> I have a Yaesu FT-101E that uses an 82pf 1KV molded mica
capacitor to couple the 12BY7 to the 6JS6C finals tubes.
The rig switches a 1000pf 1KV dipped mica in parallel with
the 82pf when the rig is switched to 160 meters. Thes e
capacitors not only seve in the coupling role but also as B+
blocking caps.
Frank,
For any RF application mica's generally are more stable than
ceramics, although there are exceptions. I can purchase NP0
(it's zero, not "O") ceramics that are temperature stable
within limited component availability. If you look at
transmitting doorknobs like those used in traps, you will
see the larger the value the less stable the capacitor is.
For example N750's end at about 300pF in a doorknob, while
1000pF DK caps are often about N3300. This is why traps and
tank circuits drift when large value ceramic transmitting
doorknob caps are used.
Mica's also generally have lower equivalent series
resistance for similar construction and cost, so they don't
heat as much when required to carry current. Mica's are
generally multi-later and that means parallel sections
increase current carrying capacity. Of course there are
exceptions. I can buy multilayer ceramic chip capacitors now
that are superior to SM caps in many applications.
Back when the 101 was manufactured, temperature stable HV
wire lead ceramics were not easily found and they were
considerably more expensive than equal performance
silver-mica's.
73 Tom
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