Old sprague capacitors - polarized or not? - Page 1
Forum section: electronics / beginners

author: Grey_Dawn_Breaking
author_rank: Newbie
timestamp: July 30, 2025, 07:39:01 pm
content: Hi all, I'm looking to replace these old caps from a system dating from the 1960s-70s and I feel like I'm going crazy.Are they polarised or not? Please see the schematic pic for where the 3 caps are.I was wondering if these film radial Cornell Dubilier Knowles caps rated for 0.1uF 200V DC would work:https://www.digikey.ca/short/qhr4rf41I've seen conflicting symbols online and I feel like I'm going nuts

author: Gyro
author_rank: Super Contributor
timestamp: July 30, 2025, 08:24:09 pm
content: With a value of 0.1uF @ 200V I would say they are certainly non-polarized, especially looking at theapplicationcircuit configuration. They are also hermetically sealed (probably paper) so I would be very surprised if there is anything wrong with them, even after that number of years.Yes, the caps you listed would be suitable replacements - if the value is small enough that they are available non-polarized then it doesn't matter if the originals were polarized or not.Old cap symbols can be a pain (especially US ones), sometimes the asymmetric symbol indicates polarized electrolytic (although they would almost certainly also include a '+' sign), sometimes it indicates the outer foil (where relevant, eg RF or high impedance) and sometimes it's just because!

author: TimFox
author_rank: Super Contributor
timestamp: July 30, 2025, 08:50:13 pm
content: When those metal-can hermetically-sealed parts were popular, it was well-known that the curved side indicated the metal case, even though the part was not polarized.It was less common to find a metal case with two insulated terminals, not to be confused with multiple capacitors sharing a common metal “bathtub”.Obviously, if one side of the capacitor is grounded, it should be the metal can.In WW II military equipment, the hermetic paper capacitors did not fail due to moisture, while similar units with wax coating or later ones with plastic shells often developed excessive leakage. In a receiver, the screen bypass caps often developed high leakage, reducing the screen voltage (fed through a large resistor), reducing the gain of the pentode amplifier.

author: factory
author_rank: Super Contributor
timestamp: July 30, 2025, 08:59:15 pm
content: They are Sprague 118P, a hermetic sealed paper/film type.https://www.technicalaudio.com/pdf/Electronics_Catalog_Extracts/Sprague_capacitors_1966_REM_E30.pdfDavid

author: TimFox
author_rank: Super Contributor
timestamp: July 30, 2025, 09:14:22 pm
content: In the full Sprague catalog linked, roughly the first half of the pages are tantalum or aluminum electrolytics with metal cans (polarized), followed by paper, oil-filled paper, metallized paper, paper/film, etc. in metal cans (non-polarized). Past that are plastic-cased paper and film units.When I fixed a Tektronix 130 L,C meter, I had to replace all of the similar plastic-cased parts (which had very lowQ), using modern polypropylene axial-lead parts.

author: Grey_Dawn_Breaking
author_rank: Newbie
timestamp: July 30, 2025, 11:00:58 pm
content: Thanks all. I'm glad to hear that my confusion wasn't just a "me" issue...I was seriously starting to doubt reality.I was going to put in a new thyristor and diac in that circuit already, and since I have those 3 new caps (the Cornell Dubilier Knowles') on-hand I might just go ahead and do those too since I'm in there anyway.

author: Konkedout
author_rank: Frequent Contributor
timestamp: July 31, 2025, 04:24:43 am
content: If these indeed are paper type (for example) and not any sort of electrolytic:Many wound foil (or metallized) type capacitors used to have one terminal marked "outer foil". In theory, if doing audio work, it was better to connect the outer foil to ground or to whatever might be the less-sensitive point in the circuit. Many axial wound Cornell Dubilier capacitors had a bar printed towards one end to indicate outer foil.

author: TimFox
author_rank: Super Contributor
timestamp: July 31, 2025, 04:39:06 am
content: Quote from: Konkedout on July 31, 2025, 04:24:43 amThose outer foil marks are now rare. If not grounded, I prefer to connect the outer foil to the lower DC voltage for safety, rather to the less-sensitive point.For example, in a tube amp a 100 nF capacitor would couple the plate of V1 at 150 V DC to the grid of V2 at 0 V DC. Many would consider the plate to be “less sensitive” ifRp= 10 k, but the grid resistor = 470 k, but there is 100 nF from the grid to that plate.Therefore, my preference is outer foil to grid in that case (lower DC voltage).

author: Grey_Dawn_Breaking
author_rank: Newbie
timestamp: July 31, 2025, 01:55:55 pm
content: Re: the outer foil and audio work - good to know for future work! A subtle but important detail to know that I wasn't aware of.In the current particular case it's a motor circuit, so I don't think it matters to this extent.