I recently picked up some very cheap 0.047 630V caps from China.(branded MKT).
These are the yellow ones, but I noticed they are about one third of the physical size of the Philips ones I am used to using.
They measure OK and test OK with 1000 Volts across them, but I'm suspicious.
Has anyone any experience with these and what about their robustness in valve radio restoration.
JJ

MKT is not a branding, it's a construction type code ostensibly meaning metallized polyester film.

I say ostensibly because, given the source and price, there's no guarantee of that, or of conformance to specification.

It would be interesting to see how they go under test at 630 volts over a period of time.

Yes, I,m familiar with MKT, Should have mentioned in fine print "cary" .
Have tested a couple of them on my University Valve mega/puncture tester, at 1000v dc.
I'll leave it switched across it next time in the workshop, so I can monitor it.
I'll also put one in a home made 3 valve regen receiver that I built as a teenager, and use as workshop background music.
Its over 50 years old, and it has a fuse in the HT line. Maybe a 10 ohm 1/4W resistor in line as well, and just let it run.

Should have mentioned in fine print "cary"

Sure it's not scary?

In most cases, brand names of these cheapies mean nothing. I think they go through a dictionary of English names.

I looked up cary electronics, and the caps I have look almost identical but slightly different logo.
I'm suspicious, and at this stage won't be using them for a customers repair.
Maybe I should open one up, and probably find cigarette tissue and aluminium foil rolled up, in epoxy filled tube. .
JJ

Attempts at passing-off are not unknown in that part of the world.

It would be interesting to dissect one or two.

Would also be interesting to know if Cary Electronics are aware of them.

I have used the Cary brand before. Their small size makes them good for hollowing out the old capacitor and inserting them inside.

Somewhere I have data on them, from memory just slightly over spec μF, zero uAmp leakage 480V and ESR was similar to other brands.

I've used them less than a year so I don't know of their longevity.

Can look for my capacitor data if you're interested.

How did you go with your puncture & receiver tests?

So far so good, but have not done any serious testing, too busy with a thousand and one other projects.
I was just very suspicious of their physical size.
To me to get those capacities in a small size you need to get the plates closer together. And to do this means a lower flashover voltage.
So I don't know what sort of dielectric they have used, and how does it age.
JJ

My thoughts exactly but they test Ok.

I need to construct a voltage doubler or trippler so I can take them to flashover.

I am sure I have megger tested them at ≈1200VDC with no leakage.

I ran a 0.01 at 1kV for a couple of hour OK. It flashed over at 2kV, but that was expected.

Cheap parts out of China are not always counterfeit. Most of the time they are cheap because they are end of production run surplus stock.

Yours are probably OK, but remember MKT (metallised plastic) caps are not rated to handle high currents such as you'd find with a TV boost cap, across the secondary of a vibrator power supply transformer or in a speaker crossover network rated for power levels over 50 watts RMS.

They'd be OK for all positions in a valve radio, there is nothing there that would stress them.

MKT caps, because the metal is plated onto the plastic foil, are much smaller than conventional wound foil parts. Nothing wrong with them per se if they suit the application.

A couple of years back I used some MKTs in series with extraction fans in our new house to slow them down and reduce the noise. Worked fine for an hour or so then failed. As I thought they might. They were being asked to pass peak currents of an amp or so at mains voltage.. Replaced them with physically larger caps - worked fine.

I have used MKTs for years in radio restos, often sealed inside original cardboard tubes melted off the old caps for visual authenticity. Never had one fail.