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Old 26th July 2012, 02:21 PM   #1
alecu7 is offline alecu7  Canada
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Default capacitor voltage for tube amp power supply

Hi all,

I'm building a power supply for a tube amplifier.
the power supply is :transformer(350V), bridge rectifier, 100uF, 5H, 100uF.
My B+ is 450Vdc according to PSU Designer II @150mA. I found the following capacitor: DS371506-CA which is rated for 370Vac. I think it's ok since 370Vac x 1.4 ~ 500Vdc, but I like to get confirmation from the experts.

Thanks,
Alex
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Old 26th July 2012, 03:04 PM   #2
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I am not an expert, but it should withstand. I used oil caps for AC with luck in PSUs. However, check if peak voltage at turn on does not exceed maximum parameters for the cap. Next thing is temperature - only 70 degrees Centigrade.

Last edited by yagoolar; 26th July 2012 at 03:08 PM.
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Old 26th July 2012, 03:11 PM   #3
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Hi!

Which factor can be applied to the AC rating to get the DC number depends on the construction of the capacitor. I once discussed this with an engineer from Jensen Capacitors. He said that the minimum you can apply is factor 1.4. In some cases even much more. But if you do not have the DC rating from the manufacturer, use 1.4 and you are on the save side. So in your case. No problem to use it

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Old 26th July 2012, 03:16 PM   #4
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Most likely that capacitor is an aluminum can motor run capacitor, They are rated in volts AC.

You should be good to go.

Follow the usual caution procedures the first time that you fire it up.
I use clip leads to the DVM to monitor the voltage and a variac to ramp up slowly. I also cover the whole thing up to contain the mess if it goes bang.

Confession: I once connected a 10uf electrolytic capacitor in reverse polarity. It sounded like a firecracker going off and sprayed gick everywhere. It was covered, none of it got in my face or eyes.

DT
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Old 26th July 2012, 03:18 PM   #5
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350v ac times 1.41 gives younthe maximum dc voltage on the capacitor. Add 10 percent
And you are save.
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Old 26th July 2012, 03:34 PM   #6
alecu7 is offline alecu7  Canada
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Thank you all for the answers.

Alex
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Old 26th July 2012, 03:40 PM   #7
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Quote:
Originally Posted by alecu7 View Post
Hi all,

I'm building a power supply for a tube amplifier.
the power supply is :transformer(350V), bridge rectifier, 100uF, 5H, 100uF.
My B+ is 450Vdc according to PSU Designer II @150mA. I found the following capacitor: DS371506-CA which is rated for 370Vac. I think it's ok since 370Vac x 1.4 ~ 500Vdc, but I like to get confirmation from the experts.

Thanks,
Alex
Check the voltage with a lower load to see how high it is . psu II will give you a peak voltage on cap one check that . The second after the choke is going to see less peaks it ok at this output. What diodes are you using ss would be 350 X 1.4 about 490 that close up to the limit. The first one to should be upped to 440 ac or 600 dc plus very safe. As has been stated before 70c is a low temp going higher on the voltage gives some room on the use of the cap . Cost difference is small maybe 3 dollars on the first cap that's cheap safety . Longer life is also there for under using the margin of operation on the cap .

Last edited by Triodethom; 26th July 2012 at 03:42 PM.
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Old 26th July 2012, 03:53 PM   #8
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by alecu7 View Post
...g capacitor: DS371506-CA which is rated for 370Vac. I think it's ok since 370Vac x 1.4 ~ 500Vdc, but I like to get confirmation from the experts.
No way. that cap would fail quickly. The filter cap must be higher then the peak B+ voltage. You can buy 500V caps.


If you only have lower voltage caps you can use two in series. That gives you double the voltage but 1/2 the capacitance value and you will need to voltage equalizing resisters too.

Weber sells some high volt caps at a fair price.
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Old 26th July 2012, 03:56 PM   #9
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by Triodethom View Post
Check the voltage with a lower load to see how high it is . psu II will give you a peak voltage on cap one check that . The second after the choke is going to see less peaks it ok at this output. ...
That only applies in the steady state case after the amp has wormmed up. At first the tubes will not be conducting so the resister in the power supply will not drop the voltage to the second cap. So EVERY time to power the amp on you are running the second cap over it's rating. It will fail eventually.

Buy caps with the required voltage. I'd say to buy then with a 20% or at least 10% margin so if you have a B+ of 450V buy at least a 500V cap
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Old 26th July 2012, 04:08 PM   #10
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The cap can withstand 1,75 times max voltage for approx. 60 s. Yet you may:
- simulate using stepped current source as a psu load
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