My biggest surprise with Ampohm tone caps is that they actually work with audible differences in sound quality. Essentially there are two camps. Wax and Oil. Within each group you can chose Tin, Aluminum, and Copper as your conductor of choice with paper-in-wax or paper-in-oil as the dielectric.
On all guitars I tried each family of cap to get the feel for what sounds great and then fine tuned the metal. As a general rule I would say the Paper In Wax / Tin Foil or Paper In Wax / Aluminum Foil were the best for single coil pickups and the sound of Fender. While the Paper In Oil caps seemed to work better in guitars that had humbuckers. The Ampohm tone caps, both oil and wax, do tend to give the effect of making the tone control taper smoother with a greater degree of subtle variation. It can’t change the taper but I think what happens is that subtle harmonics get killed by cheap caps. The extended frequency of the Ampohm caps allows you to hear tonal changes that are usually swamped by the tone circuit.
Another general statement I’m comfortable with is that in wax or oil, the tin and aluminum were very close if not indistinguishable from each other, while the Paper In Oil / Copper Foil caps seem to provide a bit more clarity and high end. If your pickups are bright the wax paper and foil give the best results. If the guitar and pickups are on the dark side I think the oil and foil are the best way to go.
I can generalize it even more. If you have a Fender go with wax, if you have a Gibson go with oil. I compared the oil and foil caps with the repro “bumblebee caps” being sold by a few companies at about $60 each. This of course includes a fancy box and treated cotton insulation on the leads. I felt that the Ampohm Paper In Oil / Aluminum Foil cap was the closest sound match for these caps. I obtained a Gibson “bumblebee” cap as used in their historic reproduction series and found them to sound very different. Subtle use of a hammer revealed why. The Gibson caps are fakes. It’s just a poly cap in a plastic shell that looks old school.
I don’t know why the oil caps seem to like Gibson style guitars and the wax works in Fender. The only observation I can make is that generally humbuckers are higher impedance pickups and use 500K tone pots while Fender single coils are lower impedance and usually use 250K tone pots. Perhaps the loading of the entire tone circuit is the deciding factor.
One plus with Ampohm over the Vitamin series of caps is that the leads are insulated from the casing at both ends. In my opinion the Vitamin Q, T, or whatever should be treated as polarized for the sake of safety. If you use them in high voltage situations putting them in backwards will put potentially lethal voltage on the shell of the cap and if installed correctly the entire shell could be at ground potential while surrounded by high voltage leads. With Ampohm there is a little more safety due to the phenolic end caps and a non-conductive plastic wrap around the shell.
While I was doing all this testing I was also trying to find out if high end guitar cables are worth it. Changing to the Ampohm oil and foil caps in my Les Paul delivered the same sonic improvement as using a $150 patch cord. Combining the two was like getting a new set of pickups.
Now I should caution you that there are a lot of people out there that can’t hear the finer details. If you play a MESA BOOGIE Triple Rectifier for example, changing the caps in the guitar or amp is a waste of money. Not even dramatic changes in preamp tubes produce much of a tonal change in these amps.
Audiophiles, studio guys, vintage collectors can all hear the difference with their own opinions on what is best. The main thing is that they all agree the cap upgrade is a good thing.
The size is an issue that people will have to come to terms with for certain guitars. Caps in the 0.02 to 0.047 / 630v range are no problem in Gibsons and I managed to fit them in all the other guitars. The 0.1uf caps are impossible but the only guitars I know of that used them were the 1950’s Fender Stratocaster. They were wax and foil. It would be really nice to have 100v caps just for physical size but it will not make a sonic difference since the signal strengths might max out at 150mv for passive pickups. None of the caps were tested using active pickup electronics. (An insightful article in Premier Guitar discusses capacitor values, voltage ranges, and options beyond what come stock in most guitars, and the reasons why you may want to try smaller caps anyway.)
Initial test in amplifiers show very little difference between a NOS Vitamin Q poly-oil-foil and the Ampohm wax and foil. My suspicion is that amp builders will likely want to use the Paper In Wax caps in the tone shaping stages of the amp and switch to Paper In Oil / Copper Foil caps for the back end of the amp where you don’t want the tone to be changing at all. Oil and Copper foil provided the clearest most uncolored tone of all caps tested while the wax and Tin or Aluminum tend to warm up single coils and give them a really nice organic tone.
Guitars used (and unfortunately abused in one case)
Vintage Fender P-bass
Yamaha AES 820 (custom test guitar with humbuckers, single coils and hum cancelling single coil combinations)
Gibson Les Paul Special with overwound P90 pickups
Gibson Les Paul Standard with Classic 57 humbuckers