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Old 19th December 2008, 12:32 AM   #1
tmhajw is offline tmhajw  United Kingdom
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Default capacitors in series- what value balancing resistors?

Hi There
I'm a bit of a newbie so apologies now if this is a dumb question.
I've put 2 capacitors in series to increase their voltage capability. They are 20uF 400v pio. (I'm building a Berman 76/6sn7 pre and these are going to decouple, one lot of 2 per channel) They will see 405v. What value balancing resistors across each capacitor should I use to equalise voltages? I have already wired them up. One channel the voltages are fairly evenly spread (240v and 160v) the other channel one is 330v and 70v!
Thanks in advance
Thomas
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Old 19th December 2008, 01:12 AM   #2
KBK is offline KBK  Canada
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This suggests severe internal impedance issues, and I would suggest trying to arrange the capacitors again, to see if one of them has an internal impedance issue,as your situation suggests rather strongly.

In the two that are not balancing properly, label them 1 and 2, and then try them with the other two caps separately.

You may get a better balance that way, but I doubt it.

The situation is saying that you need at least one new cap, as at least one of them is considerably different than the other three.

After the caps balance out the voltages in a near perfect fashion on their own, then you can begin to look into any resistive loading/balancing. The big issue is dynamic and surge (power up) voltage levels and distribution, but getting a solid voltage balance between series run caps is critical from the get-go.

What you have right now, is a situation that is primed to 'pop' one of those caps fairly soon, from over voltage. The one that is not perfect, is the one that reads the 330V (high internal impedance), and the 70V one has low internal impedance. maybe.

It may be that one is really low (internal impedance) and the other is fine or one is really high (internal impedance), thus the imbalance, and thus the need to check how many are bad, either it is one or both right now. Checking against the two that balance well is the key to figuring that out.
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Old 19th December 2008, 02:44 AM   #3
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Depending on the voltages involved,I've used anything from ~220k up to ~1Meg.
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Old 19th December 2008, 03:20 AM   #4
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Default Re: capacitors in series- what value balancing resistors?

Quote:
Originally posted by tmhajw
What value balancing resistors across each capacitor should I use to equalise voltages?
In a case where two capacitors in series are pretty far out of balance, it is necessary to get the equalizing resistors below the leakage resistance of the capacitors. You may need to go as low as 100K ohms or lower at 1 or 2W. This will depend on the age and/or brand of capacitor used. It may also be that they will charge up and form if they are NOS that have sat around for years. In this case their leakage will decrease with use. I remember seeing at least one commercially made tube amplifier that used 50K 5W resistors across the caps because they were using poor imports that had high internal leakage. So try charging them up for a while and don't be afraid to use a lower value resistor pair.
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Old 19th December 2008, 03:23 AM   #5
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220k is very commonly used, although if I read your post correctly
these are supposed to be decoupling capacitors? I would strongly recommend
that you just find some decent 630V rated capacitors. At roughly 200V per cap,
you will still have 1 mA flowing through these resistors, which depending
upon the input impedance of the next stage could produce a significant
DC voltage offset. I would also worry quite a bit about the noise from
these resistors in the signal path.
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Old 19th December 2008, 11:21 AM   #6
tmhajw is offline tmhajw  United Kingdom
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Thanks gentlemen for all your replies, certainly all valid and food for thought...
The caps are cheap Russian pio from epay...nos, so probably 20yrs old or so.
Yes, the relatively severe imbalance in one pair looks a bit ropey. From your replies it seems if they were not decoupling caps ie p/s ones instead, I'd certainly try put balancing R's across them, but yes, they are more directly 'in the signal path'. So I guess ditch them for the time being and get decent ones...! I was doing this on the cheap to see how it sounded. It sounds fantastic, by the way- beats my naim...!
I've now another question..! The pre I've built is a 76 cathode followed by 6sn7 wgt. Anode voltage sn7 196v. Kath voltage sn7 206v. Heater to Kath sn7 206v!! Berman doesn't reference his heaters in the original 'Sound Practices' schematic. Seems pushing it a bit to me, though. What damage could be done by having the H to K so high?!
Thanks again for your replies, learned gentlemen & ladies...
Thomas
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Old 20th December 2008, 06:19 PM   #7
tmhajw is offline tmhajw  United Kingdom
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Iswapped the capacitor 'pairs' around yesterday- the capacitor that had taken the 330v now went down to a more equal 209v with its partner 191v- much better. The capacitor taking only 70v of the 400v stayed the same with it's new partner - so I guess this is the duff one...
Thanks again for previous replies.
Any thoughts on heater to Kath voltages being at/ slightly over max stated limits?! It doesn't hum or anything!
Thomas
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