8/12/2014

What You Should Know About Jazz Guitar Amplifiers

By Tanisha Berg


Guitar amplifiers usually have several signaling pathways (channels) with different sound characteristics. Clean channels only have a volume adjuster; sometimes it is even combined with a tone setting. Jazz guitar amplifiers channels with more distortion usually has a gain adjuster that affects the strength of distortion degree, coupled with a volume adjuster for the output volume. The channels can be selected by pressing a button on the amplifier. Combos incorporate one or more loudspeakers mounted in a common housing.

Guitar amplifiers can be classified into the following categories: according to type, including combo - open or closed, amp head, preamp, power amp as well as rack system. This in addition to electric bass, acoustic guitar, practice, studio and stage amplifier. The use of reinforcing members, such as electron tubes, semiconductors and hybrid is common. Commercially available sound equipment can be virtually combined with a focus on one or more categories. This explains the great diversity of supply.

The signal flow and arrangement of components and adjusting elements becomes clear. The individual components of a guitar amp are the input (signal input), mostly realized by a connection with a 6.35-mm mono jack, preamp with volume control, tone control (equalizer) to boost or cut individual frequency ranges (bass, middle, treble and linear distortion), output amplifier, speaker system and power supply (PSU).

This can be achieved for example with an EL84 5W output power, while the two EL84 in push-pull mode is typically capable of 15 W. Often several tubes are connected in parallel to increase the overall performance. Power of 100 W or more can be achieved for example with multiple EL34 parallel push-pull operations.

Chance of additional effects such as chorus and tremolo can be integrated. Guitar amps come in various power levels of about 2 watts (Roland Microcube) reaches over 350 watts (Marshall Mode Four) output power. Greater power outputs are not useful in general, as in bigger stage installations, the instruments are transmitted via the PA system.

The first tube amps from the 1950s could hardly control the precursors, but the distortion was achieved by setting the amp as loud as possible. This means the power tubes were overridden and the transformer went into saturation. This is what influences the sound of guitarists such as Jimi Hendrix and Alvin Lee (Ten Years After). Technically spoken messages are added to the signal in single-ended tube, which increases even-numbered harmonics (overtones) and the signal is increasingly soft (soft clipping).

The auditory impression of distorting tube amp is described as dense, loud, rough and groovy. This sound is important in all areas of rock music and a typical guitar sound is a vital part of music styles of hard rock and heavy metal. This is probably due to the well-configured override behavior and the higher internal resistance.

If this is not desired, the loudspeaker can employ a resistor network connected upstream, which converts the major part of output signal feeding only a fraction of the output power (power soak). Alternatively, it performs a power reduction during the output stage (for example, by reducing the operating voltage of tubes) to deliver the desired result. In all cases, however, it must be remembered that this inevitably leads to decline of overall volume of nonlinear distortions of the loudspeaker, which also contribute to distortion on the output stage.




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