What Does This Thing Do? -- Coupling Capacitors

Written by
Dave Hunter
Published on
September 10, 2021 11:38:36 AM PDT September 10, 2021 11:38:36 AM PDTth, September 10, 2021 11:38:36 AM PDT

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 fourth part of Mojotone’s series What Does This Thing Do? we’re taking a look at coupling capacitors and their very real affect on the voice of any amplifier.

Although tube-amp fans often like to think of the tubes as the most significant components in the circuit, it’s easy to overlook another small part that usually plays a more direct role in voicing any amplifier: the coupling capacitor (aka signal cap, or tone cap, or just cap for short). 

Coupling caps are different from the electrolytic caps (aka filter caps) that we looked at in Part One of this series, in that rather than filtering electronic noise from the DC power supply, they connect signal-carrying components within the main part of the circuit. As such, they pass the guitar signal along in the direction it needs to go while, in many cases, simultaneously blocking DC voltage from heading where it doesn’t belong. 

If you look at the schematic for a simple 5E3 tweed Deluxe, for example, you’ll see that the power supply is delivering high voltage to the plates of the first 12AY7 preamp tube via a 100k-ohm resistor feeding each half of the tube—resulting in around 150VDC on each of those pins—but none of that DC voltage is present at the volume potentiometer for each channel. That’s because the .1µF coupling cap that connects that tube’s output to the input of each volume pot is blocking the DC voltage, while passing along the AC voltage that carries your guitar signal. Follow any tube guitar-amp circuit along from start to finish, and you’ll see other coupling caps performing similar duties all along the way.

In other places, you’ll see similar capacitors—though often much smaller—acting as “bright caps”, where they’re not asked to block any DC voltage, but simply to pass a certain high-frequency portion of the signal along to the next stage for voicing purposes. As such, these caps are working like bypass caps—the bright cap on a volume potentiometer, for example, which lets more highs into the signal when the knob is in its lower range (as you turn up that knob, the bright cap has less of an effect because more of the full signal is passing through the potentiometer anyway). Such bright caps can be connected directly between the input tab and wiper tab on a volume pot, as with the 47pF cap on the volume control in the Vibrato channel of a Deluxe Reverb, or on a switch that lets you take it in and out of the circuit, as with the 120pF bright caps in a Super Reverb and the like.

Coupling caps in general are often referred to as “tone caps” because they also play a big part in shaping the frequency content of the signal as it passes from stage to stage, and therefore in determining your tone. A lower-value cap passes less low-frequency content in the signal, while a higher-value cap passes more bass. Note that this “low vs. high value” thing can get a little confusing when you start looking at a variety of capacitors: for example, a .01µF cap has a higher value (often referred to as “larger”) than a .005µF cap. Because we’re often working with decimal-point values, you have to remember to take that “.” into consideration. When dealing with caps of the same manufacturer’s make and model, size usually follows value, so it’s usually easy to note that a .047µF cap is smaller than a .1µF cap, and so on. 

In any case, any thoughtfully designed amp will be voiced by the consideration its maker has taken in which coupling caps are used at which stages, according to the frequencies that they accentuate or attenuate, in combination with the frequency response of the tubes affecting that stage, as determined by its plate and cathode resistors, any cathode-bypass caps used (as discussed in Part One), and so forth. The final sonic result of any chain of signal-influence components always depends, however, on the cumulative effects of those several different parts. Swapping a tweed Deluxe’s .1µF coupling caps in the preamp for lower-value .022µF caps will certainly help to make that classic combo a little less bassy (a mod that many players undertake with great results), but it won’t immediately make a 5E3 sound like a Plexi, simply because the classic Marshall prominently uses .022µF caps in its preamp too.

Some aficionados also pursue specific makes of caps for their perceived sonic characteristics, and the consideration of the “sound” of caps of different compositions opens up an entirely different can of worms. Most thoughtful makers will tell you that the primary consideration is selecting the right value cap for the stage in which it is used; after that, however, many do agree that different makes and compositions of signal caps can influence fine points in the amp’s overall sound. Different types and makes might enhance warmth or brightness, perceived depth and richness, or very slightly alter the speed at which the amp reacts to your pick attack, and so forth. A cap that sounds great in one circuit, however, might sound wrong in another—whether it’s too dull, too bright, too gritty, too brittle, or what have you—so it’s rarely a case of being able to say, “this is the best tone cap there is, end of story!”

The best plan for approaching coupling capacitors for the DIY builder or modder is to follow your schematics for cap values, perhaps try a change or two of value to voice a stage if you’re getting a little too much or too little of something in the final results, and not worry too much about the “make and composition” variables if you’re using parts of good quality in the first place. Understand your caps and their role in shaping the amp’s frequency response, and you’re a long way toward dialing in your ideal tone!