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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 28th Feb 2021, 10:13 am   #1
stevehertz
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Default Capacitor voltage?

For use in a tight space in a Pye M78F that I'm restoring (see other thread in wireless section) I want to use this 0.01uF cap (marked '103' = 10,000pF = 0.01uF)). The problem is, it doesn't show a voltage rating. Does anyone know what it's voltage rating is, or is likely to be? Its position in the circuit is C7, HT feed decoupling. See circuit diagram. My HT into the set will be 74V. But that is dropped via R12 ((680ohms) and R2 (10K) to get to C7. Trader valve voltage table also shown - bear in mind the voltages quoted are based on the HT being 67.5V.

Is this cap suitable?

Otherwise it's not the end of the world, I'm pretty sure I may be able to find a cap that I can shoe horn into the very limited space but the cap I'm talking about would be ideal size wise. The set was build layer by layer, and there's a number of short wires etc lying over the cap in question that will be a pain to unhook or even replace. It's all very tight, like performing open heart surgery on a mouse. Originally the cap would have been fitted first, then those obstructive wires.
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Old 28th Feb 2021, 11:28 am   #2
Herald1360
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Default Re: Capacitor voltage?

Looks like a multilayer ceramic job. Probably 50V though could be 100V but with no ID who knows?

It would probably be OK, failure would be a nuisance, not a disaster but with something like these available:

https://www.ebay.co.uk/itm/0-01uF-10...edirect=mobile

why risk it?
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Old 28th Feb 2021, 11:47 am   #3
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Default Re: Capacitor voltage?

Quote:
Originally Posted by Herald1360 View Post
Looks like a multilayer ceramic job. Probably 50V though could be 100V but with no ID who knows?

It would probably be OK, failure would be a nuisance, not a disaster but with something like these available:

https://www.ebay.co.uk/itm/0-01uF-10...edirect=mobile

why risk it?
Looks like a good option, thanks. Let's see what others have to say.
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Old 28th Feb 2021, 12:06 pm   #4
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Default Re: Capacitor voltage?

That size cap is 50V and probably borderline for what you want.

I would go with the 100V ones.
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Old 28th Feb 2021, 12:26 pm   #5
snowman_al
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Default Re: Capacitor voltage?

If the blue cap is 50 volts working, then it should be fine.
From the voltage table the bottom of R2 has 33 volts on it (V2 screen) so add 5 volts for safety (higher HT and digital meter) and you are still less than 40 volts. It will only 'see' more for the fraction of a second before the valves start to conduct at switch on...

You could try putting one in series with a 10kΩ resistor across your 74 volt HT and see what happens?
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Old 28th Feb 2021, 12:29 pm   #6
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Default Re: Capacitor voltage?

Those radial ceramic packages are prone to have their legs tear off at the slightest stress.

David
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Old 28th Feb 2021, 4:09 pm   #7
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Default Re: Capacitor voltage?

Quote:
Originally Posted by snowman_al View Post
If the blue cap is 50 volts working, then it should be fine.
From the voltage table the bottom of R2 has 33 volts on it (V2 screen) so add 5 volts for safety (higher HT and digital meter) and you are still less than 40 volts. It will only 'see' more for the fraction of a second before the valves start to conduct at switch on...

You could try putting one in series with a 10kΩ resistor across your 74 volt HT and see what happens?
I've done this. Due to the cap being open circuit to DC, all of the power supply voltage is being dropped across it, 74V. Anyway, it's been sitting there for a couple of hours at that voltage and it's not blown up?
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Old 1st Mar 2021, 9:41 am   #8
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Default Re: Capacitor voltage?

All good then...
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Old 2nd Mar 2021, 10:06 am   #9
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Default Re: Capacitor voltage?

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Originally Posted by snowman_al View Post
All good then...
It would appear so! Time will tell!
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Old 2nd Mar 2021, 12:49 pm   #10
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Default Re: Capacitor voltage?

These were used as TTL rail (5V) decoupling capacitors.
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