different type caps sound different?

[thehoj]

I try to avoid them too, but the amp I'm working on right now is AC30 inspired, and that has a 47pF cap in the tone stack. Not being able to find exactly what I wanted, I ended up paralleling in a 36pF "good" Silver Mica with a 20pF ceramic. Curious as to how that will work out. The "good" SM might very well go bad in this amp too.

Out of curiosity, is there a reason you wouldn't be interested in the Polystyrene Film caps of that value?
I recently picked up some 47pf polystyrene caps (haven't used them yet) but I will be shortly, and I'm curious to see how they'll work. Generally I have always used silver mica's for that range of values..

[Rhodesplyr]

There seems to be general agreement that different type caps (ceramic vs polypropelene vs polystyrene vs paper-in-oil etc.) sound different.

What is the physics of this? (In advanced layman terms, not Feynman terms )

Because people have strong opinions about this, let me first say that I am not offering this as anything resembling a statement of positively known fact. I'm simply repeating something I've heard that struck me as an interesting idea.

I have heard it speculated that at least some of the occasionally perceived differences among individual capacitors--not general groups or types--has to do with the integrity of the contact between the lead and the foil or plate.

Please note the very conscious use of the word 'occasionally' above.

[Rhodesplyr]

While we're at it: Is there anything we can do to this lot to make them more reliable? Cook them, leave them in low voltage for a month, etc? They sound great, it'd be nice if we could save them.

Paper capacitors of any type have many enemies: the pH of the wood pulp used to make the paper, the unwanted presence of tiny metallic particles in the paper from the machinery used to make it, the stability of the chemical impregnant used (different manufacturers had their own proprietary recipes), problems with the foil/lead connection, and (#1) moisture. It's apparently impossible to drive all the moisture from paper without destroying it in the process (or was in the 1950s). And paper is very hydrophilic--it likes to absorb water.

Ever wonder why paper capacitors tend to increase in measured capacitance with age? Water has a very high dielectric constant. Acid in the paper, combined with moisture and the presence of an electrical charge can cause several nasty things to happen--like migration of dissolved metal molecules through the dielectric. In literature from the 1950s, it's openly discussed that a major problem for capacitor developers was their difficulty in predicting how their formulations were going to hold up over time. They were making progress--the last generations of paper capacitors were much improved--but the introduction of mylar film spelled the end for most paper capacitors in commercial products.

Why do I know all this? I wanted to put an end to all the snake-oil claims made about the original capacitors on Hammond Organ tone generators. No, they're no longer working like they were when new (as part of a series-resonant filter)--nor were they anything special, but, if you like they way they sound, leave 'em. A certain dealer buys generic Panasonic mylar caps and charges about ten times for what you'd pay for the same thing via Mouser or Digikey for a "matched set" to recap the generators--neglecting to tell customers that the capacitance value really needs to be to matched to the individual inductor.

[Steve Conner]

Out of curiosity, is there a reason you wouldn't be interested in the Polystyrene Film caps of that value?

The ones that look like a tiny roll of clear plastic film with silver foil inside? I like those, I've always used them in tube circuits in preference to ceramic or silver mica.

the integrity of the contact between the lead and the foil or plate

That is a good point, in many older capacitor designs the lead was simply stuffed into the foil and the whole thing rolled up, relying on pressure to make a contact. I think that's how the above mentioned polystyrene caps are made.

But a bad contact here, I'd class that as a fault, even if it makes the amp or guitar sound good somehow. I guess it would cause high ESR.

I also agree with the comments about paper being hygroscopic. I think people are crazy for still wanting to use paper caps, and I can't believe they pay good money for NOS ones.

[thehoj]

The ones that look like a tiny roll of clear plastic film with silver foil inside? I like those, I've always used them in tube circuits in preference to ceramic or silver mica.

Yea. It just seemed like they'd be a good alternative to silver mica or ceramic.
I hadn't realized they were available in such low values until recently.

[Groover]

A tech put one of those in my Tweed Princeton tone circuit. I didn't know what kind of cap it was until I read the above post, so thanks.

[Rhodesplyr]

I also agree with the comments about paper being hygroscopic. I think people are crazy for still wanting to use paper caps, and I can't believe they pay good money for NOS ones.

I'm glad you agree that paper is hygroscopic. I'd have had to Fedex you a roll of paper towels otherwise ;-)

The paper capacitors sealed in metal tubes are not too failure-prone, and my 1960 Scott 299 integrated amp still has some plastic encapsulated ones (Pyramid IMP brand) in its preamp section. I checked them recently when I had the amp open for maintenance. Still no leakage. But the wax-dipped? Forget it.

What's less well known is that mylar/polyester is also hygroscopic--it's 50% as hygroscopic as paper. In fact, when I mentioned to a grad student in materials engineering that polypropylene, polystyrene, and teflon were well-regarded dielectrics, he noted that they were all hydrophobic to some degree. In practice, I have had to replace a few mylar capacitors--not many but a few. I don't think I've run across a failed polypropylene or polystyrene capacitor yet.

I read one evaluation of picofarad value capacitors in which the polystyrene capacitors tested had the lowest measured distortion at audio frequencies. It's major drawback is its low melting point--it can be easily damaged by incautious soldering. Mica and C0G ceramic discs weren't too far behind, tied for second place, but both these types are more rugged--and have advantages at radio frequencies since they aren't wound (less inductance).

What I don't entirely understand is the way the markets for C0G/NP0 ceramics and mica have diverged. Cornell Dubilier is still producing mica capacitors in very robust (and pricey) packages, while nicely made C0G types like the Cera-mite line seem to be disappearing--or were the last time I checked.

[tedmich]

What I don't entirely understand is the way the markets for C0G/NP0 ceramics and mica have diverged. Cornell Dubilier is still producing mica capacitors in very robust (and pricey) packages, while nicely made C0G types like the Cera-mite line seem to be disappearing--or were the last time I checked.

Mouser still stocks over 1000 values of leaded COG / NPO caps! And they age quite well so NOS is AOK