The weird quirks and intricacies of tone caps are discussed

Hello, and welcome back to “Mod Garage” for our last installment on tone caps for Stratocasters. I’ll also take a moment to finish the story about how our obscure tone cap turned out.

After receiving the Orange Drop cap back from the lab, I replaced it with an NOS Sprague “high-voltage” ceramic cap from the ‘60s to get the Strat as close as possible to stock ‘60s specs. Our customer was happy, and he’s still playing the guitar as his number one axe. I kept the Orange Drop cap for further testing and as a souvenir of a lesson learned.

Unfortunately, situations like this—when something is working, but nobody can tell you why—really bother me. We had traced a problem and found a workaround, but that’s an unpleasant situation. So I decided to dip deeper into the obscure world of film and foil caps.

Let’s resume: we have a 100-volt Orange Drop film and foil cap, formerly made by Sprague, today built by SBE with the original Sprague machines and toolings. These caps are produced by taking a long narrow strip of insulating material and placing a strip of metal foil on both sides of it. The two pieces of foil become the plates of the capacitor, and the insulator the dielectric. This long strip is then wound into a cylindrical shape, two metal leads are attached to the two foils, and the entire construction is potted and sealed in some type of material designed to keep moisture out of the capacitor and to keep the capacitor mechanically stable. Since the capacitor is wound into a cylindrical shape, one of the foil sides is on the outside (referred to as the “outside foil” end), and the other on the inside (“inside foil” end).

Standard film and foil caps don’t have a certain orientation like electrolytic caps, so, in theory, the way they are installed inside a guitar should not make any difference in tone. So why is this important when it doesn’t make a difference, tonally? Well, the outer-foil side can be used as a shield against electric field coupling into the capacitor—very important for tube amps. In order to take advantage of the shielding properties of the outside foil, the cap must be connected in the circuit in a particular orientation, which is the low impedance side of the amp circuit.

In a passive guitar circuit, there is no low-impedance side because we use the tone cap as a bypass cap to ground, so the outside foil should be connected to the grounded side in this case. The outside foil will act as a shield against electric field coupling into the capacitor, so you want it to have the lowest impedance return path to ground. With this rule in mind and all the caps connected this way, a tube amp will be much less susceptible to interference from fluorescent lighting and hum, oscillations or frequency-response peaks due to unwanted feedback from nearby signals within the amp.

So, I started to reverse the tone caps inside different guitars, and to my surprise, I discovered the following:

  • In some guitars, the same cap makes a noticeable difference if you reverse it, and in other guitars not—heaven knows why! Stratocasters seem to be most responsive to this, followed by Telecasters and Les Pauls.
  • Because of their construction, single-layer caps like ceramic or silver mica caps do not have an outside foil, so reversing these caps makes no difference. The same goes with high-voltage film and foil caps.
So why is he talking about all this, you may ask? I simply want to encourage you to keep your ears open! If you have a good-sounding guitar that has a bad amplified tone, try reversing the tone cap(s) before you blame the pickups and spend a lot of hard-earned bucks on a new set.

Unfortunately, identifying the outside foil end of such caps isn’t always easy. Some caps have a mark to indicate this side, but because the procedure takes time, most manufacturers don’t mark their caps. To bust another internet myth, I spoke with a SBE engineer who had also worked for Sprague in the past. He said that neither Sprague nor SBE marked the outside foil on Orange Drop caps! (He did say that there are plans to offer that option in the future for custom production runs).

According to the same engineer, the banded mark you can find on older Sprague Orange Drop caps was related to the production process Sprague used at this time, whatever that means. I crosschecked this with several old Orange Drops and can confirm the banded mark is not indicating the outside foil side.

So what if the cap is not marked? To find out where the outside foil side is connected, you will need some know-how and a good scope. I don’t have the space to detail this testing procedure, but if you are interested, please send me an email. That said, since there are only two terminals, you have a 50-50 chance to get it right from the start.

The moral of this story? If you love to mod your guitars and your guitar is loaded with Orange Drops, Mallorys, Roedersteins, WIMAs or similar caps, listen to the guitar’s amplified tone, reverse the tone cap(s) and listen again. Chances are that you will hear a noticeable difference in tone, as long as you aren’t using single-layer or high-voltage film and foil caps. Who knows—maybe you will discover one of your old guitars again, unplayed for years because of a less-than-great amplified tone!

Next month we’ll return to Stratocaster mods, and I will detail a very cool mod called the “vintage coiled guitar cable simulator.” Until then, keep on modding!

Dirk Wacker lives in Germany and has been addicted to all kinds of guitars since the age of 5. He is fascinated by anything related to old Fender guitars and amps. He plays country, rockabilly, surf, and Nashville styles in two bands, works regularly as a studio musician for a local studio, and writes for several guitar mags. He is also a hardcore DIY-er for guitars, amps, and stompboxes, and he runs an extensive webpage——on the subject.

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