Tubes

Here is some useful info about tubes,and a link to a good site on amps and tubes,and also some cool videos
Tube Types And Usage
Tube amplifiers are based primarily on two types of tubes – preamplifier tubes and power
tubes. The tubes used in preamplifiers (12AX7, 12AU7, 12AT7, etc.) are smaller than the
power tubes. These tubes amplify the signal from your instrument and shape the sound. They
are inherently microphonic (they can mechanically pick up and transmit external noises). Since
these tubes are used in the critical first stages of a tube amplifier’s circuitry, it is very important
to use high-quality, low noise/low microphonic tubes for this application. Although tubes
of this quality may be difficult to find and typically cost more than “off-the-shelf” tubes, the
improvement in performance is worth the investment.
Preamplifier tubes are also used to drive the power tubes. When used in this application, a
12AX7 will produce a more distorted tone than a 12AT7, which produces a clearer, sweeter
sound. A 12AU7 is even cleaner and brighter than a 12AT7, 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.

The power tubes are the largest tubes used in an amplifier. These tubes convert the low-level,
conditioned signal from the preamplifier into a level that is sufficient to drive the speakers.
There are several types of power tubes available, each of which offers a different performance/
sound characteristic. For example, the EL34 power tube produces a great classic rock
sound. When an EL34 is driven into distortion, it produces a unique sound (“crunch”). When
compared to the 6L6, the EL34 distorts more quickly, exhibits a “looser” low-end response
and produces more harmonics at mid and high frequencies (“creamier” sound). These differences
become more noticeable at higher volumes.
The EL84(my Favorite) is similar to the EL34 but produces less output power. It can be easily driven into
distortion and is characterized by a smooth, sweet tone with excellent touch sensitivity.
6L6 tubes produce a big low-end thump and have a very good dynamic range. They offer a
more traditional “American Rock” sound. The 6V6 tubes produce a creamy sound with nice
distortion. On the other hand, the KT88 produces a big low-end but sounds more like an EL34
in the mid and high frequencies.
6550 power tubes are more rugged 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.
Some tubes are available in matched sets. These tubes have been extensively tested for optimum performance and longevity.
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***Update... Premier Guitar article on Preamp tubes... http://www.premierguitar.com/Magazine/Issue/2009/Jan/Everything_You_Ever_Wanted_to_Know_About_Preamp_Tubes.aspx