Many tube guitar amplifiers on the market today come with a feature that can either switch or vary the maximum power output of the amplifier section. Simply put, this allows you to adjust the maximum output of the amplifier, making it higher or lower at the flick of the switch. If you have a tube amp with a lot of clean headroom and plenty of power for any gig, you may be wondering why you can’t just use the volume knob to get it to the level you want.
The difference lies in how a tube guitar amplifier’s power section distorts and the interaction between the preamp and power amp. There are many ways that guitarists describe the sound of a tube amp pushed into clipping- overdrive, saturation, crunch- but the causes are quite different when it comes to preamp clipping and output power tube clipping.
The sound of your overdriven guitar is shaped by a combination of your picking, finger attack, preamp volume level, and the volume of the amplifier’s power amp. Really cranking an amplifier saturates the power tubes, causing your signal to break up. Clipping a power section of EL34 power tubes (a common choice in amplifiers) results in natural compression and increased harmonics and touch responsiveness, but to get the full effect, your amplifier has to be cranked up full.
At the same time, playing guitar with a heavy pick attack, using a clean boost pedal, or turning up the input volume will get the preamp tubes working, adding gain and drive. Since the preamp essentially takes your guitar signal and amplifies it to a level that the power tube section can output, running both hot can work to give you a sweet overdrive sound.
However, the overall volume that results from your amp being cranked can be overkill for some smaller venues, and typically will blow the band off the stage. Unless, you are the rock god that owns the show that everyone came to hear, this is where the wattage switch comes in. Like with Carvin Audio’s V3 series, you can switch down from 100 watts to 50 or 25 watts. This means that you can send the power amp into clipping at a lower output level for a nice crunch without blasting your ears off. In most situations, you are on average using very little output amp wattage, so it is very difficult to get the power tubes into distortion. With the switching power level feature switched down to 25 watts, you can achieve power amp clipping distortion more often. This distortion is very different from the preamp distortion generated using the drive knob turned up and master volume turned down. The power amp distortion is not continuous and adds flavor to the peaks. In the days before master volume knobs this was a big part of the distortion heard. Because it only happens on the peaks, it is very dynamic and you can call on it with more intense playing or back off to clean it up a little.
If you’re a guitarist who plays in a wide variety of different venues and needs an amp that can provide versatility and consistent great tone, an amp with switchable wattage is a great option.
In solid state power amplifiers, clipping should be avoided. In tube amplifiers the output tubes and output transformer still produce rounded clipping for most of the peaks. In solid state amplifiers, clipping very quickly becomes square waves which can damage speakers very quickly.
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Even if you’ve matched your bass head and cab properly impedance wise and set your amp for clean sound, sending simply too much power to your bass cab can result in blown speakers. This often happens when you are using a rig you are unfamiliar with, as we tend to know the limitations of our own equipment and have chosen that setup for a reason. Borrowing another bassist’s amp or using a backline rig only to blow it up is definitely not a great feeling.