Of Tubes and Transistors
In 1883 Edison discovered that electrons would flow in an evacuated lamp from a suspended filament. Years later, in 1905, Fleming expanded on Edison’s discovery with his “Fleming Valve”. But when Dr. Lee de Forest added a third component – the grid – to Fleming’s Valve in 1907, he opened the door to the world of electronic amplification with the development of the vacuum tube.
During World War II intensive research of the detectors used in radar systems led Bell Telephone Laboratories to the invention of the first point-contact transistor. This reliable little device gained quick support as the new component for amplification. The death of the vacuum tube seemed imminent as designers, scientists, and engineers reveled in the idea of replacing large, fragile glass tubes with these small, rock-solid devices.
However, there were (and still are) many serious listeners who realized that the sound produced by a “transistor” amplifier differs greatly from that of a tube amp – even in amplifiers that were otherwise identical in design specifications. These new solid state devices often produced a sound that was hard brittle and lifeless compared to their tube counterparts. It was determined that solid state devices produced a different, less musical set of harmonics than tubes. When pushed past their limits, they tend to mute the tone and emphasize the distortion.
Tubes are not that way: they produce a more musical set of harmonics, the intensity of which can be controlled by the player. This characteristic adds a warmth and definition to the sound which has become the trademark of tube amplifiers. When pushed into clipping, the harmonic overtones can be both sweet and pleasing or intense and penetrating, depending on the musician’s musical taste and playing technique.
Over the years, applied technology has resulted in a number of outstanding solid state amplifiers which sound very, very good. Some use special circuitry which enables them to simulate the distortion characteristics of tube amps. There is no argument against the reliability and consistency of these “maintenance free” units. Nevertheless, the tube is still held in the highest esteem by many thus establishing a successful melding of old and new technologies resulting in musical equipment offering musicians that classic “vintage” tube sound in a contemporary market.
Tube Types and Usage:
There are basically two types of tubes in a tube amp: Preamp tubes and power tubes. The Preamp tubes are smaller; usually types such as 12AX7, 12AU7 or 12AT7. These are the tubes that shape the sound and amplify the signal of your instrument. Preamp tubes tend to be micro phonic; that is they can mechanically pick up and transmit external noises. Since these tubes are in the first stages of an amp’s circuitry, it is very important to use high quality, low noise/low micro phonic preamp tubes. Although tubes of this quality may be difficult to find and can cost a bit more than micro phonic tubes, the added investment is worthwhile. It is extremely important to use a 12AX7 tube of this caliber in the first gain stage of an amplifier (where distortion is set) as well as in each high gain stage.
Preamp tubes can also be used as the drivers for the output devices (typically the power tubes). In this application, a 12AX7 will produce a more distorted tone than a 12AT7 which would produce a clearer, sweeter sound. A 12AU7 is even cleaner and brighter, giving more definition to the sound. (In some cases it is possible to change the sound by changing the type of preamp and/or driver tubes – as with any mod it’s best to consult a qualified service center first.)
The largest tubes inside the amp are the power tubes, which convert the signal from the instrument into the power necessary to drive the speakers. There are several types of power tubes available each offering different performance characteristics. For example, EL-34 power tubes tend to distort quickly and easily, giving a “creamier” sound with a “looser” low end response. These tubes are primarily manufactured outside America and have been responsible for the “British” tube amp sound favored by many players. 6550s are more durably built and stay cleaner sounding even at full power. When they do distort, the sound produced is more solid and has a tighter low end; more of a “heavy metal” type distortion with lots of power. 6L6 tubes have a very good dynamic range and offer a more traditional “American Rock” sound while 6V6 tubes produce a creamy sound with smooth distortion. Most tube amps are designed around a specific type of tube however; many can be modified to accept a different type of power tube in order to provide different sound and performance characteristics. There are sets of tubes available that have been extensively tested and matched together for optimum performance and longevity. (Check your service center about your particular amplifier before changing tube types.)
The Nature of Tubes: Why (and When) to Replace Them:
Tubes are made up of a number of separate mechanical components that have been vacuum-sealed in a glass envelope or bubble, making them somewhat fragile. Various factors, such as how hard and how often an amp is played, vibration from the speakers, road travel, and repeated set up and tear down all contribute to reduction of the tube’s longevity.
Any time you notice a change in your amplifier’s performance, check the tubes first. With the power off, carefully examine the tubes in a strong light and check them for cracks or white spots inside the glass or any other apparent damage. Then, with the power on view the tubes in a darkened room and look for preamp tubes without glowing filaments (like light bulbs that have burned out) or power tubes that glow excessively red.
Since the power tubes work harder in an amplifier than the Preamp tubes, they are nearly always the first to wear out. If the sound from you amplifier starts to grow weak (lacks punch, fades in and out, loses highs or lows, or produces unusual sounds), the power tubes most likely need replacing. Since power tubes work together in an amp, it is crucial that they all be replaced together with a matched set of the same type.
If worn out power tubes aren’t replaced they can ultimately fail, possibly causing severe damage to the output transformer and other components of your amplifier. If you notice your amp acting “funny”, especially if it’s been a while since the tubes were replaced, a checkup is recommended. This could save a lot of trouble by establishing the need for replacement before the tubes fail completely.
When replacing the power tubes, always have the amplifier’s bias voltage checked by a qualified service center. Improper biasing will cause degradation in performance and possibly damage the tubes and/or the amplifier. (See the section below for more information about biasing.)
If your amplifier squeals, makes noise, loses gain, starts to hum, lacks “sensitivity”, or feels as if it is working against you, the preamp tubes may need replacing. Remember to use only good quality, low micro phonic tubes in the preamp section.
Whenever you replace the power tubes, replace the driver tube(s) as well. This is the preamp type tube which controls the power tubes, and has to work almost as hard as the power tubes.
As a suggestion, if you’re on the road a lot, it might be a good idea for you to carry a matched set of replacement power tubes as well as a spare driver tube.
The Importance of Proper Biasing:
Power tubes require a certain voltage setting (referred to as the bias voltage) in order to perform properly. The factory will set the bias according to the type of power tubes the amp is equipped with. Whenever tubes are changed the bias voltage must be checked and readjusted as needed to accommodate the new tubes. Even if the replacement tubes are of the same specifications as the originals, their operating parameters may differ enough to require bias adjustments.
If the bias is set too low, the power tubes will run hot (the plates inside the tubes may glow red due to excessive heat) and the sound from the amp will lack power and punch. The excessive heat greatly reduces tube life; to a matter of days or even hours in severe cases. If the bias is set too high, the sound from the amp will be distorted at all levels. For the best performance and longest tube life, proper biasing is imperative.
Even if your amplifier doesn’t exhibit any of the symptoms described above, the bias should always be checked after tube replacement since tubes can often have different performance characteristics even if they have the same part numbers.
Survival Tips for Tube Amplifiers:
To prolong tube life, observe these tips and recommendations. –
Tube replacement should be performed only by qualified service personnel, familiar with the dangers of hazardous voltages which could be present when changing tubes! Do not risk exposing yourself to such hazards by performing tube replacements yourself if you are not qualified!