The 222-B was originally equipt with Pyramid IMP capacitors for interstage coupling applications. Pyramid IMP is a blend of Mylar and Paper dielectrics, known as "DiFilm". Difilm was a late 50's innovation which offered the reliability of Mylar, but with characteristics somewhere in between Mylar and Paper.
When rebuilding a Scott, you must take into consideration what type of capacitor dielectric was used (provided your goal is to retain the original sound characteristics).
The optimum replacement for the Pyramid IMP (or Ceracap) is the Mallory PVC series, or the Cornell-Dubilier WMF series (both Mylar film and foil types). The 222-B's circuitry is rather sensitive with regards to dielectric types, and one could say that the circuits were tuned around the limitations of Mylar.
The sensitivity increases with stage impedance. Here, the 222-B uses 12AX7's (high gain and high impedance dual triode), coupled to a 6BL8 paraphase phase inverter (inherently high impedance). Thus, substituting dielectrics in this particular model can vary the sound signature grossly. Polypropylene would be the worst choice, if harshness is to be avoided.
The statement above in reference to the Mallory PVC capacitors sounding overly "dull" is not a valid observation, as no two amplifier topologies will respond alike. Perhaps in some models the PVC series would sound dull, but not in the 222-B (or any Scott, for that matter).
For the 222-B, I'd suggest using only a Mylar replacement.
In reference to the capacitor types, below is a description:
-Mallory PVC Series - Mylar Film, comes in both 400 and 600V versions. The 400V version, is of course, less inductive and is ideal for H.H. Scott amplifiers.
-Cornell-Dubilier WMF Series - Another high quality Mylar Film capacitor that is THE direct replcement for Ceracaps, Black Beauties, and Pyramid IMP types.
-Cornell-Dubilier MET Series - Metallized Mylar construction. A very nice compromise to the two mentioned above.
-Russian K40Y9 - Mylar Film type in a hermetically sealed package. Comparable to the CD WMF series in sound. The 1KV version uses an oil impregnant as an insulator. These are high quality, good sounding, affordable capacitors.
The basic idea here is to preserve the original sound traits, as engineered by H.H. Scott. Essentially, the parts were chosen to deliver a flat frequency response within the audible range. Altering the construction of the parts changes certain circuit parameters (ESR, DF, DV/DT), and therefore, can alter the sound characteristics. Depending on the model, the characteristics may be subtle, or they may be severe. Whether the change is an improvement is in the "ear of the beholder".
The later Scott models (299-C, LK-72, 222-C, LK-48) used a different phase inverter topology (cathodyne), which is lower impedance. Here, using Polypropylene in place of Mylar yields subtle variations. The 12AX7 circuits remained the same, which means that using a Mylar capacitor will retain the original sound characteristics of the preamplifier stages.
Capacitors are not the only components that influence sound. Resistors also play a role. Like capacitors, resistors have certain electrical variables that influence sound (coefficient). Carbon film or carbon composition would work best for retaining the original sound, while metal oxide and similar types are typically of lower noise (but can sound overly "shrill").
