Frequently Asked Questions
Below are some frequently asked questions we receive about tubes.
If you have any further questions or need help, please feel free to reach out to us directly.
What does Matched Pair mean?
Vacuum tubes are manufactured to perform in a broad electrical range based on design characteristics. Because of the variance during manufacturing there may be as much as a 25% difference in test results in any given production run. Amplifiers that have one bias adjustment for multiple tube applications are required to use matched power tubes to optimize tube performance as well as maximizing the life expectancy. On amplifiers that have individual bias controls it is not usually necessary to have matched vacuum tubes.
Although our experience over the last 30 years has indicated that sonic performance is enhanced when each vacuum tube exhibits the same electrical characteristics in any given circuit design. Our matched vacuum tubes go through rigorous test procedures to enhance performance during their operation.
Each vacuum tube is burned-in for a minimum of 48 hours dissipating full power with filament, plate and screen voltages that are normally used in amplifier applications. Thus insuring that the various electrical parameters have stabilized and the matching of each vacuum tube will not drift during the life time of each audio tube.
Our power tubes are matched within 5% variation on all parameters. We start the process by using audio tubes that are tested for bogey values and reject those that are on the outside limits of their design. Each vacuum tube is then tested on our fleet of wt-100a original rca production testers for plate current, screen current and transconductance (gm). We also check for grid current and leakage between elements.
When you receive vacuum tubes from Penta Labs that are matched you can rest assured that you are receiving a product that has been thoroughly tested and will perform at levels unsurpassed in the industry.
Are our vacuum tubes burned in and tested before we ship them out?
YES. Each vacuum tube is burned-in for a minimum of 48 hours dissipating full power with filament, plate and screen voltages that are normally used in amplifier applications. Thus insuring that the various electrical parameters have stabilized and the matching of each vacuum tube will not drift during the life time of each vacuum tube.
Our power tubes are matched within 5% variation on all parameters. We start the process by using vacuum tubes that are tested for bogey values and reject those that are on the outside limits of their design. Each vacuum tube is then tested on our fleet of wt-100a original rca production testers for plate current, screen current and transconductance (gm). We also check for grid current and leakage between elements.
When you receive vacuum tubes from Penta Labs you can rest assured that you are receiving a product that has been thoroughly tested and will perform at levels unsurpassed in the industry.
Why are vacuum tubes still used in Audio?
1. Guitar amps
In general, only very low-cost guitar amplifiers (and a few specialized professional models) are predominantly solid-state. We have estimated that at least 80% of the market for high-ticket guitar amps insists on all-vacuum tube or hybrid models. Especially popular with serious professional musicians are modern versions of classic Fender, Marshall and Vox models from the 1950s and 1960s. This business is thought to represent at least $150 million worldwide today, perhaps more (it’s hard to determine, as most of the makers of vacuum tubes and vacuum tube amplifiers today are private companies who are secretive about their sales.)
Why tube amplifiers? It’s the tone that musicians want. The amplifier and speaker become part of the musical instrument. The peculiar distortion and speaker-damping characteristics of a beam-tetrode or pentode amp, with an output transformer to match the speaker load, is unique and difficult to simulate with solid-state devices, unless very complex topologies or a digital signal processor are used. These methods apparently have not been successful; professional guitarists keep returning to tube amplifiers.
2. Professional audio
The recording studio is somewhat influenced by the prevalence of vacuum tube guitar amps in the hands of musicians. Also, classic condenser microphones, microphone preamplifiers, limiters, equalizers and other devices have become valuable collectibles, as various recording engineers discover the value of vacuum tube equipment in obtaining special sound effects. The result has been huge growth in the sales and advertising of vacuum tube- equipped audio processors for recording use. Although still a minor movement within the multi-
billion-dollar recording industry, tubed recording-studio equipment probably enjoys double-digit sales growth today.
3. High-end audio
At its low point in the early 1970s, the sales of vacuum tube hi-fi equipment were barely detectable against the bulk of the consumer-electronics boom. Yet even in spite of the closure of American and European tube factories thereafter, since 1985 the sales of “high-end” audio components have boomed. And right along with them have boomed the sales of vacuum-tube audio equipment for home use. The
use of vacuum tubes in this regime has been very controversial in engineering circles, yet the demand for tube hi-fi equipment continues to grow.
What is Single-Ended, Class A, B, AB, ultralinear, etc?
1. Class A means that the power tube conducts the same amount of current all the time, whether idling or producing full power. Class A is very inefficient with electricity but usually gives very low distortion. * There are single-ended class-A, or SE, amplifiers. They use one or more tubes in parallel, which are all in phase with each other. This is commonly used in smaller guitar amps and in exotic high-end amplifiers. Many audiophiles prefer the SE amplifier, even though it has relatively high levels of even-order distortion. Most 300B high-end amplifiers are SE. Negative feedback, which can be used to decrease the distortion of an amplifier, is felt by some people to sound inferior.
Most SE amps have no feedback.
Unfortunately, the high saturation currents in SE-amp output transformers tend to limit the bass-frequency response of such amps. Unless you use a subwoofer with a separate driver amplifier, the SE amp will tend to give inferior performance. We also feel that keeping distortion down is important (though not as important as engineers tend to think). SE amps made with low-mu triodes can have very low distortion, IF they are properly designed. Beware of some SE amps that are designed entirely “by ear”, especially those made by small companies run by audio-guru egomaniacs. The amps made by such firms might NOT do everything well, and might not be the sound you would prefer.
* Push-pull class-A amplifiers also exist–they use two, four or more vacuum tubes (always in pairs) which are driven in opposite phase to each other. This cancels out the even-order distortion and gives very clean sound. Push-pull Class A operation usually involves low plate voltages and high plate currents, compared to Class AB operation below. The high currents might tend to wear out the tube cathodes faster than in an AB amplifier.
* There are two kinds of class-A operation, which can apply to single-ended or push-pull. –Class A1 means that the grid voltage is always more negative than the cathode voltage. This gives the greatest possible linearity and is used with triodes such as the 300B, and sometimes with audio beam tetrodes and pentodes. (Such amps are rare in the modern trend-driven and ego-ridden world of high end
audio today–most gurus lean either toward Class AB beam-tube amplifiers, or toward SE triodes.)
–Class A2 means that the grid is driven MORE POSITIVE than the cathode for part or all of the waveform. This means the grid will draw current from the cathode and heat up. A2 is not often used with beam tetrodes, pentodes or triodes like the SV300B, especially in audio. Usually a class-A2 amplifier will use vacuum tubes with special rugged grids, such as the 811A or SV572 triodes. Class A2 also requires a special driver circuit, that can supply power to the grid.
2. Class AB applies only to push-pull amplifiers. It means that when one tube’s grid is driven until its plate current cuts off (stops) completely, the other vacuum tube takes over and handles the power output. This gives greater efficiency than Class A. It also results in increased distortion, unless the amplifier is carefully designed and uses some negative feedback. Most guitar amps are push-pull Class AB. The biggest high-end amplifier manufacturers love to build big Class AB amplifiers using 6550s or KT88 type beam tubes–primarily
because their wealthy customers often have very inefficient speakers, such as electrostatic panels or Magnepans. (There are class-AB1 and class-AB2 amplifiers; the differences are the same as were explained above–the vacuum tube’s grids are not (AB1) or are (AB2) driven positive. AB2 is rare in today’s high end market, primarily because the egomaniacs who often design such amps do not how to design a reliable and good-sounding AB2 amplifier….)
3. Class B applies only to push-pull amplifiers in audio; it SOMETIMES applies to RF power amplifiers with one vacuum tube. It is like Class AB, except that the vacuum tubes idle at or near zero current. This gives even greater efficiency than Class A or AB. It also results in increased distortion, unless the amplifier is carefully designed and uses some negative feedback. If careful design is not undertaken, the result may be crossover distortion, which appears at the midpoint of the output waveform and has very bad-sounding effects in audio. Most solid-state audio amplifiers use class B, because the transistors undergo less heat stress when idling. Class B amplifiers can sound very
good, if well-designed. The classic Altec 1570 is an example.
4. Ultralinear operation is usually considered to be invented by David Hafler and Herbert Keroes in 1951. It uses only beam tetrodes or pentodes, and special taps on the output transformer. The taps connect to the screen grids of the vacuum tubes, causing the screens to be driven with part of the output signal. This lowers distortion considerably. It is usually seen only in hi-fi amplifiers that use power tubes such as the 6L6GC, 6550/KT88, EL84 or EL34. All classic Dynaco amplifiers used ultralinear connection. It is VERY hard on the screen grid of the vacuum tube, especially the EL34 type. So good-quality EL34s are critical in such amplifier circuits.
How often should I replace the vacuum tubes in my amp?
Practically speaking, you should only replace vacuum tubes in an audio amplifier when you start to notice changes in the sound quality.
Usually the tone will become “dull”, and transients will seem to be blunted. Also, the gain of the amplifier will decrease noticeably. This is usually enough of a warning for vacuum tube replacement. If the user has very stringent requirements for observing vacuum tube weakening, the best way to check vacuum tubes is with a proper mutual-conductance-style tube tester. These are still available on the used market; though new ones have not been manufactured in many years. One tester is being manufactured today, the Maxi-Matcher. It is
suitable for testing 6L6, EL34, 6550 and EL84 types. If you cannot get your own vacuum tube tester, speak to a service technician for his recommendations.