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Old 29th December 2002, 05:37 PM   #11
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So I guess we can forget the claim that BG is the fault here ?
Only 15 V from the maximum rating in steady state is what I call "narrow designed". Some will call it wrong designed. Most probably any electrolytic will blow as well if tested.
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Old 29th December 2002, 05:40 PM   #12
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Default *HIT HAPPENS.

Hi,

Easy enough to find out:

Just replace the 100V BG with an other manufacturers' 100V elco and see if that survives.

Cheers,
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Old 29th December 2002, 11:39 PM   #13
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I suspect the failure is related to the AC signal riding on the cathode. If the dc bias was 75 volts running just 30 volts AC into the 300B input would over voltage the caps. The driver stage most likely can swing at least twice that voltage to properly drive a 300B.
The cathode voltage swings with the grid. That is how a cathode follower circuit works.
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Old 30th December 2002, 09:55 AM   #14
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Dick has placed a warning about the Black Gates on his site.
He is going to replace them all with 160V types.

Martin
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Old 30th December 2002, 10:27 AM   #15
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Quote:
Originally posted by jean-paul
So I guess we can forget the claim that BG is the fault here ?
Only 15 V from the maximum rating in steady state is what I call "narrow designed". Some will call it wrong designed. Most probably any electrolytic will blow as well if tested.
If you choose to have so "tight" rating you must "know" the component. Some brands are not promising too much and some other are modest in their ratings. If you are unsure you could always test the cap before it's put into the circuit. When the leakage current is starting to rise quickly there you will have the maximum voltage.

70-80 VDC , it is not smart to have a 100 V type! This would be very unprofessional. Big risk for disaster. I try to have the rating 50-100% over the actual voltage.

If you are totally sure that the DC voltage PLUS AC never will go over the rated voltage, the caps will probely last but why save a buck and then risk high costs in repairs, warranties etc.(if it's a commersial product)?
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Old 30th December 2002, 11:15 AM   #16
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I had a 100V Nichicon cap fail in the same position (300B cathode
bypass) in my first DIY effort. Now that I know better, I realize 100V is not enough headroom here. The momentary AC + DC voltage will certainly be over 100V above a certain volume level. The failure is *not* the fault of the cap!
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Old 30th December 2002, 02:54 PM   #17
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Default STARGATE

Hi,

As dutchman Triode Dick reports on his webpages the Black Gates conduct fully when faulty.

In any circuit that is a very dangerous situation no matter what.

For a cathode bypass it can easily take cathode resistors and even the tubes with it.

So caveat emptor!

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Old 30th December 2002, 03:01 PM   #18
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Hi frank,

This dutchman can tell you not only Black Gates conduct fully when faulty. Some other "normal"caps sometimes do and explode.
I never forget measuring a defective Musical Fidelity A100. These amps I repaired a few times including the later A1. These are dangerously built amps I can tell you. They overheat so much that the volume pot and selection switch sometimes melt !

In this A100 I had plus 35 V and minus 15 V supply voltage. When I was searching what the cause of this was I suddenly felt that one of the Nichicon 10.000 uF caps was very hot. In a reflex I moved away from the unit very fast and tried to pull the plug in my dive. Within a second the cap exploded and I still remember a lot of paper and aluminium particles floating in the very silent room with only the sound of my beating heart
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Old 30th December 2002, 03:23 PM   #19
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Default CAP FEST

Hi,

What these other caps do is that the electrolytic inside starts to boil.

When pressure builds up,and they don't have a safety valve built in,they then explode.

Now if you need to remove the paint from your amps this method works very well.

So while these caps explode they open the circuit,at worst they take the cathode resistor out as well.
That's the only damage you'll have.

The BGs however conduct fully since they short the plates internally...this shorts the cathode out and the tube will go into thermal runaway...till it dies.

Thanks,but no thanks,
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Old 30th December 2002, 03:30 PM   #20
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OK, Frank but you have to agree with me that using components above their maximum ratings never is a wise thing to do. Please don't think I am a BG advocate. I just think it is very easy to put the blame on a component ( used beyond specs ) and it will be very difficult to loose this stigma afterwards. You know how it works under audiophiles: "Pst, did you hear the BG's fully conduct in tube amps ? " I heard this kind of nonsense too much especially by guys that had never used the particular component in question.
I am 100% sure a 160 V type will survive this "torture"and will give the best possible sound. Better than film caps I want to add.
These components are among the best sounding I know and attaching a label on them for failure in 1 ( ! ) type of a particular DIY amp isn't right.

The unexpected behaviour of this BG series in this apparently extreme situation is not nice. Just wonder what other normal electrolytics do in this amp. Maybe the situation is so extreme ( DC range too close to the max including a fair amount of AC upon it ) that other electrolytics will behave the same ?!?! Ever tried to put 50 V on a 35 V electrolytic ?

The nichicon's I was talking about also had the safety vents but still exploded. This behaviour I have seen before with Teapo caps on Abit mainboards. They simply conduct and take the board with them.

Regards,

Jean-Paul
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