Replacing mica capacitors - best modern equivalents? - UK Vintage Radio Repair and Restoration Discussion Forum

Voxophone · 15th Sep 2016, 3:22 pm

A pre-war valve set I’m currently restoring has a number of very fragile mica capacitors of a type without any wax or moulded encapsulation. Essentially they appear to be metal strips with a mica wafer held in place by a thin paper sleeve.

One way or another I’ve managed to damage at least two of them: a 5pF MW aerial trimmer and a 230pF IF bypass. The 75pF oscillator control grid cap may also be suspect.

My question is what type of modern capacitors would be best suited to replace these? Would it be best to go with silver mica as per the original design or would ceramic be adequate? I suspect the IF bypass would be more forgiving in than the aerial trimmer.

Obviously some level of realignment will be necessary.

Cheers

Liam

MrBungle · 15th Sep 2016, 3:25 pm

I'd go for modern cheap NP0 ceramics if the voltage isn't too high. CPC sell them in packs of 10 for about £0.15 to £0.40 a pack.

Silver mica if you need higher voltage.

David G4EBT · 15th Sep 2016, 3:47 pm

Silver mica were originally used for their close tolerance and stability, especially in tuned circuits, and whilst today, other types of caps will often suffice, why not simply replace then with modern silver mica 1% ones? There are countless listings of 1% silver mica caps on ebay, many of them mil spec, or if you prefer to buy from a large well known UK company, the likes of Farnell do them:

http://uk.farnell.com/silver-mica-capacitors

Good luck with the restoration Liam.

Nicklyons2 · 15th Sep 2016, 8:39 pm

I agree with David; best replacement for silver-mica is silver-mica

kalee20 · 15th Sep 2016, 10:22 pm

I've used polystyrene capacitors successfully, though you do have to watch the voltage rating.

The oscillator control grid capacitor should not be at all critical.

Obviously, capacitors forming part of a tuned circuit will be critical though. Agree silver mica would probably be best.

Occasionally, temperature coefficient may be important, to compensate for an opposite tempco in a coil, but that's something you will have to find out the hard way.

Voxophone · 16th Sep 2016, 3:28 pm

Thanks all. I ended up ordering silver micas from RS. Hopefully they'll do the trick.

15th Sep 2016, 3:22 pm	#1
Voxophone Hexode Join Date: Oct 2015 Location: Sunderland, Tyne and Wear, UK. Posts: 340	Replacing mica capacitors - best modern equivalents? A pre-war valve set I’m currently restoring has a number of very fragile mica capacitors of a type without any wax or moulded encapsulation. Essentially they appear to be metal strips with a mica wafer held in place by a thin paper sleeve. One way or another I’ve managed to damage at least two of them: a 5pF MW aerial trimmer and a 230pF IF bypass. The 75pF oscillator control grid cap may also be suspect. My question is what type of modern capacitors would be best suited to replace these? Would it be best to go with silver mica as per the original design or would ceramic be adequate? I suspect the IF bypass would be more forgiving in than the aerial trimmer. Obviously some level of realignment will be necessary. Cheers Liam

15th Sep 2016, 3:25 pm	#2
MrBungle Dekatron Join Date: Jun 2016 Location: London, UK. Posts: 3,687	Re: Replacing mica capacitors - best modern equivalents? I'd go for modern cheap NP0 ceramics if the voltage isn't too high. CPC sell them in packs of 10 for about £0.15 to £0.40 a pack. Silver mica if you need higher voltage.

15th Sep 2016, 3:47 pm	#3
David G4EBT Dekatron Join Date: Feb 2003 Location: Cottingham, East Yorkshire, UK. Posts: 5,761	Re: Replacing mica capacitors - best modern equivalents? Silver mica were originally used for their close tolerance and stability, especially in tuned circuits, and whilst today, other types of caps will often suffice, why not simply replace then with modern silver mica 1% ones? There are countless listings of 1% silver mica caps on ebay, many of them mil spec, or if you prefer to buy from a large well known UK company, the likes of Farnell do them: http://uk.farnell.com/silver-mica-capacitors Good luck with the restoration Liam. __________________ David. BVWS Member. G-QRP Club member 1339.

15th Sep 2016, 8:39 pm	#4
Nicklyons2 Octode Join Date: Jan 2013 Location: Barnsley, South Yorkshire, UK. Posts: 1,517	Re: Replacing mica capacitors - best modern equivalents? I agree with David; best replacement for silver-mica is silver-mica

15th Sep 2016, 10:22 pm	#5
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,081	Re: Replacing mica capacitors - best modern equivalents? I've used polystyrene capacitors successfully, though you do have to watch the voltage rating. The oscillator control grid capacitor should not be at all critical. Obviously, capacitors forming part of a tuned circuit will be critical though. Agree silver mica would probably be best. Occasionally, temperature coefficient may be important, to compensate for an opposite tempco in a coil, but that's something you will have to find out the hard way.

16th Sep 2016, 3:28 pm	#6
Voxophone Hexode Join Date: Oct 2015 Location: Sunderland, Tyne and Wear, UK. Posts: 340	Re: Replacing mica capacitors - best modern equivalents? Thanks all. I ended up ordering silver micas from RS. Hopefully they'll do the trick.