The widespread love of genuine tube tone in the age of reduced stage and studio volumes has driven many players to seek creative volume-reduction solutions while, ideally, trying to retain the full girth and depth of their sound. The output attenuator has become a particularly popular method of volume reduction in recent years, and on paper it might appear the perfect solution: Set the amp’s controls where you like them, connect the attenuator between output and speaker, and it all sounds the same, but just a little quieter. There are several factors to the function and performance of these units that are worth understanding, and in practice the sonic result of most attenuators is rarely “exactly the same, only quieter.”
Thanks to a boom in the attenuator field, many different types are available today. Some passive units use purely resistive loads, some are reactive loads (designed to interact with the amp the way a speaker does), and some active types tap the signal on its way to the load, then re-amp it via a solid-state power amp. I won’t try to define a “worse-better-best” scale along these lines here. There are ardent fans of each type, and whatever works best for you, works.
The function of all passive attenuators (resistive and reactive types) is to drain off a portion of the amp’s output power and pass the rest along to your speaker. This means that your amp is producing the same wattage at its output that is produced when you achieve your ideal “cranked” tone, but the speaker is getting less of that wattage. This makes the amp quieter in the room, but it also means the speaker is pumping less air, and the resultant sound may be perceived differently as a result. What would seem, on paper, to be “the same sound, only less of it” actually hits the ear sounding slightly different, thanks to several complex interactions between decibel levels, frequency range, and the way the ear interprets it all.
The result is usually heard (or recorded) as a slightly duller overall tone, with reduced highs and a less presence-y feel. Still, these detractors might be only slight, and if they let you crank your amp into the sweet spot, rather than constricting it to somewhere far short of the tone zone, many players feel it’s a fair compromise. The more you attenuate the output, the more dramatic these effects seem to be, and in all honesty, few attenuators can claim to have no effect on tone at their more extreme settings. Some of the more complex attenuator designs include circuits that let you dial back some of what is lost, but you can often just add more treble and presence at the amp to replace what the attenuator shaves off.
By hitting the speaker with less power, an attenuator also lessens, or perhaps entirely removes, the effect of speaker distortion from your sonic brew, and this is another way in which tone can be affected. When connected to an amp working near full volume, most speakers will clip a little in their own performance, creating a hairier sound that can be an important part of some cranked amp tones (think tweed Deluxe, with its low-efficiency Jensen alnico speaker raging to beat the band). So just be aware that the more attenuation you use, the less speaker distortion will factor into your tonal stew.
Attenuators can also have an impact on tube life and the overall safety of your amp, although this might be less significant than is sometimes reported. A common perception is that output attenuators will help you burn through output tubes faster, but that might largely be because an attenuator inspires you to play the amp itself at higher output settings simply because you can get away with it—meaning you run the tubes hotter and harder. Some makers warn against using attenuators with their amps (and a few might even threaten to void your warranty if you do so), but there is little hard evidence to say that attenuators themselves will wear out tubes or output transformers any faster than merely playing your amp at the high settings.
Despite these relatively minor caveats, the recent popularity of output attenuators speaks to the success of these units in achieving cranked tube tone at tolerable volume levels. Bottom line: If one gets you where you need to be, any such minor compromises are probably worth putting up with.