As printed in Premiere Guitar magazine: “Mallory caps have a tubular shape and yellow color. You can find these caps in a lot of amps, and also inside guitars as tone caps. These caps have a great fat and punchy sound, pronouncing the mid frequencies while keeping the treble alive. This is the ideal tone cap for rock guys, and a lot of our customers choose them for their Les Pauls and SGs. They also sound great inside Fender guitars, and Leo Fender put these caps inside all of his “Music Man” amps, manufactured exclusively for him by the Plessi company. This is a good overall tone cap, and a great place to start; it seems to work well with both humbuckers and single coils.”
Mustards are great polyester film/foil capacitors. Film/foil construction will always be better for audio applications than metalized film which is the more common cap construction these days. Mustard caps were the common cap that was available in the 1960s to mid 1970s
Consider the benefits of film/foil over metallized designs. The construction of film/foil requires aluminum foil and a dielectric film to be stacked, rolled, and wrapped. The leads are then welded directly to the foil. Due to the required thickness of the dielectric, film/foil capacitors can be recognized by their generally larger size over metallized types.
The electrode of metallized types are actually a very thin layer of metal which is sprayed directly onto a thin dielectric. The leads are welded onto the thin metal layer.
So, in comparison, the film/foil will generally be more robust in build and quality as well as in performance over the metallized type. Since its electrode is a solid sheet of foil with sizable surface area, the film/foil can withstand higher voltages and currents over the metallized type at the same frequency. Think of a heat sink. The larger the sink, the better it is at dissipating heat. The film/foil design maintains better stability under use, so the capacitance value does not travel as much over time, and it can handle faster duty cycles at higher voltages.