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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.
Test Results
All tubes posted on our website are tested for the following:
- Shorts
- Gas
- Leakage
- Microphonics/noise
- Section balance (where applicable)
- Voltage drop (rectifiers)
- Current draw (mA)
- Transconductance (Gm)
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).
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 important 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 Equipment
Amplitrex AT1000
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
Vintage Testers
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.
Matched Tube Sets
Standard Matching
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. If this is not possible for you, rest assured that our standard matching is probably going to work just fine for you.
Test Result Classifications for Vintage Tubes
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 show 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.
Photos
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.
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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.
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