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Old 22nd February 2012, 05:39 PM   #1
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Default fixed bias capacitors

Hello,

I just realized I put in my 10uF filter(?) capacitors for the fixed bias circuit in backwards. This is in a Bassman clone I built. The schematic shows the positive ends on both of them going to ground. I put them in the standard way-negative to ground instead, but it works. I have -50v going to the grid @ about 47mA. This is in line with the 70% rule I followed. (I know there are many opinions about this 'rule' and don't want to get into a discussion about it here-please)

Is what I did going to be okay? The amp is now 2000 miles away with my nephew.

Can anyone explain why the caps are reversed here?

Thank you,

Daniel
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Old 22nd February 2012, 06:22 PM   #2
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Electrolytic capacitors are polarized elements. In a few time, when reversed, they will leak and perhaps explode. You MUST observe correct polarity, and if bias is negative respect to ground, then the positive of the capacitor also must be grounded. Or, in last use bipolar or nonpolarized units.
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Old 22nd February 2012, 07:14 PM   #3
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Default thanks...

I see, I have instructed my nephew to not use it until I can have his Dad switch them around.

He's had it for a few months and plays it every day, why wouldn't they pop by now?

And, how can they work being in reversed like that?

Daniel
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Old 22nd February 2012, 07:48 PM   #4
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Generally electrolytic capacitors fail VERY, VERY quickly when connected the wrong way round, but presuming these are in the low current bias circuits?, then they won't die in any spectacular fashion. It's probable though that they have already failed, and should be replaced not just turned round -it's likely that the bias is running considerably at the moment.

As already mentioned, the capacitors are the other way round because it's for a negative voltage rail.
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Old 22nd February 2012, 08:17 PM   #5
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I played the amp for a couple of months, had a touring guitarist check it out for at least an hour and I checked and rechecked the bias voltage and the current before I sent it.

I'm not sure how this worked for so long if they failed though. Replacing them-yes, I'll have that done-good idea.
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Old 23rd February 2012, 12:13 AM   #6
Enzo is offline Enzo  United States
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Who in this business hasn't inadvertantly installed a cap backwards, I admit to my mistakes. Usually they fail right away, not always. It is possible the little caps managed to "reform" themselves when powered up in reverse, so they are finctioning as caps. But how reliable will they be? And how long will they live? The question is, for the price of a couple dollar caps, when do we want them to fail? WHen the kid is playing a gig? Or sometime after they have been removed?

They are working, so the amp will work. As long as they are not refusing to filter that low current bias supply, the amp will work as expected, and no one would be able to tell the difference. But one day... FFFFFFTTT.
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Old 23rd February 2012, 03:02 AM   #7
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That makes more sense, thanks everyone. I'm glad I noticed it now and not while he's showing it off...
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Old 23rd February 2012, 12:12 PM   #8
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Quote:
Originally Posted by Enzo View Post
Who in this business hasn't inadvertantly installed a cap backwards, I admit to my mistakes. Usually they fail right away, not always. It is possible the little caps managed to "reform" themselves when powered up in reverse, so they are finctioning as caps. But how reliable will they be? And how long will they live? The question is, for the price of a couple dollar caps, when do we want them to fail? WHen the kid is playing a gig? Or sometime after they have been removed?

They are working, so the amp will work. As long as they are not refusing to filter that low current bias supply, the amp will work as expected, and no one would be able to tell the difference. But one day... FFFFFFTTT.
I believe that Daniel gets luck because the impedance in series between power supply and caps surely was high, but in the case these resistors are low or donīt exist, the caps would explode instantly. I several times at my job put a diode, transistor, IC or cap reverted, but in almost cases this conduct to a device failure in few seconds.

Be careful because such an error not only can destroy the equipment, you can have personal injury when the cap container explodes.

Best regards.
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Last edited by Osvaldo de Banfield; 23rd February 2012 at 12:15 PM.
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Old 23rd February 2012, 03:07 PM   #9
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Quote:
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Who in this business hasn't inadvertently installed a cap backwards, I admit to my mistakes. Usually they fail right away, not always.
I had a small one (10uF or so) 'rocket' off the board one day, narrowly missing my eye and hitting the ceiling
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Old 23rd February 2012, 03:09 PM   #10
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Je je je, I have two!!!
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