What Does This Thing Do? - Bright Caps

Written by
Dave Hunter
Published on
February 10, 2022 at 10:17:17 AM PST February 10, 2022 at 10:17:17 AM PSTth, February 10, 2022 at 10:17:17 AM PST

There are a lot of mysterious components within any guitar amp, many of which remain puzzling even to hobbyists who have built a DIY project or two. In the ninth part of Mojotone’s new series What Does This Thing Do? we’re examining bright caps, those enigmatic little chunks of dielectric that have such a big impact on your tone’s top end.

Guitarists who haven’t wired up any amps themselves are perhaps most likely to have heard of a bright cap...

...(short for capacitor) in responses to queries about how to make an amp sound less bright. These are often blamed for the icepick trebles experienced in anything from a Marshall 2204 to a Matchless Lighting to a reissue Vox AC30CC, and in many cases the advised cure is “clip it out!” The fact is, though, that a bright cap can be a powerful tone-tweaker as part of any well-thought-out amp design, which is why un-tweaking it from the picture makes a surprisingly dramatic effect for the removal of just one small part. 

A bright cap can take all kinds of configurations, but the most common—and the one we’re discussing here—takes the form of a small, low-value capacitor that is either connected between the input terminal and wiper terminal on an amp’s volume potentiometer, or on a switch that makes such a connection between those points when in the “on” position. Rather than the entire signal flowing through this capacitor as it does with a coupling cap (discussed in What Does This Thing Do #4), the bright cap is used such that it passes only a frequency-determined portion of the signal around a particular stage. Which is to say, it sneaks more of the highs from the input of the potentiometer to its output. In many classic amplifiers that carry two channels labeled Normal and Bright (or similar), the only real difference between them is a bright cap attached to the Bright channel’s volume potentiometer.

As such, this small cap delivers a proportion of the high-end frequencies at a determined cutoff point to the output of the potentiometer, frequencies that otherwise might be lost from the signal without it.

Those highs are all there in the signal in the first place, it’s just that they wouldn’t arrive at the other end of the potentiometer in their full sparkling glory when that pot is turned down from full. When the pot is turned up high, on the other hand, all of the frequencies coming in should be headed out the other side (other than a small proportion that is lost on the way through the component), and the bright cap itself is redundant.

In basic terms, the high-frequency content passed from the potentiometer’s input to its output is determined by the value of the capacitor, which is to say its rating on the scale used to determine the functional “size” of these electrical components. The lower the cap’s value—within the range of acceptable bright-cap values in general—the lower the proportion of highs it will bypass into the signal the other side of the pot. The legendary tweed Fender Bassman, for example, has a 100pF (aka .0001µF) bright cap on the volume pot of its Bright channel, which is the only difference between that and the Normal channel. A cap of that value generally provides a noticeable but subtle lift in highs, and few players ever object to its inclusion. 

The bright cap on Marshall Master Model 2204 and 2203 Lead amps, on the other hand, are a whopping 500pF (.0005µF), which introduces a lot of highs into the signal at the lower end of the potentiometer’s range. Experienced Marshall-o-files will tell you that these amps are designed to be turned up anyway, and when you get that first volume control wound up high the frequency content will be more balanced. If you traditionally play yours at lower/cleaner settings, however, the advice is often simply to clip that cap from the pot and remove it from the circuit entirely.

Anticipating that a fixed bright cap might not be to all tastes with all guitars in all playing situations...

...many makers have attached it to a switch that will take it out of circuit as desired. Such was the case with all of Fender’s larger blackface and silverface amps, which used a bright cap of a slightly higher value, but made it switchable so you weren’t stuck with it. 

Here are some bright-cap values used in a range of classic amps:

  • Fender Deluxe Reverb – 47pF (hard-wired)
  • Fender Super/Twin Reverb etc – 120pF (switched)
  • Fender 5F6-A Bassman – 100pF (hard-wired)
  • Marshall JTM45 – 100pF (hard-wired)
  • Marshall Model 1987 “Plexi” – 500pF (hard-wired)
  • Marshall 2203/2204 – 500pF (hard-wired)
  • Matchless Lightning – 120pF (hard-wired)
  • Matchless DC30 – 180pF (hard-wired)
  • Mesa/Boogie Mark IIC+ – 180pF (switched)
  • Mesa/Boogie Mark III – 100pF (switched)

Vox did things somewhat differently, and also changed up circuit designs so often during the early years of production that the early JMI classics tend to make for confusing examples (consider the classic Top Boost AC30 of the mid ’60s had three channels, and each achieved a desirable level of brightness using different means, the eponymous Top Boost channel itself being chief among them). The Vox AC30CC reissue, however, to name but one, has a 180pF bright cap permanently fixed across the Top Boost channel’s volume pot, which induces too much high end into these amps for many players’ tastes. For that reason, the “clip it out!” mod is a popular option with these amps.

Whether you’re looking to mod an existing amp or to carefully voice a scratch build to suit your tastes, the bright cap is a component worth considering carefully. It’s usually easy to test a number of different cap values, too, to see which one works best in your particular circuit, and in a ground-up build there’s often a place to put a mini-toggle switch to make the bright cap optional. Either way, it’s a tool you can use to dial in your own tone to something closer to perfection.