For those who are wondering what the control dial on the back of your bass cabinet is for, this Tech Talk article should help.
It’s the level control for the tweeter in your bass cab.
This control goes by several names; Fender usually refers to it is a high-frequency horn attenuator (horn being synonymous with tweeter in this case); often labeled horn level on the back of Fender bass cabinets. It lets you dial in how much high-end bass signal you want to hear out of the tweeter.
So it’s a volume control for the tweeter, then?
Yeah, basically, although the fact that it affects the volume of only part of the entire signal makes it a kind of tone control, too. The dial controls a specialized circuit that makes it possible to increase or decrease the loudness of the tweeter. This specialized circuit includes a device called a crossover, and to understand more clearly what it does to your bass sound, a look at what’s going on electrically in your bass cab might help.
Generally speaking (there are always exceptions, of course), what happens in most bass cabs with a tweeter is that the entire signal from the amp is sent to the speakers. We’re talking the whole shebang—low, mid and high frequencies. Everything. It all goes to the speakers. Now, those speakers are pretty good at handling that entire spectrum, especially the low and mid frequencies. High frequencies can be another story, though. Your bass cab’s speakers probably do OK with them, but just OK rather than really great. To really get the most bang for your buck in the higher-frequencies department, your bass cab could use some help.
That’s where the crossover, the tweeter and that mysterious dial on the back of your cabinet come in. The crossover is a built-in electronic filter that takes the whole signal and removes the lower and middle frequencies, leaving only the higher frequencies. The crossover sends this newly separated high-frequency portion of the signal to the tweeter, which is a driver specifically optimized for that portion of the frequency range.
The result is more well-defined and pronounced brightness and treble; a fuller and more dynamic range of bass sound.
The high-frequency horn attenuator control—that dial on the back of your cabinet—comes after the crossover and before the tweeter. It offers variable electrical resistance to the signal passing through it, which means that once the crossover “determines” the high-frequency signal that will be sent to the tweeter, you can use the dial control to set how much of that signal is actually used.
There are no rules about where to set the high-frequency horn attenuator control; it’s purely a matter of personal preference. You won’t hurt anything if you max it and leave it that way (Fender bass cabs typically offer internal protection, usually a fuse, that will shut down a tweeter before it’s in too much danger).
Several Fender bass amps, such as some mid-2000s Bassman combo models, have a high-frequency attenuator control in the form of a three-position rocker switch rather than a dial. This switch enables the player to turn the tweeter off, get full high-frequency response or a decreased response (usually -6 dB).