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
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
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.
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
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
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.
As already mentioned, the capacitors are the other way round because it's for a negative voltage rail.
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.
I'm not sure how this worked for so long if they failed though. Replacing them-yes, I'll have that done-good idea.
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.
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.
That makes more sense, thanks everyone. I'm glad I noticed it now and not while he's showing it off...
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 don´t 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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I had a small one (10uF or so) 'rocket' off the board one day, narrowly missing my eye and hitting the ceiling 😱
I repaired an old B&W TV years back, can't remember the make.
The main electrolytic was mounted horizontally under a steel clamp to the chassis.
The can rocketed forwards (despite the clamp), crumpling completely against the back of the CRT.
All the paper and foil inside went backwards, forcing the back partially off, and filled the set and back with debris.
Cleaning it up was a right job, the actual repair took a few minutes just changing the cap, but the clean-up took a few hours.
The main electrolytic was mounted horizontally under a steel clamp to the chassis.
The can rocketed forwards (despite the clamp), crumpling completely against the back of the CRT.
All the paper and foil inside went backwards, forcing the back partially off, and filled the set and back with debris.
Cleaning it up was a right job, the actual repair took a few minutes just changing the cap, but the clean-up took a few hours.
So far I've not managed to blow an electrolytic this way (I know I'll regret saying this...).
I did however manage to impale a lump of plastic into the centre of my forehead by mistaking a no-built-in-resistor indicator neon for one with the resistor. At work, with my boss immediately behind me, with smaller fragments peppered around my eye sockets.
I then went home and had a cd drive suddenly fly open very quickly and the disc, still spinning at about 48X frisbee out and slice my forehead open across the exact same spot the plastic had hit earlier.
And I'm still doing this stuff, just a bit more carefully.....
I had a LARGE electrolytic explode on me when building a transistor amp - on my bed at my mums 😀
I hate cleaning up exploded caps, I use to work on emergency generators and pump stations and the big motor run/start caps would explode inside the control panel and just create a real mess (especially oil filled).
Anyway the reason why the positive goes to ground is because ground is at 0v and that is more + than -50v.
Anyway the reason why the positive goes to ground is because ground is at 0v and that is more + than -50v.
Dear Famous...
I understand now, thanks for that description. "Ground is more positive than -50v." That is all I needed to jog my understanding.
I understand now, thanks for that description. "Ground is more positive than -50v." That is all I needed to jog my understanding.
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