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Old 11th August 2008, 12:06 AM   #1
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Default Filament Snubber Cap Explodes! What happened?

Okay, so I was just relaxing and listening to the radio when suddenly, I hear a nice loud crackle coming from my equipment rack and I look up to see smoke billowing from my amp! Upon later inspection, I see that a filament snubber cap blew up. The amp is a S-5 8w push-pull with a 12v filament. I put in 1uf Tantalum caps for snubbers. I am a real noob with snubbers and am wondering, what did I do wrong? Are these the wrong caps? Should I remove them?
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Old 11th August 2008, 12:22 AM   #2
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Quote:
Are these the wrong caps?
Yes

Quote:
Should I remove them?
Yes

Snubbers should be non polarized caps and usually a value much lower than 1uf. I would question if they are even needed on a filament supply.
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Old 11th August 2008, 12:31 AM   #3
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Tantalums are bad juju in my opinion. Some tantalum caps, especially if they are old, just spontaneously short I have pieces of analog scientific gear from the late 70s/early 80s that use gobs of tantalums for power supply bypassing. Just one of these tantalums popping brings down the entire power supply rail. If the power supply is current limited, the cap damage may be limited to a barely noticable pinhole in the capacitor's epoxy coating, making the bad one hard to find. As a result, I have some instruments gathering dust on my shelves waiting for me to pull all the old caps (~50 of them per instrument) and replace them with a mixture of film caps and modern electrolytics. Modern tantalum capacitors are better, but they still don't respond to voltage surges very well (they die), and you pay dearly for the dubious privelege of having them in your circuit.
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Old 11th August 2008, 01:13 AM   #4
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whoops! just shows what a noob I am.

could my amp be damaged?
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Old 11th August 2008, 03:22 AM   #5
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Originally posted by ambience exists

could my amp be damaged?
I'm with wrenchone. Tantalums are a very common failure mode in electronic equipment. They were/are liked for their small size and cost.

I seriously doubt that your amp was damaged. But I have seen damaged circuit boards from caps burning up.
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Old 11th August 2008, 03:52 AM   #6
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If tantalum caps were used as snubbers across the diodes wouldn't they have mostly AC across them? Polarized caps don't enjoy that sort of thing.

I've had small surface mount caps go off like a firecracker and shoot glowing embers. In a former life I had an SE 211 amp with caps rated for 65vdc across the cathode resistor. Not knowing how different an 845 is bias wise, I plugged one in and a while later *BOOM*. The force was so strong it bent the metal end lead on a 50 watt resistor. Not much left except for a metal end cap and some stinky white smoke.
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Old 11th August 2008, 05:26 AM   #7
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Yes, exactly.

Tants are polarized, and besides that, have a very high failure rate even when used properly.

Your amp is not damaged.  Replace the tant with a 1-10µF nonpolarized Panasonic or Nichicon.  That cap is valuable, it keeps the hi-frequency nasties from the wall out of the heater, and thus the audio circuit.  I always use them.  Not tants.

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Old 11th August 2008, 08:32 AM   #8
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I put in 1uf Tantalum caps for snubbers.
I second everything said above.. Tantalums are just bad news. Nearly burnt down the house once,when one exploded on my bench,and lit my stack of papers on fire. I *NEVER* use them anymore.Ever.
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Old 11th August 2008, 09:25 AM   #9
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Quote:
Originally posted by wrenchone
Tantalums are bad juju in my opinion ... and you pay dearly for the dubious privelege of having them in your circuit.

http://www.cellular-news.com/coltan/
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Old 11th August 2008, 05:11 PM   #10
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Quote:
Originally posted by HollowState


I'm with wrenchone. Tantalums are a very common failure mode in electronic equipment. They were/are liked for their small size and cost.

I seriously doubt that your amp was damaged. But I have seen damaged circuit boards from caps burning up.
There is a new class of organic polymer tantalums designed to avoid the fire issue common with older tantalums. Tantalums were used primarily in applications where low esr, good high frequency performance, or extended temperature range operation were a concern - they are also very compact relative to older aluminum types for a given value - but in their heyday they weren't necessarily (or usually) the cheapest option.

They should never be used at more than about 60% of their rated voltage, and never in situations where they will be exposed to voltage transients. Never, ever reverse bias a tantalum for any reason. They have a variety of insidious failure mechanisms due to the way they are made, minor events may not cause immediate failure but result in catastrophic failures at some later date. Issues in manufacturing can also create areas with unusually high leakage currents which result in localized heating, additional damage and then a cascade failure which usually results in either an exploded cap or a fire.

There is no real reason to use them except in rf bypass applications such as in cell phones, and I imagine the newer organic types would generally be used. They are rather expensive so I would not expect to see them in widespread use.

A lot of older car stereos (stereo cassette/radio) used tantalums because of their compact size and wide temperature range. (AL types used to freeze...) They sound terrible as audio coupling caps, and invariably the smoke gets let out. (I've had several - -)
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