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Thread: Current handling capability: big F&T power supply caps vs new design Nichicon

  1. #36
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    Quote Originally Posted by Leo_Gnardo View Post
    As our friends in UK say "stand well back." Often when lighting big firecrackers on Guy Fawkes Day. Coming up soon, only 5 weeks!
    At a previous company, once in a while we'd get a new laser from the German factory, plug it in, turn it on, and get a nice "BAMM". They usually worked just fine, but we swore they put a cap in there backwards just for our enjoyment.

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    The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

  2. #37
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    measure voltage drop across that. Can I use my old 'scope to do that?
    As ripple current is not sinusoidal but pulsed and is specified as RMS value, a true RMS meter would be fine.

    Re peak, turn-on, so, suppose amp has been sitting for some time, and the caps are "empty". Turn the on-off switch "on", its that first spike that comes across the transformer and hits the filter cap?
    That would be cap inrush current. It only happens once and is typically of no concern with tube amps as the PT limits inrush current to safe values.
    Trobbins is speaking about peak VOLTAGE at turn-on. Before tubes are fully heated and draw current there may be a voltage overshoot.

    Is this due to the on/off switch arc that occurs just as the contacts are about to close
    The switch arc only happens when contacts are opening (not when closing) and current is interrupted. The magnetic energy involved is too low to noticeably increase ecap voltage.

    power transformer charging?
    If you mean the magnetic turn-on surge current, this is only a primary side effect. No related surge at the secondary.

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  3. #38
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    Thanks Helmholtz. I will post values when I get the amp wired up.

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  4. #39
    Bent Member Chuck H's Avatar
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    What Helmholtz said

    And just to clarify, it's the UNLOADED voltage that is the figure we're working with here. Short surge, not really a problem, but the unloaded voltage can stand across the caps in a couple of ways and hold there. So if your amp has 450V caps and an unloaded voltage of 515V there may be a problem at some point. The truth is there are a shit ton of amps that do this and we don't read about caps blowing up in those amps under circumstances where the voltage isn't loaded down. In fact I'd say I've NEVER read about it here or experienced it personally. Still, on my second to last build there was 540V or so unloaded so I designed totem pole arrangements with 350V caps with parallel 220k resistors on all nodes just to be on the safe side. This is the sort of thing you may be about to do. It's not really a justified precautionary measure (in the real world), but it makes sense at face value. Many amps, including some built by major manufacturers with cheapo modern capacitors, have the filters rated above B+ and often NOT above unloaded voltage (which would occur while the caps are charging and before the tubes are warm, regularly) and those amps don't suffer from failing power supplies.

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    Bed time reading for the terminally bored.
    https://www.nichicon.co.jp/english/p...f/aluminum.pdf

    BUT
    I like this older one from Evox Rifa,, it even has a Valve (Tube) Amp Calculation Example. Must be for HiFi as they only calculate for a 2V variation on B+, 5% is good enough for a Git Amp.
    http://materias.fi.uba.ar/6648/archi...c_appguide.pdf

    Cheers,
    Ian

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    Last edited by Gingertube; 10-01-2019 at 04:37 AM.

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    As from ecap literature the failure mode associated with overvoltage is increase of leakage current > cap heating up > outgassing, I assume short events of moderate overvoltage like during warm-up won't do much harm at least in short term.
    But if caused by an amp defect (e.g. open OT primary) or during repair work power tubes are inoperative, there may be (unnoticed) overvoltage for a considerable amount of time.
    So it makes a lot of sense to choose the rated voltage of the filter caps at least 20% higher than nominal loaded voltage (see RIFA guide).

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  7. #42
    Senior Member trobbins's Avatar
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    Yeh the electrolytic app guides have been quite devoid of good insight about over-voltage operation - but the Nichicon guide that Ian posted shows an informative Fig 2-4. A manufacturer will set the design/operating voltage with some margin for when leakage current increases to some significant level, and taking in to account the statistical spread of their product - perhaps a blend of commercial risk and technical risk with setting margins. The other key aspect is the operating temperature - where percentages favour a cool capacitor.

    Another comment is that preamp stage filter capacitors that are 'protected' by dropping resistance may well have some inherent peak voltage limiting if the leakage current starts to avalanche. It would be really interesting to see some internal test documentation for B+ type caps. The link indicates the type of degradation/failure occurring as voltage pushes past the rating, and perhaps indicates that using a microphone or your ear during turn-on could be a good test for the onset of internal sparking. I've often heard older polyprop caps of circa 10uF 600V start 'popping' as they are tested for insulation.
    https://pdfs.semanticscholar.org/8a7...218f114870.pdf

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  8. #43
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    Quote Originally Posted by trobbins View Post
    Another comment is that preamp stage filter capacitors that are 'protected' by dropping resistance may well have some inherent peak voltage limiting if the leakage current starts to avalanche. It would be really interesting to see some internal test documentation for B+ type caps. The link indicates the type of degradation/failure occurring as voltage pushes past the rating, and perhaps indicates that using a microphone or your ear during turn-on could be a good test for the onset of internal sparking. I've often heard older polyprop caps of circa 10uF 600V start 'popping' as they are tested for insulation.
    The way I see it, and I sure could be wrong but I always appreciate corrections that educate , if there's a condition or fault that creates a no or very low current situation in an amp then the dropping resistors won't drop much voltage. Potentially almost none relative to just the caps ESR. A bad ground, tubes pulled for testing, broken filament circuit, etc. would all result in high voltage sitting on top of the capacitor regardless of the resistors in the circuit. So I have to wonder if whoever is saying that the preamp nodes are protected by the dropping resistors is aware of this.?.

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  9. #44
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    Quote Originally Posted by trobbins View Post
    That UCY Nichicon has a very long life rating, and at 105C.
    To insert my own 2 pennies into this topic, this parameter would be the biggest determining factor in choosing any electrolytic cap. (IMO)
    In fact, this is the main reason I use film caps wherever possible. I think that the low cost and availability of suitable high voltage/high capacitance film now make them worth designing around (in new builds).

    Of more concern is that the amp doesn't impose a peak voltage at turn-on that exceeds the cap rating - as in pretty much all modern caps, there is no +10% surge capability, so taking the time to measure the peak voltage achieved at turn-on is probably of most importance in choosing a suitable cap.
    Right on. In some high power tube amps the unloaded peak voltage at turn on can be well over a 150V higher than when the B+ falls to it's nominal voltage at idle (for example in some vintage Matamp/Oranges, among others)

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    Last edited by SoulFetish; 10-02-2019 at 01:48 PM.
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  10. #45
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    Yes if filter and coupling caps have no leakage, then at power on all those caps are likely to see max B+ across them unless the amp includes some bleed resistance down towards the preamp stages and hence there is some voltage drop across the B+ ladder resistors. A similar situation exists for some direct coupled preamp stages, like a triode anode connecting to the grid of a following cathodyne stage - where the cathodyne triode's grid sits at B+ but the cathode sits at 0V.

    A relatively minor amount of capacitor leakage could cause a noticeable voltage droop, depending on where in the circuit this is. I guess screen bypass caps in preamp pentode stages may see some voltage droop, as the B+ supply resistance is often circa 1Meg. And of course connecting a voltmeter, even if it is 10Meg input impedance, can droop some voltage levels in a valve amp. And the topic of how an electrolytic may react when subjected to voltage over its rating indicates that some B+ droop may occur towards the input stages.

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  11. #46
    Senior Member trobbins's Avatar
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    Just read through a very interesting paper that provides some example measurements that support the likely survivable level of turn-on peak voltage applied to a first filter capacitor.
    https://pdfs.semanticscholar.org/58f...0d6f2ac4a6.pdf

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    Thanks.

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  13. #48
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    Quote Originally Posted by trobbins View Post
    Just read through a very interesting paper that provides some example measurements that support the likely survivable level of turn-on peak voltage applied to a first filter capacitor.
    https://pdfs.semanticscholar.org/58f...0d6f2ac4a6.pdf
    Nice paper. Thanks for sharing

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