Tube amps have a few methods for volume attenuation, some work really well, others do not. For most that have a simple full/half switch, it is simply changing how the power tubes are configured. The switch changes them from pentode to triode mode usually. Running a power tube in triode mode lowers the power and also changes how it distorts. Some will simply turn off a set of tubes and swap in a load resistor so the primary impedance still works for the reduced number of tubes. This is almost identical to simply pulling two tubes from your amp with the only real difference being the primary impedance seen at the tube's output. Another method is to do what is marketed or buzzworded as " Power Scaling ". Power scaling is the reduction of the power rail that feeds the power tubes. This can be done in a couple of ways. You can use an alternate secondary tap on the power transformer ( Mesa is popular for doing this ) that provides less voltage, or you can use a power regulator circuit that dissipates the energy away from the power tubes. Power scaling is the new hot topic and is possibly the most neutral sounding way to reduce power with modest power reduction levels. When used with high or extreme levels of power reduction, there can sometimes be issues.
The biggest problem is that amplifiers are designed to produce high levels of output, so trying to make them only produce a fraction of what they would produce is challenging. As you can imagine, it is sort of like asking a top fuel dragster to putt around town. It isn't going to do it. This is kind of the manufacturer's fault for pushing the sales of their big amps whence upon a time. If you don't have a full stack, you aren't cool. Fast forward to the early part of the 2000s and lunchbox amps became a craze. That craze has sort of ended and the market has changed to want a full-sized amp that can do full-sized amp duties, yet still do what lunchbox amps can do.
I feel that 30 watts is about the perfect wattage for nearly any amp to be. If you need more volume than that can provide, you are already too loud. 15 watts can do it if you don't mind a little breakup. 1 watt is almost too much power for the bedroom. Most musicians have no idea what the conversion of the watts to the realized output means. When you buy a speaker ( like a Vintage 30 for instance ), it will have a sensitivity. The sensitivity is the SPL ( volume ) that will be produced with 1 watt of power. For most guitar speakers this is going to be roughly 90-95db. That is pretty loud and it only takes 1 watt of power to produce that output. If you play through a 4x12, the efficiency is raised even more, to where you may have 100db or more in SPL for that same 1 watt of input power. For most bedroom players, you may only need 1/4th to 1/2 of a watt to have more than enough volume. This is why making a 100-watt amplifier that quiet is so hard. It is designed to produce SIGNIFICANTLY more output.
Designing an amp to have less output power while still having the same core sound is difficult, if the sound the amp makes, is dependent upon its power section. If the core of the amps sound is from the preamp, it is a little easier, but most amps still rely on one thing that I tend to dislike about amp designs, the feedback loop. Most amps employ a negative feedback loop that ties the output of the amp back to the phase inverter. The loop is designed for operation at a performance level, not bedroom level volumes. When the amp is turned WAY down there simply isn't enough negative feedback to smooth out and make the amps sound correct. The reason amps warm up and sound better as you turn them up isn't because you are working the tubes, it is because the feedback loop is actually working and is smoothing out the frequency response of the amp. Amps that don't use negative feedback are less prone to this but are also not typically 100+ watt amplifiers. When you don't have a negative feedback loop the amp is prone to exposing its own deficiencies as in it " has a sound " that is easily noticed. The feedback loop evens out the frequency response of the amp often masking the deficiencies of the amps preamp section. This is where I think most amp designers have failed. It isn't that an amp is incapable of playing at low volume levels, it is that they are so stuck on stupid with cookie-cutter amp designs that they don't spend the time making an amp more usable from each extreme. The feedback loop is one area of huge disappointment when it is not working as it is intended.
The long and short of it is, buy an amp for its intended application and you will spend much less time trying to make it be something it isn't. It is very difficult to design an amp that works the same quiet, as it does loud. The amps that do both quiet and loud well, are typically amps that are lower in wattage and honestly use less sophisticated and complicated circuitry. Because the amp is closer to both extremes it is just that much easier to tune it and make it work for those purposes. The lunchbox craze was an important part of modern amp design. It was the eye-opener to manufacturers to see that users really do want the sound at a reasonable level. We are finally beginning to see the technology and fruition of that need come to life. If you NEED an external attenuator, it is because you are fixing a problem that probably exists only to you. It is the whole of the idiom of having the right tool for the wrong job. When you need a jewelers hammer, a 5-pound sledge doesn't always work.
cool to hear. I was about to buy the HT-5, because I was so impressed by its clean tone (compared to the Boss Katana and other amps in the showroom)
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