INTRODUCTION TO VACUUM TUBES
Electric guitar and bass players over the past 50 years have been linked by a single critical piece of equipment: The tube amp. With all of the advances in transistor and digital technology, professionals still prefer tube amps as part of their musical arsenal from studio to stage. There are many reasons for this, and we will explore many of these factors in plain language, throughout in this guide.
Tubes have provided the foundation for musical and audio circuits that have produced much of the worlds’ greatest rock, jazz, blues and country music during the last century. Ironically, tube-driven circuits are not necessarily a perfect “reproducer” of sound. They are actually a “producer” of part of the musical output by adding definite, subtle, tactile and sonic characteristics to the signal path of instrument amps.
So, why do tube amps sound and feel different?
First, tubes are imperfect. They each have unique performance characteristics and designs. They respond to signal input in a softer, slower and less precise manner than digital circuits. This provides the soft feel, or touch that many players enjoy in great tube amps. This is due to the natural “compression” that tube inefficiency creates. Unfortunately, they also degrade over time and use, which is why many professionals re-tube their amps before each tour and maintain spares for road use.
Second, tubes tend to distort in a very non-linear way. If you look at tube circuit distortion on test equipment, you would see a slightly rounded waveform as the tube starts to distort. This is different from a solid-state or digital waveform which tends to appear square, distorting all frequencies at the same time.
In tube circuits, low frequencies tend to distort first, and higher frequencies stay clean longer. Because of the way we hear sounds, this is more pleasing, less harsh. The harmonics that tend to distort in a tube circuit are also more musical and are the reason that when you “grab” a note on a guitar at high volumes and the amp distorts, it will feed back in a musically useful and pleasing way.
Finally, even though tubes are made of glass, and have little metal parts inside, they are actually pretty durable, and with proper testing, rather predictable. Over the years, there are many specific amp designs that have been associated with an “American” sound, a “British” sound, etc… Many high-end boutique products use these reference sounds from tubes in their circuit designs to achieve signature tones.
What is a Tube?
A tube is an electronic gain producing device consisting of a minimum of four active elements: a heater (filament), a cathode, a grid and a plate. These are all sealed in a vacuum glass enclosure to prevent parts from burning. Once heated, the cathode begins to emit electrons, which flow from the cathode (which is negatively charged) toward the plate (which is positively charged). The grid’s purpose is to control this flow, in effect, acting as a “valve”. This is one reason that tubes are called “valves” in the UK.
What is the difference between preamp tubes and power tubes?
When the guitar’s pickup produces a small voltage (the result of the string vibrating in the pickup’s magnetic field), this signal goes into the “preamp” part of an amp circuit. It is applied to the grid, which causes a larger current flow from the cathode to the plate. Preamp tubes are usually the main tone generators in an amp circuit and you can experiment with the gain and tone of you amp easily by trying different preamp tubes. GT makes it easy by providing clear, intuitive descriptions of these tubes.
Power tubes provide the major horsepower in an amp circuit, and will have their greatest impact on tone as they distort. This means you have to run amps pretty loud to get the maximum benefit from the tube distortion in the power amp section. That explains the popularity of small, lower power tube amps which distort nicely at lower volumes. Keep in mind, there are several styles of power tubes available. You must use the specific style (EL34, 6V6, 6L6, etc…) that the circuit was designed to accommodate.
What is “biasing”?
Bias refers to the adjustment (generally in the power tube section) that controls the voltage of the grid of a tube. When the grid bias is properly set, the tubes are balanced in the circuit and produce a clean, powerful signal. It is like matching the engine RPM to the proper gear in a race car to achieve optimum performance for the desired speed.
If the bias is not set correctly, the amp may not perform properly, or the tubes may wear out more quickly. If you install a set of tubes with very different outputs or strengths from each other, it may not be possible to adjust the bias to achieve maximum performance from all power tubes. You will get a compromise on all tubes, and none will perform to their proper potential or specification.
Fortunately, many amp manufacturers use power tubes that fall in the same specification as “Medium” (4-7 fine scale) of Groove Tubes as their stock tubes. The amplifier’s bias control is adjusted at the factory for tubes in this power range. Tubes in this range are widely available at all times from Groove Tubes.
If you want to change the performance of your amp to distort more quickly, or stay clean longer, you can use Groove Tubes in the Low (1-3 fine scale), or High (8-10 fine scale). If so, you should have a qualified service tech re-bias your amp for the new tubes. The good news, is that once this is done, you should be able to replace your power tubes with Groove Tubes of the same type and rating without bias adjustment.
Preamp circuits and many tube power amp designs today feature an “auto’biasing” circuit. You can check with the amp manufacturer to see if your amp has “auto-bias”. If so, no further adjustments are needed.