Unknown capacitors. Any idea about them?

[Miguel Lopez]

Hello

Have to say that althought the title says "unknown capacitors" they are referred as unknown to me only, because I have never seen them before in Cuba. That's why I would like to ask if somebody have used them, and to have an opinion about their quality (leakage, voltage standed, lasting)

A friend of mine have them unused and gave them to me for my experiments on valve electronics.

Any comment welcome

Note: When I say "voltage standed", I'm not referring to the voltage labeled in the component but to the voltave that the component really stand. Forgive any wrong spelling.

[Ed_Dinning]

Hi Miguel, they look like "mouldaseal " types, usually a paper based dielectric with a moulded plastic cover. They were the successor to waxed paper types but not much better at developing leakage when there was a crack in the covering.
Typically used in valve sets with up to 300v HT.

To check apply rated voltage from a DC power supply via a high wattage 10K resistor and measure leakage. Any leakage , even 1uA could be a problem in the feed to a signal grid. That level of leakage would not be a problem as a screen grid decoupler in practice.

Not sure how they would have been affected by the Cuban climate.

Ed

[Refugee]

If the outer is like the plastic milk bottles are made of they will be OK for 100V now but stick to 25 volts for biasing circuits.
They were better than wax ones but not as good as modern ones.

[Murphyv310]

They might be tropicalised types and less prone to leaks.
Do ED's test first and check the leakage current. You might well be lucky with them. They look better than our Mouldseal caps that weren't wonderful, as your ones seem to have no join in them, look like a proper moulded outer covering so less likely to be effected by humidity, like I say possibly tropicalised.

[Miguel Lopez]

Thank you very much for your answers. I will do the test and will let you know the results.

[Miguel Lopez]

OK
Yesterday I did the test and results were very good. I connected in series the HT PSU, the cap under test, a 4K7 wire resistor and my M2005 microamperemeter. After the initial charging spike I couldn't detect any leaking current on any of the types tested. they were:

0,27uF/400V tested at 300V
0,33uF/250V tested at 220V
0,56uF/250V tested at 220V

Using the 10uA range of the instrument with 200 division scale, I couldn't notice even a single dicision deviation of the pointer fron zero, so I guess the caps are OK. I tested five capacitor of each value with similar results.

On the other hand I tested a 0,5uF/160V Soviet paper cap (tested at 120V) and I had a leakage of about 50uA. So I think that those plastic capacitors are very good for valve electronics, don't you think?

[Refugee]

Those are good capacitors and should be reliable.