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palcal

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Joined: 12/10/2011
Location: Australia
Posts: 1993

Posted: 01:22am 14 Sep 2019

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I am restoring a valve amplifier I built in 1967. I am replacing all the components
and there are a couple of phase correcting capacitors and the article stipulated Plastic or Ceramic NOT HIGH K. So how do I know what to use, are ordinary ceramic capacitors low K. I know that polystyrene are OK but hard to get. RS components have them but I have to buy 10 at nearly $3 each.

"It is better to be ignorant and ask a stupid question than to be plain Stupid and not ask at all"

Warpspeed
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Joined: 09/08/2007
Location: Australia
Posts: 4406

Posted: 02:21am 14 Sep 2019

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Hi K ceramic capacitors are made with ceramic that has a very high dielectric constant which makes the capacitor very small and low cost, but the value can be highly inaccurate and changes with temperature. They are used where the actual value is not really critical.
Ordinary ceramic capacitors of stable accurate value are often marked as NPO type.

Good plastic film types might be a better choice in the higher values, but these can sometimes become large and expensive.

Polyester are low cost general purpose, and good enough for most things, probably your best bet for audio. �They are usually dipped and often green or brown in colour.

Polystyrene are much higher quality, higher accuracy grade and lower leakage, good for instrumentation for example. Can be a really expensive, often silver in colour.

Polycarbonate are for high energy, high frequency and pulse applications, these too can be expensive in the larger sizes, but they are very robust, often yellow in colour.
Edited 2019-09-14 12:27 by Warpspeed

Cheers, �Tony.

plover

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Joined: 18/04/2013
Location: Australia
Posts: 306

Posted: 03:44am 14 Sep 2019

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If it is an Audio Amplifier and especially in the signal path as I understand the ceramics have an impressive high noise factor and are very microphonic. If you have an Oscilloscope you can hook up a cerammic and give it a tap and see what happens.

I also think the polarity is important, "ground" side should probably go to the lowest voltage point.

palcal

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Joined: 12/10/2011
Location: Australia
Posts: 1993

Posted: 05:15am 14 Sep 2019

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I removed the existing polystyrene caps and although they are over 50 years old they measure to be very accurate to their value so I think I will reuse them.

"It is better to be ignorant and ask a stupid question than to be plain Stupid and not ask at all"

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