.33uF polyester cap suitable? - UK Vintage Radio Repair and Restoration Discussion Forum

Bufo Bill · 25th Oct 2020, 5:55 pm

Hi, I need to replace C30 in the circuit of an Acoustical (Quad) QCII see image below.
Will a 0.33uF polyester 200V cap be suitable? I have a Mica capacitor in that spec, but I seem to remember being told that film caps are best for audio purposes?
Thanks from Bill.

paulsherwin · 25th Oct 2020, 5:58 pm

Are you sure you have a 0.33uF mica cap? That is a very big value for mica.

Polyester should be fine.

bikerhifinut · 25th Oct 2020, 6:10 pm

I'd agree with Paul that seems large for silver mica but if you have one in a suitable value and voltage rating it will be fine in an audio coupling application, after all the phono stage is probably littered with them.
its a 0.25 uF on the circuit so either a 0.22 or 0.33 uF will be fine there as its the output coupling capacitor.
0.22 being the modern E6 preferred value equivalent of the 0.25.

I take it the original is still in and is a leaky old PIO?

Andy.

Maarten · 25th Oct 2020, 6:13 pm

If it looks like a mica and really is 330n, it's a paper capacitor and should not be used.

Bufo Bill · 25th Oct 2020, 6:17 pm

Thanks guys.
Andy, this is my second replacement, I b******d the first with a badly positioned soldering iron. The original was indeed paper in oil.
Cheers both, Bill.

Bufo Bill · 25th Oct 2020, 6:19 pm

MAARTEN, I will keep on the safe side and use a polyester.
Cheers from Bill.

Bufo Bill · 25th Oct 2020, 6:28 pm

Just had a proper look under a work light, its PICOFarad!
Cheers from Bill

Maarten · 27th Oct 2020, 12:54 pm

In that case I would doubt it's even defective, but a ceramic capacitor would do the job.

bikerhifinut · 27th Oct 2020, 3:07 pm

I wouldn't use a ceramic capacitor as an output coupling capacitor or indeed anywhere in the signal chain of a high quality audio amp., which the Quad most certainly is. Full stop.
Stick with a polywotsit film cap Bill, you can't go wrong.

Andy.

Maarten · 28th Oct 2020, 12:46 pm

Ah, I missed that it was in the signal chain. Ceramic can be reasonable good, especially if the signal is small compared to the bias, but it can still introduce some distortion.

Radio Wrangler · 28th Oct 2020, 3:47 pm

Some ceramic capacitors have all sorts of nasty behaviours... some are even microphonic, but other types are perfectly fine. C0G and NP0 ceramics are OK, though they're only generally available up to about 2200pF.

Mylar film capacitors suffer from dielectric absorption. You can charge one up, leave it for a while, discharge it, and then watch it charge itself back up! weird! Once you see this antic you think "I'm not putting my audio anywhere near one of those!" but they're used often enough and no-one seems the wiser. DC blocking capacitors are used to maintain a constant amount of charge, so such things shouldn't show up anyway.

Anyway, 0.33uF polyester ought to be fine.

The deeper you go into any component, you find imperfections of various sorts and then you have to decide which imperfections to accept. Perfectionists go stark raving bonkers at this point. Audiophilia Nervosa.

David

25th Oct 2020, 5:55 pm	#1
Bufo Bill Heptode Join Date: Jan 2016 Location: Wolverhampton, West Midlands, UK. Posts: 777	.33uF polyester cap suitable? Hi, I need to replace C30 in the circuit of an Acoustical (Quad) QCII see image below. Will a 0.33uF polyester 200V cap be suitable? I have a Mica capacitor in that spec, but I seem to remember being told that film caps are best for audio purposes? Thanks from Bill. Attached Thumbnails

25th Oct 2020, 5:58 pm	#2
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,943	Re: .33uF polyester cap suitable? Are you sure you have a 0.33uF mica cap? That is a very big value for mica. Polyester should be fine.

25th Oct 2020, 6:10 pm	#3
bikerhifinut Octode Join Date: Nov 2013 Location: Penrith, Cumbria, UK Posts: 1,993	Re: .33uF polyester cap suitable? I'd agree with Paul that seems large for silver mica but if you have one in a suitable value and voltage rating it will be fine in an audio coupling application, after all the phono stage is probably littered with them. its a 0.25 uF on the circuit so either a 0.22 or 0.33 uF will be fine there as its the output coupling capacitor. 0.22 being the modern E6 preferred value equivalent of the 0.25. I take it the original is still in and is a leaky old PIO? Andy.

25th Oct 2020, 6:13 pm	#4
Maarten Dekatron Join Date: Dec 2007 Location: Haarlem, Netherlands Posts: 4,199	Re: .33uF polyester cap suitable? If it looks like a mica and really is 330n, it's a paper capacitor and should not be used.

25th Oct 2020, 6:17 pm	#5
Bufo Bill Heptode Join Date: Jan 2016 Location: Wolverhampton, West Midlands, UK. Posts: 777	Re: .33uF polyester cap suitable? Thanks guys. Andy, this is my second replacement, I b******d the first with a badly positioned soldering iron. The original was indeed paper in oil. Cheers both, Bill.

25th Oct 2020, 6:19 pm	#6
Bufo Bill Heptode Join Date: Jan 2016 Location: Wolverhampton, West Midlands, UK. Posts: 777	Re: .33uF polyester cap suitable? MAARTEN, I will keep on the safe side and use a polyester. Cheers from Bill.

25th Oct 2020, 6:28 pm	#7
Bufo Bill Heptode Join Date: Jan 2016 Location: Wolverhampton, West Midlands, UK. Posts: 777	Re: .33uF polyester cap suitable? Just had a proper look under a work light, its PICOFarad! Cheers from Bill

27th Oct 2020, 12:54 pm	#8
Maarten Dekatron Join Date: Dec 2007 Location: Haarlem, Netherlands Posts: 4,199	Re: .33uF polyester cap suitable? In that case I would doubt it's even defective, but a ceramic capacitor would do the job.

27th Oct 2020, 3:07 pm	#9
bikerhifinut Octode Join Date: Nov 2013 Location: Penrith, Cumbria, UK Posts: 1,993	Re: .33uF polyester cap suitable? I wouldn't use a ceramic capacitor as an output coupling capacitor or indeed anywhere in the signal chain of a high quality audio amp., which the Quad most certainly is. Full stop. Stick with a polywotsit film cap Bill, you can't go wrong. Andy.

28th Oct 2020, 12:46 pm	#10
Maarten Dekatron Join Date: Dec 2007 Location: Haarlem, Netherlands Posts: 4,199	Re: .33uF polyester cap suitable? Ah, I missed that it was in the signal chain. Ceramic can be reasonable good, especially if the signal is small compared to the bias, but it can still introduce some distortion.

28th Oct 2020, 3:47 pm	#11
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,868	Re: .33uF polyester cap suitable? Some ceramic capacitors have all sorts of nasty behaviours... some are even microphonic, but other types are perfectly fine. C0G and NP0 ceramics are OK, though they're only generally available up to about 2200pF. Mylar film capacitors suffer from dielectric absorption. You can charge one up, leave it for a while, discharge it, and then watch it charge itself back up! weird! Once you see this antic you think "I'm not putting my audio anywhere near one of those!" but they're used often enough and no-one seems the wiser. DC blocking capacitors are used to maintain a constant amount of charge, so such things shouldn't show up anyway. Anyway, 0.33uF polyester ought to be fine. The deeper you go into any component, you find imperfections of various sorts and then you have to decide which imperfections to accept. Perfectionists go stark raving bonkers at this point. Audiophilia Nervosa. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done