Tube Preamp Project: advise and discussion

[johngovan1234]

> Ceramics are as much as possible be avoided.

"Ceramic" covers a w-i-d-e range of stuff.

Think of a baked clay flower-pot versus the glaze on a fine dinner plate.

A 0.1uFd, even 0.01uFd, has historically been a "salted" ceramic, like X7R or Z5U, with funny adulterants to raise the value per unit of space. These high values tend to vary with voltage, temperature, and mechanical stress.

A 1,000pFd or smaller can be made with a "glass-like" ceramic which really is good as glass (but easier to make thin) and shows very-low variation. Under 1,000pFd there's no reason to use "salted" ceramics, the pure stuff is small and cheap. FWIW, small ceramics are good-enough for the Jensen 990, a very high-quality audio studio op-amp.

Thank you PRR for clarifications and corrections. I'll take note of this and continue to study more. I think my statement is too general about caps. A big enlightenment indeed.

[anotherjim]

I was finding the ceramic discs as the usual dull baked clay form - maximum 100v in NPO. Then a jump to at least 1kv with a shiny dipped coating. Nothing at all in between.
This 2kv YP5 type...
https://www.rapidonline.com/suntan-ts16003d221ksbpb0r-220pf-10-2kv-y5p-ceramic-disc-capacitor-08-1538
Replaced a Mica that should have been ok, but was doing a very good imitation of a conch shell to the ear as soon as things warmed up.
But anyway, here's an example Mica that should be perfect. Price isn't way huge, but is high compared to most passive components...
https://www.rapidonline.com/suntan-ts23002h221j3b000r-220pf-5-500v-silver-mica-capacitor-12-0057

[PRR]

> ceramic discs as the usual dull baked clay ........ 1kv with a shiny dipped coating.

The outside stuff is NOT the dielectric, just something to hide the raw guts.

The dielectric runs from NP0 (En Pee Zero) which is nearly perfect, to X7R which _I_ can sometimes hear in a preamp, to Z5U which is pretty dubious stuff.

Silver Mica is as good as it gets. However the page you linked shows a 100V 100pFd poly cap for a much lower price. Rapid's site discouraged me from finding a higher-V version of the same thing.

[anotherjim]

Yes, Rapids parametric search only works properly within types of cap, not all caps.
However, I just tried the page header search simply for "220pF capacitor", got all types of that value. Then got parametric selection on the sidebar. Picked all the HV options and found this...
https://www.rapidonline.com/wima-fkp2j002201d00hs-fkp2-220pf-2-5-630v-radial-polypropylene-capacitor-64-2491
Not an ideal package either for tag/turret or build-on-pots construction. Those Wima caps are usually supplied with very short legs.