Guitar Tube Amps
Guitar Tube Amps Fast Facts
A tube is an electronic device consisting of a minimum of four active elements: a heater (filament), a cathode, a grid, and a plate. All of these are sealed in a glass enclosure with its air removed – a vacuum – to prevent the parts from burning. The filament is heated in order to warm the cathode. 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. If the grid were absent, this movement of the electrons would be uncontrolled, much like water rushing from a faucet that’s opened all the way.
A Brief History of Guitar Tube Amps
Guitarists both today and yesterday are linked in many cases by one piece of equipment: a tube amp. In fact, the tube amp is currently enjoying its greatest popularity with musicians, even though there have been great strides in transistor amp technology over the past 20 years. Some guitarists prefer tube amps, while others are equally fond of transistorized, or solid-state, models. But regardless of how close their sound may be, for many guitarists there remains a certain mystique about tubes.
Why do designs built around tubes sound different from those following the solid-state approach? Simply, tubes work differently. What follows, then, is an explanation of their construction, function, and applications.
When a small signal is applied to the grid, it causes a larger change in the current that flows between the cathode and plate accordingly. In effect, it acts as a valve. A portion of the amp’s electronic circuitry, the grid bias control, adjusts the proper voltage setting of the grid. The amount of bias varies from tube to tube, depending on its sensitivity, and it acts to keep the tube “idling.” When the grid bias is properly set, the tube is balanced to the circuit, and therefore produces a clean, powerful signal (proper biasing also extends the life of the tube).
For optimum performance, the bias setting should be checked whenever power tubes are changed – preferably by a qualified technician using an oscilloscope. A bias adjustment is a relatively simple operation, and can be performed for a minimal bench charge. Some symptoms of improper bias setting include the amp running too hot, excessive hum after it’s been on for a short while, or distortion that just doesn’t sound right. The amp doesn’t necessarily have to sound bad for its tubes to be incorrectly biased, and these symptoms may indicate other problems. However, if your amp is behaving in an extraordinary manner, a trip to the shop may head off damage to it, regardless of the cause.
No tube primer would be complete works in an amplifier. Imagine a small guitar amp with no volume or tone controls: just a guitar input, one tube, an output transformer, and a speaker. The guitar’s pickup produces a small voltage, the result of the string vibrating in the pickup’s magnetic field. In general, this signal is applied to the grid, which in turn causes a large current flow from the cathode to the plate. Thus, a correspondingly large voltage now appears at the plate. This plate is connected to an output transformer, which matches the tube’s output impedance to that of the speaker. (Because there is a great disparity between the impedances of the tube amplification circuit and the speaker, the transformer must act as a buffer to interface the two components.) Thus, a small, low-power signal from a guitar’s pickup can produce a high powered signal to drive the speakers.
Naturally, amps don’t all sound alike. This is due to variations in the type of tube that is used, the quality of the tubes, and the specific circuit design of the amp. In other words, some tubes amplify more than other tubes under similar conditions. Also, the amount of gain a tube produces varies with the circuit design. This is why different makes of tube amps can sound very different, even though they use the same tube types. In addition, certain amps use completely different types of tubes. A good example of this is the English-type Marshall using European EL-34 tubes in its power amp section, compared to the U.S. type, which employs American made 6550 power tubes. The U.S. and English styles sound and play very differently, reflecting the character of their power tubes. That is, the English EL-34 tubes yield more distortion than their American counterparts, although they produce roughly the same amount of volume. With internal bias modification (which mostly involves changing some resistance values), any U.S. Marshall amp can use European EL-34s, and vice versa.
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