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ampage archive
Vintage threads from the first ten years
Electrolytic ratings
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 6/2/1999 9:46 PM |
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| tubegirl | Electrolytic ratings I've always been intrigued at how manufacturers like Fender and Vox got away with using 450v rated power supply caps while also using large chokes. The schem for the AC30.36 for example, shows a 10-20H choke used in a PI configuration with 450v rated caps. However a EWB sim of this setup produces initial inductor charging peaks of 520-550v. Similar results with many of the Fender schems (some peak at 600v). What's going on ? Assuming the sim. is accurate, why didn't the electro's blow ? I thought one should AT NO TIME exceed the electrolytic rating. I notice Fender later upped the ratings to 600 or 525v (and some of the reissues use series electro's) but even this doesn't seem to give the old 80% safety margin. Any ideas ? Is there some real-world trickery going on here ? |
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| 6/2/1999 11:09 PM |
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Bruce |
Yeah, I think once the power tubes start conducting real bias current at idle, the total RC time constant isn't there to let the caps charge up to full peak voltage anymore so the caps only see that high voltage for a few seconds out of the whole time the amp is turned on. Caps aren't suddenly blowing up at 451vdc and holding just fine at 449vdc. Notice that most of the ones that will sufferd from that over voltage, later had 70uf/350v caps in series as the first filter. Bruce Mission Amps |
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| 6/2/1999 11:46 PM |
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| Richie |
On most electrolitics there is a surge voltage rating,some more than others.And they are wired different to get more voltage protection from the startup.Like Fenders, having two caps series, with parallel resistors. I'll try to explain, if I don't confuse myself. If you have two caps in series say 100uf at 350 volts.. you would end up with half of the capacitance, 50uf but the voltage it would be able to handle would be 700 volts. But the problem with this.[Doc and I taked about this] Is in most fenders, when you have a cap failure it is most common to see one of these. Because one may weaken over time,and the other has to carry more of the load, so it will go first.If you have them in parallel.. say a 16uf at 525 it will double the capacitence and be 32uf but the voltage will still be 525.Some use both series/parallel in some amps.Most caps have about a 50 volt surge rating. Thats why people like the slow turn on of a GZ34 it builds up slow. Where the others are a fast buildup in voltage. Doc could probably answer this more technical than I can.But I like to use a bigger rated cap for voltage.when they are run on the edge.. you will run into trouble alot faster. I have learned that no matter what the uf cap you use, if it gets close or above the voltage rating it will still hum.I think its telling someone its hurting!! I fought a hum in an amp for a while, and found out it needed a bigger voltage cap.It had 525 volt caps..and ran at 560.Put in a 600volt cap and cured it.Its just safer for voltage spikes and uncertian power voltages at different places.If something is on the edge,it can't stand a zap, and it sometimes don't take much. I hope this helps. And some on here will remember seeing some of the mess form the oil caps letting go!!![Richie] |
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