FAQ
Q: Why do I need matched power tubes in the first place?
A: Properly matched tubes will draw current from the power source in like quantities. Power tubes that are not matched properly will tend to distort more, and sooner. Power tubes are matched for two main variables: Transconductance and Plate Current, or Current Draw. Let’s look at plate current first.
Plate current (Ip) is the measurement of how much current the tube is drawing at idle. Normally measured in milliamps (mA’s), this reading is the tube’s natural bias point. Each tube type has a plate current range that the tube must be operated within.
Now onto Transconductance, or Gm. Transconductance is a measurement of the overall strength of the tube. This is what most good tube testers will check for in addition to other factors. Each tube manufacturer sets a transconductance value for each individual tube type. A tube tester will produce a numeric value, that, when compared to the manufacturer’s number, will give us some indication of how “good” the tube is.
A properly matched set of tubes means each one will be doing a similar amount of work in your amp’s circuit. When power tubes are sharing the load your amp sounds cleaner and tube life is maximized.
Q: Explain your tube matching process.
A: We strongly believe that the best tube sets are factory matched. What follows is a brief on the procedures followed by New Sensor. Detailed Groove Tubes information can be found under the entry, “All About Groove Tubes”.
The crucial plate current (Ip) value is measured at the plate of the tube. Most matchers that are in the field only measure the cathode current. The cathode current reading will include both the plate current and screen current. Variations in screen current will affect the cathode current reading. Screen current is checked separately and if the screen current fall within a certain acceptable range, the matching process continues to establish the Ip and Gm numbers. New Sensor’s Tube Matchers even have Sola Harmonic Regulating Transformers to stabilize the filament voltage; their price tag is in the $30,000 range.
This is their “Platinum Matching” process”. “Platinum Matching” indicates that the tubes are burned in to stabilize cathode activity and that the tolerances in the matching process are very tight. The Ip (plate current) is within +/- 1ma and the Gm (transconductance) is within +/- 100 micromhos (equivalent to 1/10th of 1ma at a grid voltage change of 1 volt). The Tube Matcher calibration is tracable to the National Bureau of Standards.
We further check power tubes out on either a Maxi Matcher or a Maxi Burn Station, running them at representative current values for 24 hours. Most tube failures occur early on; our method allows us to catch tubes that would normally fail early.
Small signal tubes are checked and tested on the Hickok 6000A using standard data supplied by Hickok. The unit has been converted to solid state and can also read plate current. Meter modifications have increased the accuracy of the unit to that of the Hickok 539 series. After a warm-up period (as much as 15 minutes for new production tubes), we check triode to triode balance for matched sections (as requested), and can also record plate current measurements.
Q: I have a tube tester, Maxi Matcher, etc. that is telling me my tubes aren’t matched ?
Most matchers only apply a very small amount of current to only a portion of the cathode. Some matchers tie the screen to the plate through a resistor, essentially testing the tube as a triode. This causes variations in the reading. The ONLY way you can duplicate readings is to EXACTLY duplicate test conditions. The tube testers generally available to the public function as good checks against shorts, leaks, and may provide some general information as to the condition of the tube. They are not the final say, however. The Maxi Matcher is a great product - we have and regularly use one. With it’s unregulated power supplies, it is no where near as accurate as the equipment available to New Sensor or Groove Tubes. You’ve heard it before: the only real check of a tube is how it performs in your amp. We understand that no one wants to use their amp as a “tester”, per se; this is why we check all tubes ourselves using our own equipment.
Q: What if one tube in a pair or quad goes bad during the warranty period ?
A: We will replace your single tube with a comparably matched one if available. If no suitable single tube replacement is in stock, we will provide a new pair or quad for you.
Q: What causes a tube to be microphonic ?
A: Vibrations from your speakers impart a force upon everything around them, including your vacuum tubes. The internal electrodes are affected by these vibrations. This causes the space between the electrodes, and hence the electrical field generated, to vary. This variation is fed back through the tube, and an annoying howl results. These vibrations can affect the electrodes through the socket or the base, or by waves of sound striking the glass envelope of the tube.
