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Old 26th July 2012, 04:51 PM   #11
tomchr is offline tomchr  United States
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Rather than running the caps right at the hairy edge, you could use two caps in series (with ballast resistors to split the charge evenly).

~Tom
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Old 26th July 2012, 04:57 PM   #12
kevinkr is offline kevinkr  United States
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I'd look for something with a higher voltage rating or contact the manufacturer to confirm that the multiplier is >1.4 for the AC to DC voltage rating.

I've had very good luck with ASC PPIO motor runs in the past.
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Old 26th July 2012, 05:17 PM   #13
alecu7 is offline alecu7  Canada
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When I checked Angela website I found:
Angela ASC Capacitor 50uF/515VDC

If you zoom in the picture you can see this 515Vdc capacitor is actually 370Vac. So the rule SQR(2) ~1.41 does apply. But maybe the input voltage from the amp is too high..

Thank you all for the answers.

Alex
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Old 26th July 2012, 06:23 PM   #14
JamesG is offline JamesG  United States
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Tomchr is correct. Place two 450V caps in a series with some ballast resistors and you will be fine. I HATE it when electrolytic caps fail and 450V caps are not that expensive. I always stay well away from the edge of their ratings. As a result, I have not seen a power supply cap failure in any of my builds.

My design philosophy is conservative (e.g. design with assumption that the user is standing barefoot in salt water). In the long run, its cheaper and easier.
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Old 26th July 2012, 06:25 PM   #15
ChrisA is offline ChrisA  United States
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This is a very good document. It is best not to site just one sentance or one number from it but to read the entire document. For example, caps use a voltage that is 35% higher then rated to 'form" them at the factory. So you mght think it is OK to go up to say 34.99% over rated voltage but then you read later that they can only withstand such abuse a few hundred times before failure. and that temperature and ripple curent also determine lifetimes. If you push all these specs they tell you how to compute lifetime before failure and the time is much lower than many of us want, under 2,000 hours (one hour of daily use)

So we "de-rate" these caps can gain considerable lifetime from them. In a tube amp temperatures is going to be high but we can control ripple current and percent of maximum rated voltage and get some reasonable lifetimes.

These "rules of thumb" we see on these forums are maybe good for a first orer design but then you have to not only read the specs for the cap you might choose but read how those specs are measured. For example "Max serge voltage" means that it can withstand that X many time before failure, not that it can withstand it forever.

And then these number are all just probabilities. It will tell you say, 3,000 hours before a 50% change of failure but what if you want to be 90% certain your amp will last for 4,000 hours? You CAN figure this out from the data sheets but kit takes some work.

This should be required reading. It's good.....
http://electrochem.cwru.edu/encycl/m...4-appguide.pdf
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Old 26th July 2012, 07:03 PM   #16
alecu7 is offline alecu7  Canada
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Chris,

Thank you for the article.


Interesting reading about ballast resistor at page 13:
"When only two capacitors are in series, balancing resistors
are seldom needed for voltage sharing. Before including
them anyway for voltage bleed-down, consider that not
using balancing resistors often increases system reliability
by reducing the temperature near the capacitors and by
eliminating components less reliable than the capacitors they
are meant to protect."

I think in the end I will reduce the input voltage a bit (~10-20V) and use 1 capacitor.


On a personal note I found very interesting how the aluminum foil is etched.
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Old 26th July 2012, 10:58 PM   #17
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The article from CDE applies to Aluminum Electrolytics. It is important to understand how all the factors interplay to affect the true lifetime in an electrolytic.

However the Panasonic capacitor mentioned in this thread is not an electrolytic. It is a Metallized polypropylene film in oil capacitor intended for the phase shift capacitor in an air conditioner. Here the ripple current is measured in AMPS (5 to 20) and is continuous. The capacitor is connected directly to the power lines and gets to eat any surge on the line. In Florida an AC unit will run at 70 to 100% duty cycle during the day and 20 to 50% at night. The caps fail, but it is not common and a 20 year life is common. A motor run cap is loafing along in a tube amp. Note that there are motor start caps that ARE electrolytics which are intended for intermittent use only and not suitable for tube amps.

The data sheet for these Panasonics is rather sparse in important things like the continuous current spec. I haven't tried these particular caps but builders of Tubelab SSE's have been using 370 VAC motor run caps in their power supplies for 7 years. I recommend a 375-0-375 volt Hammond sourced power transformer that provides a B+ from 430 volts (tube rectifier) to 460 volts (diodes). None have failed.
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Old 27th July 2012, 02:26 AM   #18
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Quote:
Originally Posted by ChrisA View Post
That only applies in the steady state case after the amp has wormmed up. At first the tubes will not be conducting so the resister in the power supply will not drop the voltage to the second cap. So EVERY time to power the amp on you are running the second cap over it's rating. It will fail eventually.

Buy caps with the required voltage. I'd say to buy then with a 20% or at least 10% margin so if you have a B+ of 450V buy at least a 500V cap
I see your point . However the 370 ac cap is effectively rated at 1.4 times 370 for dc or 515 aprox. The peak voltage of cap one will be higher than cap after the choke and less likely to exceed that 515 volt point as will the current peaks . The resistance of the the choke is not stated but at 150 ma will be of note reducing the voltage that cap two will see. If it where me I would use all 440ac cap and have a longer life for all the caps at a low cost. The turn on behavior of vacuum tube diodes like damper diodes with a 11 to 15 second delay before forward conduction is much easier on the caps than a ss diode that hit very fast with voltage and strong peaks .
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Old 27th July 2012, 09:31 AM   #19
12E1 is offline 12E1  United Kingdom
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Quote:
Originally Posted by alecu7 View Post
...
Interesting reading about ballast resistor at page 13:
"When only two capacitors are in series, balancing resistors
are seldom needed for voltage sharing. Before including
them anyway for voltage bleed-down, consider that not
using balancing resistors often increases system reliability
by reducing the temperature near the capacitors and by
eliminating components less reliable than the capacitors they
are meant to protect."
....
That's an interesting assertion in the article. I would always consider balancing resistors to be necessary, probably more so with electrolytics than with paper-in-oil and other types. The manufacturing tolerances on electrolytics can mean that voltages are not shared equally doe to capacitance variation - some can be rated -20% / +80% in C value, whereas the resistors can easily be 5% or 10% parts even when rated suitably for wattage and voltage.

The other fact is that all components age, and if one capacitor lowers its value more than the other then the voltage sharing will shift without resistors being present. The fact that the resistors will bleed the capacitors or charge when the unit is unplugged is a useful bonus in my view.

Resistors age too of course - I recently replaced caps and balancing resistors in an amplifier. The caps were nearly 30 years old, and two out of four resistors had shifted upwards in value by over 25% since I built the amp - somewhat agreeing with the reliability comment in the text quoted.

It should go without saying that dissipating resistors should be kept away from capacitors to avoid ageing by heating, but one often sees parts place badly (applies just as much to modern SS equipment).

Another point sometimes overlooked is to use resistors of adequate voltage rating. Many may only be rated to 250 or 300 VDC, Just as we are discussing putting capacitors in series to share voltage, the same can be done with resistors to help prevent breakdown in high voltage situations.

Last edited by 12E1; 27th July 2012 at 09:34 AM.
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