Capacitor selection

[Bushbaby]

Hi All,
I hope you are having a good bank holiday.
I'm showing my naivety here, in building my amp clone the vast majority of capacitors I can see that have been used on early 70s models are silver mica. Is this an absolute must to use or can I use other forms of caps e.g. polyester, ceramic assuming that they are the correct value and voltage?

Best Regards

[peter scott]

Silver mica capacitors are delightfully stable and generally live for ever.

Peter

[Bushbaby]

Would it be possible to use others such as metal film as a replacement. Silver Mica caps are good but very expensive to buy.

[peter scott]

You need to check out the various pros and cons of all the different capacitor types. They are well documented. Thermal stability, breakdown voltage, leakage current, ESR, physical size, manufacturing cost and longevity etc.

Peter

[Murphyv310]

Hi.
There is no need to fit Silver Mica caps in an audio amplifier. Granted they are stable etc but fitting them to me is OTT. In a guitar amp that has rather limited frequency response and no NFB I'd be more worried about the circuit design than fitting caps that won't benefit a design that is to be honest not anything approaching HiFi.
Vishay polypropylene caps are pretty decent and I'd defy anyone to tell the difference audibly between SM caps any Poly ones, unless the audiofools brought their super sensitive, highly responsive, 100% balanced Lugs with them.

[ppppenguin]

Any modern plastic (polyester, polypropylene) dielectric caps of the right value and voltage rating will be 100% OK. For values below 1000p where plastic dielectric caps may not be readily available I'd use ceramic. NP0 or C0G types for stability.

I'm amazed that anyone was using silver mica caps in audio amplifers at all.

[Amie]

Hi Bush

If you do go for silver mica do make sure you also fit the diamond encrusted carry handles to the finished case too

[Kalee20]

I'm amazed that anyone was using silver mica caps in audio amplifers at all.

I have an audio amplifier with a SM capacitor fitted, it's 47pF for phase compensation from a voltage amplifier anode, to 0V!

Chosen for its voltage rating (and was to hand...)

So now need to take Amie's advice on use of single-crystal tertrahedral-lattice carbon...

[Bushbaby]

Thanks for all advice - the photos I have appear to show old Lemco round caps which I was all under the impression that they were silver mica. Am I wrong with this?

[ppppenguin]

When you say "round" do mean flat, disc shaped with both wires fairly close together? Those would be ceramic.

All the SM caps I've seen are rectangular, flat, with a wires coming out of each end, either radially or axially.