Choice of small value caps

[ted01]

Hey all,

I know the subject of caps has been beat to death, but I can't seem to find much in the archives on small value caps (<1000 pF). OD's don't go down that small, and I'm hesitant to use ceramic disks due to something I read in Morgan Jones book, Valve Amplifiers. He says that ceramics have no place in audio circuits. Now, that's his opinion. I'm wondering what you use for small values. I'd probably get a handful of each, based on your ideas, and find out for myself. Problem is, I'm just not finding a whole lot of options in the sub 1000 pF range in the ol' Mouser catalog.

Thanks!
Ted

[Firestorm]

Below 1000pfs you're mostly looking at ceramics or silver micas. If it's a snubber or a shunt circuit use whatever is cheapest. If it's carrying a signal, I always liked the SMs, but opinions differ. You can even mix and match. BTW, plenty of silver micas available down to 10pf, maybe lower.

[lastwinj]

you can find polystyrene as well. Ebay has some selection. every once in a while, you can find some small value polypropylene or polyester caps.

what are you needing?

you can also run 2 lower voltage caps in series to get a higher voltage, but smaller value I.E. 2 1000pf 250v caps in series wold be 500pf at 500v.


germ

[ted01]

what are you needing?

Well, what I'm working on now has a couple of 330 pF caps and a 390 pF cap. I'd like to have a selection in the house, as it is a major event getting into town to the electronics store. It'd be nice to have about a dozen values from 10 pF to 1000 pF.

Ted

[David Root]

MIAL blue polystyrenes I think have nice tone. www.surplussales.com has them. I believe MIAL is an Italian company.

[krash]

2 1000pf 250v caps in series wold be 500pf at 500v.
germ

Not really. Maybe at DC you can presume they will be 500V.

But if you expect a high voltage AC signal (which is usually NOT the case when using a cap in the signal chain of a tube amp), then you have to take into account the tolerance of the cap and the fact that it may not distribute an AC voltage evenly across the two caps. This is kind of moot to this discussion but if you were, say, building a switchmode power supply and needed a snubber cap then you'd not want to count on more than maybe +25% voltage by doubling them... so two 250V caps in series would be 313V.

Again it's basically a moot point because ceramic caps below 1000pF are usually rated plenty high for tube amps, even in single caps. I use a 2200pF 1.5KV cap in some of my amps and it's like 2 cents from Mouser.

Back onto this topic, I don't think there's anything wrong with a ceramic cap in the audio signal path in a tube amp. Where are you going to use a sub-1000pF cap in the signal path? Tone controls most likely, treble cap, or bright cap. I have even heard of Ken Fischer alternately using film caps and ceramic caps for the .001uF coupling cap in 'wreck amps into the clipping stage, and some cloners finding very little if any noticeable difference in the sound of these parts when used in this circuit.

FWIW I use a ceramic cap for the tone control on all of my amps. I also use a ceramic cap for shunt resistors across plate resistors in some places. I have a heck of a time finding 2200pF film caps but I manage to root them out, that's the smallest signal path cap in any of my amps.

[mhuss]

I would not use a ceramic as a coupler in a hi-fi amp (which Mr. Jones is speaking to). Even Knight and Dynaco didn't generally use ceramics for audio path work back in the day.

However, most old Marshalls use ceramic for 220 to 470pF couplers and bypass caps -- and I think most of us can agree those don't sound horrible!

A guitar amp is not a hi-fi amp, and every weird distortion and coloring that any of its components might add to the stew is part of the overall flavor.

--mark

[roberto]

Ceramic are not linear, and this could be used to "marshallize" the sound, specially if you go near the declared voltage. For a more linear sound I use Wima caps. SM usually for cleans (or smooth leads) and only if the signal is passing throught, not for grounding or anode bypass.

[Andy Le Blanc]

on the other hand..... some lower dollar amps were built using only
ceramic caps..... it makes sence to go for the bottom dollar
they can still be sourced for as little as .11 each... even less
my ears like SM caps.....in the path......ceramic to ground... non-path application too
but it depends on your ears and the sound your shooting for
cap type can radicaly alter tone...... dont go too far with the voltage rating...
that can effect tone too.... many older amps had 400v rated caps where today
the trend is for 600v..... you can also pair caps thru the signal path
put a low value cap , your favorite type, with the larger coupling cap
to color the detail of the top

[Dai H.]

However, most old Marshalls use ceramic for 220 to 470pF couplers and bypass caps -- and I think most of us can agree those don't sound horrible!

I've never tried counting but old Marshalls had a mix of ceramic and mica for bright caps and the treble cap as well as the 47pF in the PI.

A guitar amp is not a hi-fi amp, and every weird distortion and coloring that any of its components might add to the stew is part of the overall flavor. Mr. Green

I agree. I think it's best to think of parts as relative. Unless it's dangerous or something like that there is a lot that goes.