FAQ

Tube testing is the measurement of vacuum tubes for various electronic characteristics such as Transconductance, Gain, Noise, Emissions, etc. Tubes can be graded based on this information to enhance the operation or quality of a vacuum tube audio system. The selection of tubes into sets of similar measurement is called tube matching.

A tube tester provides mechanical and electronic interface for vacuum tubes and an operating environment. Power supplies are provided including filament power for the heater, plate voltage, bias and screen voltage. These voltages are maintained across a tube type as a “test regime” so that the performance of a given tube can be compared against all others of its type. All tube testers create unique test settings for a given tube type, and therefore test results between testers are rarely the same, though they are often proportional and can be correlated.

Plate Current is the amperage that flows across a tube from the cathode to the plate. In the case of an audio tube, the control grid and screen grid mediate the amount of current that flows from cathode to plate. Here is an excellent link to further in-depth discussion of how tubes work. http://www.john-a-harper.com/tubes201/

Bias Voltage is used in signal and power tubes to create a difference in potential between the cathode and control grid in order to control the flow of current across a tube. Here is an excellent link to further in-depth discussion of how tubes work. http://www.john-a-harper.com/tubes201/

Gain is the multiplication factor of a tube; it is the ratio of the output to the input. It can be voltage gain, current gain, or power gain. Usually tubes are characterized as having voltage gain, stated as a dimensionless number. A 12AX7 for instance typically has a gain of 92. But they can be as high as 110 (EH 12AX7-WXT) or as low as 85 (early Mullard). This number does not change very much over the life of a given sample tube and is somewhat harder to use a differentiation value.

Transconductance is a derived result calculated from several measurements. It is a measurement of how much AC current will pass through a tube at a given plate voltage, bias voltage, and signal input. Mathematically, it’s the result of the AC output current divided by the AC input voltage. Different results will occur given different driving environments for a tube. You can see this on a Maxi-Matcher by changing plate voltage or bias voltage and watching the transconductance numbers go up or down as a result.

When driving a tube, reducing the grid voltage generates more current flow, which makes transconductance results go up. Increasing the plate voltage also makes results go up. What makes it meaningful is to set up a similar test environment for a given tube type and then compare all the tubes of that type in that set up, to each other. This is what all tube testers do, from vintage Hickoks to modern Maxi-Matchers. Each different tester has its own set of values it applies to a tube, and generates its own set of measurements.

The specifications for transconductance that are shown on manufacturer’s product sheets are always maximum values; the maximum AC current a tube can pass. In fact, the specifications published by the manufacturers were used to sell tubes, not as usage values. It’s similar to maximum engine RPM or redline for a car – you don’t actually use the engine that way, but it can go that fast. Tubes are seldom used at these max values, because they would burn out in a short time.

Many vintage testers have specifications for what a given tube type should measure to be considered ‘good’ or ‘bad’. These specifications were developed when tube manufacture was closely controlled, even across manufacturers, and therefore a given tube type could be expected to show similar if slightly varying measurements. RCA, Sylvania, Phillips, Tungsol, etc all used the same spec for building a given tube.

That is not the case today, as we have widely varying measurements within a given type. This is due to lack of specifications about what a certain tube type should be. The Maxi-Matcher does not offer pass/fail standards for tubes, but functions more appropriately as a comparison test bed for tubes in your stock or available from your manufacturers.

The SmartFuse is a unique, proprietary, high-speed, automatically resettable, current cut-off safety circuit. It acts to protect the Maxi-Matcher from shorted tubes, or mis-pinned insertion, or failures while testing, or over-current episodes. The SmartFuse acts within 10 milliseconds on any excursion of current beyond 120 milli-amps in the Plate circuit of the tube socket in use. It can be activated by a shorted tube. It can be activated by selecting too little grid bias, thus increasing plate current beyond the trigger point. It can be also be activated by switching values of grid or plate voltage while at higher overall current flows, due to switching transients that surpass the 120 mA hard limit.

This means that the current that is passing through the tube at the particular settings in use is greater than usually expected. This tells you that you have a strong tube, relative to others of the given type.

There are many makes of, for instance, the 6L6 power pentode, from the originals back in the late 1930‘s to Czech, Chinese and Russian types today. There’s no real standard for what is a “6L6”. Some are stronger and some are weaker, even brand-new. A worn-out Wing-C is going to be stronger than an original RCA. This is why we don’t offer pass/fail standards for tubes, because it’s hard to say what is good and what isn’t good.

This means that the current that is passing through the tube at the particular settings in use is less than usually expected. This tells you that you have a weaker tube, relative to others of the given type.

That may be because the tube is gassy; its vacuum is soft and gas ions floating around inside the sealed bottle may slow down or impede the flow of electrons from cathode to plate. It may be because the cathode coating has gotten weak or thinned out and is transmitting fewer electrons. It may be because a mechanical structure is mis-aligned or broken internally. Or it could be that inter-electrode capacitance has changed and the tube isn’t passing signal with an neutral frequency response.

The Maxi-Preamp measures Gain, Transconductance, and Noise (dB) in signal tubes. It has LED indicators on the front panel showing whether a tube under test is operating or not, and then also displays on the LCD screen an exact measurement of the currently selected test function. If the LED is lit red instead of green, this indicates a failure or shorted tube.

The Maxi-Preamp does not have an emission test – this is often a dangerous test to make on a tube, depending on how it is done, and we don’t feel it has that much value.

The best test you can make of a tube is for Transconductance, which is a measure of how well the tube amplifies signal, how strong it is and/or gassy it might be. The higher the number, the stronger a tube is generally.

You may measure Gain (multiplication factor) correctly, and it may have low noise, but the tube is not very strong, which is what Transconductance is. This is why we suggest measuring for this factor, as it is a dynamic measurement which mimics the way a tube is actually used in amplifier.

For the bench technician, the Matcher can be used as a highly efficient, very accurate and easy-to-use test station to quickly determine the status of a tube or set of tubes. Tubes to be tested are installed in the test panel and results are read off, indicating operation status. In this case, specific settings for a tube matter less than they would in a tube-matching program, and you can use any combination of plate voltage and bias voltage that generates a mid-range current flow.

For the tube reseller, repeatability is a key criteria, and therefore settings must be held the same across a tube type in order for measurements to be relevant to each other.

For the home audio enthusiast, ease of use and comprehensibility are key elements, and therefore the Matcher has been designed to provide results that are easy to understand and easy to apply.

In general, the Matcher is best thought of as a comparison test bed for the tubes you have in hand. Considered this way, you can use your own tube stock to develop a familiarity and set of pass/fail standards that reflect the manufacture and availability of your stock. Rather then make up standards using older specifications from manufacturers that no longer exist, we allow a greater flexibility to determine what is ‘good’ or ‘bad’ in your tubes.

As tubes are installed and removed from the test sockets many times, metal shavings from the pins can build up over time under the tube sockets, on top of the PCB they’re mounted on. Odd and unpredictable behaviors can result from this. Pick the Matcher up and turn it upside down once in awhile – shaking it can dislodge these particles. Remove the bottom panel from the Classic Matchers and blow them out from time to time. On the Matcher II, vacuuming the unit with a keyboard nozzle from the top side also works to dislodge and remove the particles.

The Maxi-Matcher does not have a regulated power supply, and therefore the various DC voltages internal to the tester vary slightly depending on the power being measured. Higher current flows cause a small drop in plate and bias voltage, for instance. For those who need the highest repeatability standards, we recommend always testing tubes in the same format – i.e. always test four tubes at a time, or one at a time, or whichever setup makes the most sense for your application. This reduces variation in the power supply levels to a minimum, guaranteeing the best test results.

The Maxi-Matcher does not have a regulated power supply, and therefore is somewhat sensitive to input AC mains voltage. In some locations, AC line voltage varies during periods of high usage, sometimes as much as 5 or 10 volts. This will cause variation in Maxi-Matcher measurements. We recommend the purchase of an AC line regulator (not a line conditioner; these do not help maintain line voltage) to solve this issue. We offer a line-voltage regulating transformer as an option for those users who require the highest level of precision from their Maxi-Matcher.

We do not use a distribution network for our products. Tube testers are a niche product that is easily searched for online today, and we serve our customers directly from our factory in Seattle Washington. This keeps the purchase cost of the product as low as possible.