You know how polypropyline motor run caps…..

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Paul Barker
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#1 You know how polypropyline motor run caps…..

Post by Paul Barker »

Are rated usually but not exclusively for AC, and when I started out the old guys who gave me the nod for dc you could use it in higher voltages. Lets not say the actual voltage because opinions vary. The salient point is not affected whether its 1.414 greater tolerance or the other extreem sometimes mensioned 2.828 or whether you are a double man.

My 6em7 project because I chosen the Monkey method stands the cathode at 170 volts dc. Suddenly, I realised, my bypass paper in oil caps are rated at 160v.

Any thoughts?
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#2 Re: You know how polypropyline motor run caps…..

Post by pre65 »

What is your required capacitor capacity ?
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#3 Re: You know how polypropyline motor run caps…..

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Not fussed the capacity I have 90 uF is enough to form high pass filter starting -3db at 6 hz. I could tolerate less. But not half which would make voltage a non issue. Ill go do the maths and come back on that.
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#4 Re: You know how polypropyline motor run caps…..

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OK back already, uneven number of caps so can only series connect two which double voltage tolerance and reduces capacitance to 30, which puts -3db at 18hz which may actually make no loss because my speakers might be rolled off before that anyway.

But there is another option; stop turning the variac up after cathode voltage reaches 160v and accept less power output.

But my thought is that just like polyprop motor caps the pio cap manufacturer may have intended the voltage spec in AC terms not dc so same principles.
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#5 Re: You know how polypropyline motor run caps…..

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That aside I gues Ill make the first iteration within the 160v limit see how it sounds and enjoy it. Before I nudge the dc voltage up tp 170 and see if anything explodes. With cheap to me 6em7’s. Rather than expensive rare 1930’s valves. I estimate the em7’s will sound just as good by the way, just lower powered. I’m waiting on resistors RS are sending on 7th July. I have a virtual gas safe inspection tomorrow so Im studying ATM. I also havevto retake my qualification to work on gas (we have to do this every five years by date is 30th April. So Im also studing for that. But if status quo at home levels off I will not retake and never work as a gas man again. So I am trying to study for something I may never do again, and I have no udea what condition Diana will be in on 30th April when I have to go one way or the other. Neither do I have a vehicle. Another part if the unpredictable equasion.

So Ill build this amp to keep me sane in the mean time.
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
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#6 Re: You know how polypropyline motor run caps…..

Post by Nick »

I guess it depends on the cap. But if not specifically DC rated, I would expect 1.414 of AC voltage would be a safe limit. Not sure where the justification for higher multipliers would be from other than optimisum.

I checked motor start cap on RS, it had this in the docs

"Direct current operation
These capacitors can be used with a DC voltage not exceeding the peak value of the rated voltage."

Where they described the peak value as square root 2 times the AC RMS voltage.
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#7 Re: You know how polypropyline motor run caps…..

Post by Paul Barker »

Yes thats what I think they used to say for pio caps. So few people using them now its not a question that comes up.

The caps sold at engineering shows are polypropylene and the reason theyre sold is the equipment is modern use, to shift phase not just to start and or run motors, but to equalise the 3phases in industry because they are fined for stressing the network. The rating is ac.
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#8 Re: You know how polypropyline motor run caps…..

Post by Paul Barker »

I passed the virtual gas assessment.
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#9 Re: You know how polypropyline motor run caps…..

Post by pre65 »

Paul Barker wrote: Mon Jan 29, 2024 4:36 pm I passed the virtual gas assessment.
Well done Paul. :D
The only thing necessary for the triumph of evil is for good men to do nothing.

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#10 Re: You know how polypropyline motor run caps…..

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It was harder than a real man came to 3 jobs with you. All my career it was the same man, Phil Morris from Hull. He knew me so well it was a relaxed affair. He was my first every one in between and my last man in person. Now its AI. A sign of the times. Wander which year in the future they will find out the mistakes AI software has made and there is another software scandal. I mean we had to take them to 3 real jobs you had done. If I ran the register of people deemed competent to work on gas I would send human gas registered engineers with suitable additional training to asses the workmanship of engineers on the register. But what do I know?
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
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