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

[mikepukmel]

Ever notice the doors in places like hotels or schools? The hinges are so much more heavy duty than ones in your home. Same for the latch and knob. That stuff is meant to last years under heavy use. Clearly they are "better" hinges and knobs than ones in your house. yet the home ones have worked just fine the life of the house in many cases 40-50-60 or more years. So what would my home gain if I replaced my lesser hardware with the better stuff? Nothing really. Just my opinion, but when we micro-analyze caps like this, it is a similar matter.

Enzo, ah, ok so, we have these few brands that make electrolytic capacitors for these old tube amp applications. They look about the same, and specs, don't know. They have voltage rating, don't know about temp. Illinois, F&T, Sprague ($$$). These are huge can, thick leads. I wasn't so much trying to micro analyze, but compare the 'old style' big can, thick lead caps, with newer ones that look tiny in comparison, but that seem to have 'better' electrical specs. Not better from sound point of view, since I have no idea the impact on sound quality, but temp rating, voltage, ripple current. Was curious if something else in there would cause the new design caps to fail in a guitar amp. Or as Leo mentions above: charge them up and see what happens! Hoping for a non kablooey charging, here.

[mikepukmel]

I was recently donated a shoe box full of ancient wax & paper caps. Maybe I'll offer 'em one of these days in our Flea Market. Or go for the big bucks on EepBay or Revoib. "Get that vintage sound at a price you can't afford!" Also have a box full of huge 400V electrolytic "beer cans" from 1966. They oughta make a big hit as they go kablooey when someone tries to charge 'em up, what fun. I'll have to give 'em a go, next 4th of July...

Leo, my friend Steve's dad taught him the fine art of TV repair. This is late 60's through the 70's, when there were still many tube TV's in service. He has a standard joke, they have 2 x 100 foot extension cords. They put a tv after a repair, on an end table out in the back yard. stand 100 feet from the TV, and plug the two cords together, in case of a kaboom situation.

[Leo_Gnardo]

They put a tv after a repair, on an end table out in the back yard. stand 100 feet from the TV, and plug the two cords together, in case of a kaboom situation.

As our friends in UK say "stand well back." Often when lighting big firecrackers on Guy Fawkes Day. Coming up soon, only 5 weeks!

[trobbins]

I would suggest the query needs some more flesh with respect to the amps you intend to use the caps in. I'd anticipate that the ripple current in many valve amps is substantially below the ripple current of caps now typically available for retrofit. If that is the situation then ESR and internal temp rise is unlikely to be a concern compared to local ambient temp.

What is more likely a concern is appreciating what ripple current the cap experiences. Pretty much the only cap that would experience significant ripple is the first filter cap, and the output stage filter cap (if those caps aren't one and the same) - any other filter cap would not have a ripple issue. Measuring the ripple current by temporarily inserting a 1 ohm resistor in the negative terminal to 0V may be pretty easy to do - similarly using something like PSUD2 can get you a reasonable estimate in a few minutes.

That UCY Nichicon has a very long life rating, and at 105C.

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.

[mikepukmel]

I would suggest the query needs some more flesh with respect to the amps you intend to use the caps in. I'd anticipate that the ripple current in many valve amps is substantially below the ripple current of caps now typically available for retrofit. If that is the situation then ESR and internal temp rise is unlikely to be a concern compared to local ambient temp.

What is more likely a concern is appreciating what ripple current the cap experiences. Pretty much the only cap that would experience significant ripple is the first filter cap, and the output stage filter cap (if those caps aren't one and the same) - any other filter cap would not have a ripple issue. Measuring the ripple current by temporarily inserting a 1 ohm resistor in the negative terminal to 0V may be pretty easy to do - similarly using something like PSUD2 can get you a reasonable estimate in a few minutes.

That UCY Nichicon has a very long life rating, and at 105C.

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.

Hope this doesn't end up a dupe, I wrote up a reply, and got "do you want to leave this page", clicked yes, and the reply disappeared.

Re the intended amps: so far,just this one guinea pig Supro Thunderbolt S6420 ss rectified (6L6 cathode biased, simple circuit about 30watts).

Re measuring ripple, just to confirm, I need a big (high wattage) 1R resistor, disconnect the negative side of the first filter cap, measure voltage drop across that. Can I use my old 'scope to do that?

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? Is this due to the on/off switch arc that occurs just as the contacts are about to close, or combination of that and power transformer charging?

[mikepukmel]

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.

[Helmholtz]

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.

[mikepukmel]

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

[Chuck H]

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.

[Gingertube]

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

[Helmholtz]

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).

[trobbins]

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

[Chuck H]

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.?.

[SoulFetish]

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)

[trobbins]

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.