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

In summary: 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...except for more power.
  • #1
wolram
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I have built several guitar amps, but the last one I built burned out a capacitor, can anyone think of a reson for this? it was an eloctolytic, all is working fine now I replaced it.
 
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  • #3
wolram said:
I have built several guitar amps, but the last one I built burned out a capacitor, can anyone 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:
 
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  • #4
berkeman said:
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
 
  • #5
wolram said:
I have been trying to load the scematic, but for some reason

1573848232920.png


:smile:
 
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  • #6
Ok I can't up load the scematic but here is the address for ithttp://www.ampmaker.com/infocentre/thread-140.html
 
  • #7
p18sc01.jpg
 
  • #8
Got it, it is c19 that blew in the power supply, sorry to have messed you guys around.
 
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  • #9
wolram said:
Got it, it is c19 that blew in the power supply
wolram said:
the last one I built burned out a capacitor, can anyone 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?)?
 
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  • #10
wolram said:
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 ?
 
  • #11
275v RMS == 390v peak on low load.

I'd want a 500v rating for that cap.
 
  • #12
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.
 
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  • #13
davenn said:
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.
 
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  • #15
wolram said:
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?
 
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  • #17
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).
 
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  • #18
Asymptotic said:
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.
 
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  • #20
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.
 
  • #21
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  • #22
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?
 
  • #23
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.
 
  • #24
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.
 
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  • #25
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.
 
  • #26
wolram said:
I have built several guitar amps, but the last one I built burned out a capacitor, can anyone think of a reson for this? it was an eloctolytic, all is working fine now I replaced it.
Electrolytics commonly fail from excess heat over time. A possibility is that the voltage rating was too low. Another is that it was leaky as manufactured, or you used a very old one. Another possibility is that a rectifier is leaky giving it AC current which causes heat. Also if the capacitor value is too low it will get hot from the ripple current. Electrolytics are polarized so if you wired it wrong that could be the reason.
 
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1. Why did my capacitor blow in my guitar amp?

There are several possible reasons for a blown capacitor in a guitar amp. It could be due to age and wear, improper installation, or a power surge. It could also be a sign of a larger issue with the amp's circuitry.

2. How can I prevent my guitar amp's capacitor from blowing again?

To prevent future capacitor blowouts, make sure to use high-quality, properly rated capacitors and follow proper installation techniques. It's also important to regularly maintain and clean your amp to prevent buildup of dust and debris.

3. Can I replace the blown capacitor myself?

If you have experience working with electronics and know how to safely handle the high voltage components in a guitar amp, you may be able to replace the capacitor yourself. However, it's always recommended to have a professional repair technician handle any repairs to avoid further damage to the amp or potential harm to yourself.

4. Will a blown capacitor affect the sound of my guitar amp?

Yes, a blown capacitor can affect the sound of your guitar amp. It may cause a decrease in volume, distortion, or other unwanted changes in tone. It's important to address the issue as soon as possible to prevent further damage to the amp.

5. How do I know if a capacitor is blown in my guitar amp?

If you notice any changes in the sound or performance of your guitar amp, it's possible that a capacitor may be blown. You can also visually inspect the capacitors for any signs of damage, such as bulging or leaking. However, it's best to have a professional technician diagnose and repair the issue to ensure the safety and proper functioning of your amp.

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