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Old 14th August 2003, 05:51 PM   #1
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Default A dumb question about coupling caps.

I plan on using a couple of .33 200VDC Vitamin Q coupling caps in the design below instead of the .47 caps shown. My question is this. Are the 200VDC Vitamin Q caps rated high enough voltage wise for this position? AC wise they will be fine. I'm thinking that since there really won't be a DC voltage drop across them (since they are connected to the grids and not to ground). Am I looking at this correctly? Thanks.

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Old 14th August 2003, 06:02 PM   #2
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Use a 400V cap there. The grid is at ground potential. And the cap will see the full B+ at turn-on, not to mention what happens with large signal swings. You might have a 200V cap survive because of the current limiting inherent in the grid resistor, but I wouldn't want to chance it, since the downside will involve smoke, flame, and melting glass.
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Old 14th August 2003, 06:38 PM   #3
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Damn Sy. I must be asleep at the wheel. Thanks. I don't know why I didn't see that. Cheers.

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Old 14th August 2003, 08:25 PM   #4
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100% correct, except you'll only get voltage on the cap when the circuit is passing very low frequencies ( <50Hz), around the cutoff end of its bandwidth.

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Old 14th August 2003, 08:35 PM   #5
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Waitaminute. What do you mean the cutoff point is going to be around 50 Hz. It should be much lower than that.

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Old 14th August 2003, 11:44 PM   #6
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It will. But it'll just barely begin to roll off around 50Hz.

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Old 14th August 2003, 11:48 PM   #7
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Hi,

Quote:
It will. But it'll just begin to roll off around 50Hz.
Correct.

Which is why reducing that coupling cap in value isn't such a good idea...

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Old 15th August 2003, 12:27 AM   #8
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Don't know what you've selected for C2 (10uF/400Vdc) but the Vishay 735P series available through Mouser have extremely low ESL and ESR - using them for your B+ bypassing ought to help preserve maximum bandwidth (heck, I'd use on for the bulk filter capacitor and a smaller 1uF/0.1uF combo near the plate resistor for bypassing.

For coupling, I'd use one 1uF/400VDC 735P series cap, unless you absolutely must have paper dielectric, in which case "line to ground" safety capacitors might be worth looking into (they invariably use paper dielectric because of its unparalleled self-healing properties).
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Old 15th August 2003, 12:34 AM   #9
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Hi,

Quote:
For coupling, I'd use one 1uF/400VDC 735P series cap,
Using oversized coupling caps isn't a good idea...using the best possible quality cap is much better.

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Old 15th August 2003, 02:01 AM   #10
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Hold on a sec, isnt the 15k resistor dropping 150V at 10mA? Therefore, only 150V is seen at the coupling cap. With 50V to spare I think 200V caps would work. Or am I missing something?
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