Ceramic Capacitors - Where can I use them?

[bobskie]

Hi everyone I have a Bush AC 34 which I'm currently restoring and I was wondering which caps would be happily replaced with ceramics. I read something about distortion caused by these ceramics and was wondering if someone could tell me which ones I definitely should NOT replace with ceramics. I have a huge amount of ceramics I received from a friend and many have the correct values but I'm not sure where they can and cannot be used. This is my first radio restoration and my first dealings with electronics quite this old so it's all new to me. I tried to link to the manual but for some reason it just got started out and wouldn't work so hopefully you can find it!
Thanks for your help.

[Cobaltblue]

Hi Bobskie

The reason the link doesn't work is because your posting a link to a web site that has free circuit information.

http://www.vintage-radio.net/rules.html rule B8

This site is partially supported by the sale of service data.

You can however post relevant snippets.

Cheers

Mike T

[bobskie]

Ok read the rule i missed that before, I'm sure you can all find the PDF.

[paulsherwin]

I don't really understand what you mean when you say ceramic caps cause distortion. Different cap technologies sometimes have slightly different operational characteristics, but these will be completely inaudible in a piece of equipment like this. You can use ceramic caps anywhere if the values are correct and the voltage ratings are adequate, the only exceptions being mains related caps where class Y or X2 caps are required.

Ceramic caps aren't routinely used in radio restoration, but that's because they cost more than plastic film caps and aren't easily available in all the required values.

[bobskie]

Some people seem to say they oscillate causing an audible hum when used in audio circuits but I'm just quoting what i have read. I at first assumed they would be fine but reading into it more this is what people seem to say.

[paulsherwin]

This sounds like audiophool BS from a hifi forum. You will find all sorts of crackpot ideas in such places.

[bobskie]

Haha probably is I just wanted to make sure, I'll get those put in soon and see what others have to say. Thanks for your help!

[Boater Sam]

If you are thinking of replacing the silver mica waxed caps with anything-------DON'T.
you will wreck the tuning and alignment, silver mica almost never go duff.

[paulsherwin]

You don't normally need to change any cap with a value less than 0.001uF. Such caps are almost always mica or ceramic and rarely fail, as Sam says.

[bobskie]

I knew about the mica ones not usually going bad, what if they are under 0.001uF but are paper? Or does that never happen? Thanks.

[paulsherwin]

It almost never happens. I don't think I've ever come across one.

[G8HQP Dave]

High permittivity ceramic capacitors are quite nonlinear (and often piezoelectric too). They can be used as decoupling capacitors anywhere, and coupling caps in noncritical situations such as old wireless receivers or amateur radio gear. Best not to use them for hi-fi!

[paulsherwin]

Yes, I think this may be what the 'distortion' reference was about.

Philips used ceramic caps a lot in their early 60s designs before switching to plastic film.

[Radio Wrangler]

First off, there are several types of ceramic material that they make capacitors from. These give different tradeoffs between the amount of capacitance that can be packed into a given mechanical size of component versus its voltage ratings.

Some ceramic materials give very stable capacitance values over temperature variation but only really give you small capacitance values in typical packages and the usual voltage ratings. These types are good for use in tuned circuits, where otherwise temperature changes would put the set out of alignment. They are good replacements for mica types, but mica ones don't often go wrong. However world resources of mica are dwindling.
These materials are sometimes known as "NP0" or "C0G" though these are really temperature grades rather than names.

Other ceramic materials let you pack in a lot more capacitance at the expanse of changing more with temperature. Value-wise, they overlap with the range that plastic film capacitors have been used in, and into the range of smaller electrolytics. They aren't usually as stable as fil types. So the film types live on for the time being if precision is important.

Some of the very high density ceramics used to replace small electrolytic capacitors have very large temperature coefficients (but so did the electrolytic ones they can replace) but some of them are microphonic. Picking up vibrations and adding them electrically to the signals they are handling. They can also be quite non-linear too. In many places it isn't a problem, but you need to know about this if you're working with very small audio signals.

The hifi fraternity is full of old wives tales and all sorts of misinformation. Sometimes they've picked up something which is quit factual, but they've lost any sense of scale or knowledge of when such factors are or are not significant. Serious design engineers, the ones who really can do the mathematical analyses, keep well clear. The funny belief people are disproportionately noisy, so it's difficult for beginner to sort out real knowledge from counter-scientific lore.

David

[bobskie]

Well that's cleared that up, thanks everyone! I can see this forum will be very useful. I'll get started on the restoration and keep you all updated.

[Biggles]

One point to note is check the voltage rating of your ceramic capacitors as later ones sometimes are only rated up to about 50 volts. The voltage rating for some reason isn't always marked on small ceramics I have found.
Alan.

[Radio Wrangler]

The temperature coefficient doesn't get marked either in most cases. With surface ount oes you get no markings whatsoever.

The moral is to buy them from known sources and to keep them bagged separately with a thorough spec on a note in the bag.

David

[Diabolical Artificer]

I've heard that certain caps in a certain situation can emit a high pitched noise, not a hum due to the piezo effect. Also saw an article that purported that they can distort a signal, but your talking about an effect so small that you need an oscillator with a THD figure of 0.00001 or something daft to find it. Sorry can't link said article but it was as Paul says in connection with some audiophile nonsense.

Here's some informative video's https://www.youtube.com/watch?v=J0TjW_0xTJU https://www.youtube.com/watch?v=xlvqUts9H9c
https://www.youtube.com/watch?v=TDDoi70cxw0

Andy.

[bobskie]

Fortunately the caps I received all have voltages marked on the bags they are in, no temperature coefficients though. I have started to put them in now and should hopefully finish soon.

[Radio Wrangler]

Piezo electricity is a two way street. The ones which convert vibrational stress into electrical signals, also work to convert electrical signals into sound. Used in switch-mode PSUs they can get quite noticeable.

The distortion figures really depend on what you're doing with them. Voltage-dependent capacitance changes of over 10% are possible from the worst materials. Used as integrating capacitors in analogue to digital converters, these would ruin the performance of a DVM.

In RF applications they would cause intermod and crossmod problems if used with signal voltages across them.

If used for blocking DC in an audio signal, and large enough in value for the voltage across them to not change, they would give only small distortion BUT most of these materials tend to be poor for leakage current as well.

If used for power supply decoupling, and there isn't much ripple voltage across them, and the stage they are decoupling has a decent power supply rejection ratio, then they don't do much harm.

Their chief value is in replacing small electrolytic parts in non-critical applications, or where the last 0.01p must be saved.

You need to know these grades exist, so you don't buy any unaware of their nasty habits, and know where to not use them. They are quite commonly available.

Grades NP0 and G0G are nice, linear and stable. Use them in RF circuits and wherever they don't get too physically big.

Grade X7R is a useful decoupler grade. It has a poor temperature coefficient. Choose them for power rail decoupling when you want to fit more microfarads in than NP0 will do.

Then there are several extra high density materials with various designations. These fit a lot of capacitance into a small part. It's simplest to avoid these, or have a good think before using each one.

Different people have different views on capacitor choices. I really don't understand the audiophile theories. Some get a very Meldrewian "I don't believe it!" from me or induce rolling laughter. Sometimes they do get it right. I think I understand capacitors, though. I have to, for the day-job.

David