3
\$\begingroup\$

I realize that there is along history to mica capacitors; however, I've been unable to determine if there any modern equivalents that have the same static electronic characteristics. The short list of types that seem like they'd have similar behavior and properties over time are silicon capacitors, ceramic capacitors, film capacitors; however, I've not found a rigorous comparison anywhere.

I'm trying to redo an old amplifier with modern parts as the original parts are no longer available.

\$\endgroup\$
  • 1
    \$\begingroup\$ Old schematics often specified Mica caps because the alternative styroflex caps didn't go beyond 80°C. That's all stuff of the past. Polypropylene does and is a good alternative for most cases. Ceramic caps are for decoupling and HF. \$\endgroup\$ – Janka Jan 11 '18 at 22:21
  • \$\begingroup\$ Are the original mica capacitors missing? If not, do you have any reason to suspect they are faulty? If it ain't broke, don't fix it ;) \$\endgroup\$ – Andrew Morton Jan 11 '18 at 22:24
  • \$\begingroup\$ I had the impression the OP wants to create a copy of an existing (tube) amplifier. \$\endgroup\$ – Janka Jan 11 '18 at 22:25
  • 1
    \$\begingroup\$ @Janka I was sort-of wondering about that. Is "redo" recreate or restore? \$\endgroup\$ – Andrew Morton Jan 11 '18 at 22:26
  • \$\begingroup\$ What are the parasitic requirements you need? Like ESR or leakage current? \$\endgroup\$ – Voltage Spike Jan 11 '18 at 22:53
4
\$\begingroup\$

Don't worry about the internal makeup of the capacitor. Instead, figure out what you really need the capacitor to do. Once you have such a spec, then just look around for a cap that meets them. If you've specified it properly, then you won't really care if the dielectric is mylar, polypropylene, mica, or something else.

The two most important specs are the capacitance and the voltage. Those alone will push you towards certain technologies.

Since you mentioned audio, you need to get more specific for anything in the signal path. Many types of ceramic have some piezo effect, so aren't good for audio. Vibrations and mechanical shock on the device can end up in the signal. High-capacitance ceramics are also non-linear.

If you only need up to 10 nF or so, but have to stand off high voltage (like in old tube amplifiers), then mylar or even old paper types would work fine.

If this is just to filter out noise, like across a power supply, then the cheaper and smaller ceramics might be fine.

In short, this process starts with good specs. Without them, you're just poking around in the dark.

\$\endgroup\$
0
\$\begingroup\$

( 50V to 30kV) Silver Mica caps still exist and the suitable alternative caps for high voltage include PTFE and for RF , NP0 Ceramic which have better tolerance options.

https://www.digikey.com/products/en/capacitors/mica-and-ptfe-capacitors/64?k=&pkeyword=&pv1989=0&FV=ffe00040&mnonly=0&ColumnSort=0&page=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=25

Although the 30kV mica cap was 1000 pf 5% , with 6 in stock , it was also $1120.50 ! designed for military high voltage RF AM transmitters like 10 amps at 2MHz

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.