Tubes Matching and Selection
What is Matching?
Matching refers to the grouping of power tubes with similar or identical electrical characteristics into sets. These sets can be pairs, quartets, sextets, and more. Matching is not used for preamp tubes or rectifier tubes.
Why Match?
Every electrical component has certain manufacturing tolerances, and tubes are no exception. When multiple power tubes are used together in an amplifier (often seen in push-pull configurations), and they cannot be individually adjusted (the amplifier has a single bias voltage for all power tubes), it's essential to use tubes with identical electrical characteristics to ensure the amplifier operates stably, safely, and can deliver its maximum power.
How Does Power Tube Matching Work?
To "match" power tubes, i.e., group them, their electrical characteristics must first be determined. This is done in a measuring circuit, usually in the form of a specialized tube measuring device. Relevant values such as current or the slope of each tube are measured, which are then used as the basis for matching. In a second step, tubes with the same electrical characteristics are grouped into pairs, quartets, or other sets.
Do I Need Matched Power Tubes?
This question should primarily be answered by the amplifier manufacturer. However, we generally recommend using matched power tubes. Even in amplifiers where each tube can be individually adjusted or those with cathode bias or auto-bias. This ensures you're on the safe side.
How Precisely Should Power Tubes Be Matched, and How Much Variability Is Acceptable?
Since a tube is a nonlinear component, it's practically impossible to find two absolutely identical tubes. Therefore, a certain tolerance should always be expected in matching. However, this tolerance should be small enough that no problems such as increased hum, power loss, or premature aging occur during tube operation. In push-pull amplifiers, some asymmetry is usually not a problem; in fact, it can even positively affect the sound. However, if the currents of the used tubes are too different, a noticeable "hum" can occur. Our experiments with 6L6 and EL34 tubes have shown that audible humming starts at approximately a 10 mA difference.
How Is Matching Done at Tube-Town?
The matching of power tubes and the determination of matching values are done in our facility using a computer-controlled measuring station. The tubes are tested and measured after a burn-in period with a high anode voltage (approximately 500V for EL34) to determine their electrical characteristics under realistic operating conditions.
The following measurements are taken:
- Anode current
- Slope
- Screen grid current
In addition, the tube is tested for:
- Grid leakage current
- Gas
- High screen grid currents
This allows us to detect and sort out tubes during matching that could cause disturbances during operation.
Do Preamp Tubes Need to Be Matched?
No.
Matching primarily applies to power tubes, which are used for amplifying power. Preamp tubes are usually operated in series and are "self-biased" or "auto-biased," meaning their operating point is automatically set by the circuit. Furthermore, preamp tubes primarily serve voltage amplification (signal amplification) rather than power amplification, unlike power tubes. Therefore, selecting them based on current doesn't make much sense. Instead, internal resistance should be considered as a criterion.
Should Preamp Tubes Be Selected?
Depending on the amplifier's circuit and purpose, it's advisable to use "selected" preamp tubes that have been tested and chosen for specific characteristics. The main task of the preamp is to amplify a low-level signal, possibly just a few millivolts, to levels of several tens of volts to drive the power tubes effectively.
Since high gain factors - 100, 1000, or even more - are involved in this process, unpleasant noise can occur if the tube used has significant background noise or reacts sensitively to shocks. This noise is amplified by the same factor as the actual useful signal and can therefore have a negative impact. (Note: This is NOT the gain factor indicated in the datasheet, such as 100 for 12AX7 tubes or 20 for 12AU7 tubes).
Especially in guitar and bass amplifiers, the input stage of the preamp is a crucial but also critical amplification stage, as it amplifies the weak and high-impedance signal from the pickups very strongly. Therefore, tubes that generate little noise and have been specifically selected for this stage are of great importance. The tube in this amplifier stage is often referred to as V1.
For this reason, we label tubes that have been specifically selected for this position with the addition V1 or V1-Selection.
Our Criteria for Preamp Tube Selection
V1-Selection
- Low background noise
- No or low susceptibility to microphonic effects
Balanced Selection
- Low background noise
- Both systems have the same gain (+/- 5%)
All tests are performed at an anode voltage of approximately 250V and the grid bias voltages specified in the respective tube datasheet.