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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. When making any modification to your equipment, it is highly recommended that you consult with a qualified service center.) V18-112/V18-212 Tube Guitar Amplifier with Reverb Important Information About Tubes and Tube Products (continued): 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 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. The Nature Of Tubes — Why (And When) To Replace Them: Tubes are made up of a number of fragile mechanical components that are vacuum-sealed in a glass envelope or bubble. The tube’s longevity is based on a number of factors which include how hard and often the amplifier is played, vibration from the speakers, road travel, repeated set up and tear down, etc. Any time you notice a change in your amplifier’s performance, check the tubes first. If it’s been a while since the tubes were replaced and the sound from your amplifier lacks punch, fades in and out, loses highs or lows or produces unusual sounds, the power tubes probably need to be replaced. If your amplifier squeals, makes noise, loses gain, starts to hum, lacks “sensitivity”, or feels as if it is working against you, the preamplifier tubes may need to be replaced. The power tubes are subjected to considerably more stress than the preamplifier tubes. Consequently, they almost always fail/degrade first. If deteriorating power tubes aren’t replaced they will ultimately fail. Depending on the failure mode, they may even cause severe damage to the audio output transformer and/or other components in the amplifier. Replacing the tubes before they fail completely has the potential to save you time, money and unwanted trouble. Since power tubes work together in an amplifier, it is crucial that they (if there is more than one) be replaced by a matched set. If you’re on the road a lot, we recommend that you carry a spare matched set of replacement power tubes and their associated driver tubes. After turning off the power and disconnecting the amplifier from the power source, carefully check the tubes (in bright light) for cracks or white spots inside the glass or any other apparent damage. Then, with the power on, view the tubes in a dark room. Look for preamplifier tubes that do not glow at all or power tubes that glow excessively red. V18-112/V18-212 Tube Guitar Amplifier with Reverb Important Information About Tubes and Tube Products (continued): Whenever you replace the power tube(s): • Always have the amplifier’s bias voltage checked by a qualified service center. Improper bias voltage will cause degradation in performance and possibly damage the tubes and/or the amplifier. (See “The Importance of Proper Biasing”, below for more information). • We highly recomm...
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