Hi There,
I've bought a few Audyn MKP capacitors for my tube amplifier project with a maximum rating of 400Vdc
Yesterday i started wondering if i made a mistake buying these capacitors. The powersupply is 280-0-280 at 300mA whith a full wave rectifier. Calculating the maximum voltage results at -=395 for the first capacitor, which is just below the maximum rating.
However when i simulate the power supply in duncans amps PSU designer the voltage initially surges to around 450V for a very very short time.
Would this damage the capacitors? and should i look after a type which has higher ratings?
Thanks!
Hilbert
I've bought a few Audyn MKP capacitors for my tube amplifier project with a maximum rating of 400Vdc
Yesterday i started wondering if i made a mistake buying these capacitors. The powersupply is 280-0-280 at 300mA whith a full wave rectifier. Calculating the maximum voltage results at -=395 for the first capacitor, which is just below the maximum rating.
However when i simulate the power supply in duncans amps PSU designer the voltage initially surges to around 450V for a very very short time.
Would this damage the capacitors? and should i look after a type which has higher ratings?
Thanks!
Hilbert
Hilbert,
Caps. frequently can withstand a turn on surge that's in excess of the WVDC. Still, you are very much on the ragged edge.
If you are employing SS rectification, adding a well sized inrush current limiting thermistor could be the difference between a reliable unit and parts failure. A data sheet for GE/Thermometrics parts is here.
Caps. frequently can withstand a turn on surge that's in excess of the WVDC. Still, you are very much on the ragged edge.
If you are employing SS rectification, adding a well sized inrush current limiting thermistor could be the difference between a reliable unit and parts failure. A data sheet for GE/Thermometrics parts is here.
bereanbill said:
Cap tolerance has more to do with capacitance. The voltage rating has to do with the dielectric strength of the dielectric in the cap (polypropylene, teflon, paper, etc.) Materials with higher dielectric strength and thicker films can withstand more volts before breaking down/arcing over.
There is some margin in dielectric ratings for caps (the Panasonic TS-HA's, HB's etc have a momentary voltage rating that's 50V higher than the nominal rating IIRC), but start-up over voltage is sustained for seconds, considerably more than the rated time for overvoltage, I would imagine.
hilbert: Consider Panasonic TS-HB's for the PS electrolytics, they are rated at 450V working voltage, and have excellent ripple ratings.
Ratings
Your Caps unfortunately will not do for this application....can you return them??
You should be using caps that will see no more than 2/3s' of its WVDC rating..a 500 WVDC will still be too close. A 500 will work...but not for very long.......maybe a couple hundred hours running.
A 630 WVDC should be used.....this stretches lifetimes to tens of thousands of hours, similar to tube lifetimes.
___________________________________________Rick.........
Your Caps unfortunately will not do for this application....can you return them??
You should be using caps that will see no more than 2/3s' of its WVDC rating..a 500 WVDC will still be too close. A 500 will work...but not for very long.......maybe a couple hundred hours running.
A 630 WVDC should be used.....this stretches lifetimes to tens of thousands of hours, similar to tube lifetimes.
___________________________________________Rick.........
No I can't return them anymore, i have these for too long now. I might use them for another project, maybe a preamplifier when time comes. Too bad, i'll look around for electrolythic caps, 630V MKP capacitors are too expensive for what i want to build.
I have the book "building valve amplifiers" from Morgan Jones, he wrote something about combining an elco with a small MKP cap to get a cap which acts almost the same as a MKP cap from the same size. I'll dive into the book when i have the time again.
I have the book "building valve amplifiers" from Morgan Jones, he wrote something about combining an elco with a small MKP cap to get a cap which acts almost the same as a MKP cap from the same size. I'll dive into the book when i have the time again.
There are a couple of things I do not follow here - perhaps it is the time of night (in RSA) - but perhaps I can learn something.
Hilbert,
First, I accept that you are talking about a tube power supply with SS rectification, and capacitor input filter. You mention that you have a peak of 450V, which drops to 395V after 100mS? That would mean an initial transformer voltage of some 318V, which is almost 14% above the rating of 280V. Even including what Eli correctly said, this is quite a no-load/load percentage! I am used to rather 6 - 9% in practice.
Then it drops after 100mS to the fully loaded peak value? What loads the supply in such a short time? Not knowing your circuit, but even directly heated tubes somewhere do not come up to full emission in such a short time. Spreadspectrum mentioned the possible effect of an (input) choke; that is the only topology that I can think of to cause this. But from the description it seems that you have a capacitor input filter.
It appears that you were going to use the MKSs as only capacitors - apart from factory safety factors they do not have a surge spec as electrolytics do, so there you are not safe as others mentioned. But I will stick my head out and say the fixation in some quarters about not using electrolytics is overrated, especially with modern materials development. At best you might put poly's in parallel as Morgan Jones suggests; they are inexpensive but I have never found them essential.
Lastly, Richard Ellis, respectfully, why the very conservative approach regarding MKS/poly/whatever decent capacitor's voltage rating? What is there here that I do not know? Are you including the possibility of spikes, otherwise what else?
Hilbert,
First, I accept that you are talking about a tube power supply with SS rectification, and capacitor input filter. You mention that you have a peak of 450V, which drops to 395V after 100mS? That would mean an initial transformer voltage of some 318V, which is almost 14% above the rating of 280V. Even including what Eli correctly said, this is quite a no-load/load percentage! I am used to rather 6 - 9% in practice.
Then it drops after 100mS to the fully loaded peak value? What loads the supply in such a short time? Not knowing your circuit, but even directly heated tubes somewhere do not come up to full emission in such a short time. Spreadspectrum mentioned the possible effect of an (input) choke; that is the only topology that I can think of to cause this. But from the description it seems that you have a capacitor input filter.
It appears that you were going to use the MKSs as only capacitors - apart from factory safety factors they do not have a surge spec as electrolytics do, so there you are not safe as others mentioned. But I will stick my head out and say the fixation in some quarters about not using electrolytics is overrated, especially with modern materials development. At best you might put poly's in parallel as Morgan Jones suggests; they are inexpensive but I have never found them essential.
Lastly, Richard Ellis, respectfully, why the very conservative approach regarding MKS/poly/whatever decent capacitor's voltage rating? What is there here that I do not know? Are you including the possibility of spikes, otherwise what else?
Hi Johan Potgieter,
I'ts not the time of night, it might be my confusion.
I'll explain the situation. I am buiding an 807 tube PP amplifier, i have got the schematics from www.triodedick.com, a dutch guy, page is also in english.
His ( and mine, i've bought the same ) trafo set. The power transformer has the following ratings: 280-0-280 and is rectified using solid state. ( it has also two 310 secondaries which make it 310-280-0-280-310, the higher 310V is possibly used when swithing from solid state to tube rectification ).
In the schematics MKP capacitors are used, there was no maximum rating. When i bought mine i somehow made a miscalculation and bought Audyns with a maximum rating of 400V. Because my transformers arrived this, i started to design the casing and suddenly i realised that 280V * 1,4241 is around 395V which is very near the maximum specification of my capacitors.
So i did a simulation in PSU Designer and noticed a very small voltage spike of around 450V for a duration of 1/100th of a second ( not a 1/10th, my fault, sorry ) and started wondering whether i bought the right capacitors.
I hope i did clear things up now.
I'ts not the time of night, it might be my confusion.
I'll explain the situation. I am buiding an 807 tube PP amplifier, i have got the schematics from www.triodedick.com, a dutch guy, page is also in english.
His ( and mine, i've bought the same ) trafo set. The power transformer has the following ratings: 280-0-280 and is rectified using solid state. ( it has also two 310 secondaries which make it 310-280-0-280-310, the higher 310V is possibly used when swithing from solid state to tube rectification ).
In the schematics MKP capacitors are used, there was no maximum rating. When i bought mine i somehow made a miscalculation and bought Audyns with a maximum rating of 400V. Because my transformers arrived this, i started to design the casing and suddenly i realised that 280V * 1,4241 is around 395V which is very near the maximum specification of my capacitors.
So i did a simulation in PSU Designer and noticed a very small voltage spike of around 450V for a duration of 1/100th of a second ( not a 1/10th, my fault, sorry ) and started wondering whether i bought the right capacitors.
I hope i did clear things up now.
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