I have an axial tubular vintage capacitor that I have removed from circuit. It is a claret colour with a white band at one end & is marked as follows;

630
0.1 +- 20%
200 VDC

Many non polar caps are sold without reference on them whether they are AC or DC. I presume that in that case it can be taken for granted they are AC only, by default.

So, when a capacitor is marked as above, with presumably a DC component, what does that say about the circuit it is in (in this case a valve signal generator)?

Does this sound like a vintage oil capacitor? If it is, what would be a modern replacement? Unfortunately, I needed to cut one axial lead exactly at the end of the cap to remove it, so I am unable to return it to the circuit.

Thanks.

Specialised caps can be designed with either AC or DC in mind, but caps used in vintage valve radios are a very general type and are used to for example block DC but pass AC.
Why have you removed the cap, is it leaky?.
I’m not a believer in just replacing caps because the general call is to “recap”.
Check, test and replace in the instance of reliability and safety, as one progresses in the “restoration” or repair.
JJ

The convential wisdom, on my reading of it, is to replace all vintage caps other than mica & ceramic, but I'm open to other views such as yours, this being my first venture into restoration.

I thought all caps blocked DC, so what does the 200V DC qualification mean for the 630V cap in my OP?

The conventional wisdom is as you describe it, and it stems from not wanting to visit sets for repair twice, or more. A commercial imperative as much as a nod to efficient work procedure.

All caps do block DC, but some caps -- such as electrolytics -- are destroyed by AC.

I don't know what particular capacitor you have there, but 200 volts is probably the working voltage, and 630 volts is probably the surge (maximum peak voltage) rating, which is a particularly important characteristic in power supply filter applications.

All up I believe the markings are:-

0.1μF with a 20% tolerance and as GTC said, 200VW and 630V for the surge, meaning that the capacitor can withstand up to 630V for a very short time.

Generally, a capacitor will pass AC and filter DC. Polarised electrolytic capacitors can only be used as filters, as GTC mentioned as a reverse polarity will ensure they can no longer do anything. Specialised non-polarised electrolytic capacitors are available for things like starting electric motors and providing power factor correction for commercial electrical installations.

Conventional wisdom would have one replace the motor and gearbox of your vintage MG with a Japanese equivalent.
No more visits to the repair shop and more efficient repair?.
. JJ

Conventional wisdom would have one replace the motor and gearbox of your vintage MG with a Japanese equivalent.

Nah, but replacing anything branded 'Lucas' with anything else would be a great move.

If that capacitor looks like it's been dipped in some sort of synthetic resin, not moulded, and it's a claret colour, it'll probably be a Ducon polyester from the 70's and it'll be OK.

Paper capacitors are the ones you worry about, and they will generally have a wax coating.