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 1150V).
- Section balance (where applicable)
- Voltage drop (rectifiers)
- Current draw (mA)
- Transconductance (Gm)
Test results are available in engineering units for both transconductance (Gm) and plate current draw (mA). These results are typically reported in "percentage of specification." This method is simpler for most people (including us!) to report on the strength of the tube in relationship to 100% = new value rather than giving fractional engineering specs (58mA/54mA, etc.).
* When evaluating test results from an AT1000, is it critical to know what bias mode was used to perform the test and what scores to look at for evaluating a tube. When tubes are tested in auto bias mode, the AT1000 adjusts the bias voltage until the tube reaches spec mA readings. So tubes tested in auto bias mode will all have mA and Gm readings that are very close to 100%, but the bias voltage that was required to get the tubes to reach these scores will vary for each tube. When tubes are tested in fixed bias mode, the AT1000 uses the same bias voltage for each tube so the mA and Gm readings will vary for each tube. Since we use the fixed bias mode, our test results are reported in the actual mA and Gm numbers, not the bias number. For more information on this topic, see Interpreting Amplitrex AT1000 tube tester results.
Test Result Classifications for Vintage Tubes
We classify each tube by its test results (see classification descriptions below). The results will be reflected in the price range for the product. Click "more info" to select the classification you need for your application. In some cases, we will also list the actual test results. We use the following classifications for vintage tubes:
We are in the process of standardizing the way we list test results on our website. If you have questions about the description of a particular tube, let us know and we can offer more information.
- 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. (Incidentally, most of the tubes I use in my own audio equipment are bargain quality. These tubes tend to work fine and last a long time.)
- Display Only: These tubes do not work for one reason or another and are fit only for display purposes. Typically these are types that are in high demand—otherwise they wouldn’t be worth displaying.
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. In some cases, we will also post additional photos in the gallery.
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 test the tube’s strength (both in terms of current draw and amplification). Many tubes that bare signs of use on the pins, base, or flashing still test as strong as a new tube would test straight from the factory. And many tubes that we pull out of the correct boxes sealed from the factory test quite poorly. In addition to tubes degenerating due to use, tubes also degenerate as a result of minute losses of vacuum over time. No seal is perfect (referring here to the seal between the metal pins and the glass envelope), and inevitably tubes lose strength as the vacuum is gradually compromised. 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.
For new production power tubes, the operating conditions we use for testing and matching are listed in the product description. In some cases (EL34, EL84, KT77, etc.) we also offer higher sets of voltages for tubes that are often run more aggressively. This enables you to compare how closely our standard matching specs fit the operating conditions of your tube equipment. In most applications, these standard matches will work well.
Custom Power Tube Matching
Ideally, we’d love to be able to match our tubes to your application by duplicating your equipment’s operating conditions. 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. Just choose “unsure” whenever the option presents itself and we’ll choose a set for you using our standard matching specs. If there are issues, we’ll still be here to support you and get it sorted out.
- 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.
We also have custom test equipment that allows us to test tubes at up to 1150v so we can match power tubes at voltages that go beyond the 500v limit of the AT1000. This is especially useful for large triodes like the 211 and 845, but is also useful for matching 6550 types at specs for high voltage applications like the Ampeg SVT series (660v plate, 335v screen, etc.).
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.