My amplifier -- why did this capacitor blow in my guitar amp?

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wolram
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I have built several guitar amps, but the last one I built burned out a capacitor, can any one think of a reson for this? it was an eloctolytic, all is working fine now I replaced it.
 

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berkeman
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I have built several guitar amps, but the last one I built burned out a capacitor, can any one think of a reson for this? it was an eloctolytic, all is working fine now I replaced it.
Well, judging from your schematic and pictures, I'd say that your flux capacitor power supply had a purple-out transient. That is not uncommon, but I'm sure you know that... o0)

:smile:
 
wolram
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Well, judging from your schematic and pictures, I'd say that your flux capacitor power supply had a purple-out transient. That is not uncommon, but I'm sure you know that... o0)

:smile:
I have been trying to load the scematic, but for some reason
 
berkeman
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I have been trying to load the scematic, but for some reason
1573848232920.png


:smile:
 
wolram
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Ok I can't up load the scematic but here is the address for ithttp://www.ampmaker.com/infocentre/thread-140.html
 
wolram
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p18sc01.jpg
 
wolram
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Got it, it is c19 that blew in the power supply, sorry to have messed you guys around.
 
berkeman
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Got it, it is c19 that blew in the power supply
the last one I built burned out a capacitor, can any one think of a reson for this? it was an eloctolytic
What's the voltage rating of C19? Did it blow up at first power-up (maybe installed backwards)? Or did it fail after some amount of time? What did it look like after it failed (expanded pressure relief area on top?)?
 
davenn
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Got it, it is c19 that blew in the power supply, sorry to have messed you guys around.

it should have a rating of at least 400V as it's going to have ~ 300 - 350V across it

Is your one at least 400V ?
 
NascentOxygen
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275v RMS == 390v peak on low load.

I'd want a 500v rating for that cap.
 
Baluncore
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A transformer specified as 275 Vrms will actually give 5% more when there is no load on the secondary.
275 Vrms * 1.05 = 290 Vrms.

290 Vrms * 1.4142 = 410 Vpeak.
450 Vdc is the next higher voltage rated electrolytic capacitor.

The mains voltage may also be higher than expected by 5% or so.
At full power the ripple current will be higher than load current by maybe a factor of about 10.
 
wolram
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it should have a rating of at least 400V as it's going to have ~ 300 - 350V across it

Is your one at least 400V ?
Yes it's 500v, installed the correct way and it blew after about 2 days working, the cap had a slight bulge to one side.
 
nsaspook
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Yes it's 500v, installed the correct way and it blew after about 2 days working, the cap had a slight bulge to one side.
Who made it?
 
wolram
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If an electrolytic has been sitting on the shelf for a couple of year or more it's a good idea to connect it to a current-limited supply, and gradually ramp it up to rated voltage. Look up "capacitor reforming".

https://www.qsl.net/g3oou/reform.html gives the general idea. A number of VFD drive manufacturers publish similar procedures since a new drive might be in benchstock for years before it is called into service, and taking time to reform the DC link caps guards against premature failure (and even more down time).
 
530
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If an electrolytic has been sitting on the shelf for a couple of year or more it's a good idea to connect it to a current-limited supply, and gradually ramp it up to rated voltage. Look up "capacitor reforming".

https://www.qsl.net/g3oou/reform.html gives the general idea. A number of VFD drive manufacturers publish similar procedures since a new drive might be in benchstock for years before it is called into service, and taking time to reform the DC link caps guards against premature failure (and even more down time).
@Asymptotic that link appears to be broken.
 
144
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I can see it is not a Dynaco circuit. if you run high frequency too long it can over heat caps and blow them. It could be an engineering mistake. Double the cap voltage of your blown cap put a new cap in. If you ever have a Dynaco tube amp you will never want anything else.
 
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Since this horse is already lying down:
Baluncore mentioned 'Ripple Current'. That is often the straw that breaks the camel's back in electrolytic capacitors. It causes dielectric heating; ultimately raising the temperature of the capacitor and 'aging' (or 'cooking') it. I've seen many designs where a capacitor was of the correct voltage and nominal capacitance, but not rated for the ripple current heating that it would experience. Using a higher-than-required voltage rating is an imprecise (but often effective) way to survive high-ripple situations. Or, one could completely understand what is required of a part (before selecting it). This behavior is one of the most significant differences between an ideal and an actual capacitor.

edit: that's an ugly mixed metaphor, isn't it?
 
530
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What about a low ESR rated cap?

Electrolytics have a high failure rate in general, but a good brand makes all the difference. I like to see Panasonics and Nichicons. OP hasn’t mentioned the brand yet.
 
Baluncore
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Steam engines and vacuum tubes require special care for long life. Chemical cells and electrolytic capacitors are similar. They have limited lives that are extended by gentle treatment. Care for them like you would a pet or your favourite pot plant, and they will have a similar lifetime.
 
FactChecker
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There may be nothing wrong with the design. A lot of components will fail early. That is just the nature of the timing of component failure. Failure rates have a bathtub shape.
 

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