester will actually perform as a matched set in your equipment.
Custom Power Tube Matching
The operating conditions we use for testing and matching power tubes on the AT1000 tester are listed in the product description. In addition, we run each power tube set on our custom test equipment that enables us to check the test results and matching in a circuit that duplicates your equipment’s operating conditions to the best of our knowledge. If it is possible for you to get these specs (either by taking measurements or getting the info from the manufacturer or other online resources), that will go a long way in assuring we can send you tubes optimized for your equipment.
The minimum info we need is:
We’d also like to have:
- Plate voltage
- Screen voltage (if applicable)
If this is not possible for you, rest assured that our standard matching is probably going to work just fine for you.
- Expected plate current draw (given in mA)
- Expected (or fixed) grid voltage (given in -V)
Most of our testing is done on our laboratory grade Amplitrex AT1000 units. We use adapters to test tube types that are not supported in the stock configuration (437, 417, 7N7, 6EU7, 80, 83, etc.). The AT1000 features include the following:
- Tests tubes at real-world power levels, up to 500V @ 160mA
- Accuracy and Stability: +/-1% for heater, plate, screen, and grid supplies +/-1% for GM and emission (plate current) measurements
- Microphonics Test: Any amplifying tube, including power tubes, can be checked for microphonics
- Shorts Test: Tube is continually evaluated for shorts during the test sequence
- Gas Test: Detects effects of ionic gas current on grid circuit impedance
For tubes that do not conform to this tester, we have a number of highly regarded calibrated vintage tube testers (such as the Hickok 539C and 752A, Avo CT160, Triplett 3444, etc.) that can cover just about everything.
High Voltage Test Fixtures
We have custom test equipment that allows us to test tubes at voltages that go beyond the 500v limit of the AT1000. This is especially useful for large triodes like the 211 and 845. It is also useful for matching 6550 types at specs for high voltage applications like Ampeg SVT and Music Man amps.
TC Tubes specializes in precision testing and matching of vintage and new production vacuum tubes for guitar & bass players, audiophiles, and DIY tube audio enthusiasts. Our website features secure payment options and fast, convenient shipping around the world.