See "Valve amplifiers 3" by M. Jones, pages 211-229, especially Fig. 4.4 - various capacitors types, and the quality of them. The air plate caps are the more nearly perfect, with electrolytic Al-foil/bipolar at the "other side".
Then citation: "Possible audio uses (of air dielectric, metal plate) include:
- ~300pF variable capacitor across the input of a MM RIAA stage to allow cartridges to be optimally loaded by the pre-amplifier
- ~50 pF for trimming equalization capacitors to their exact value"And later, author uses just that - 40 pF variable cap for "trimming" the exact value of RIAA EQ cap in his EC8010 RIAA preamp, Fig. 7.37, page 568.
And please, calm down your agressive tone, one of the reasons why are all here is to avoid "rubbish" from some other forums... 
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