Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ncusack on May 26, 2016, 12:09:58 pm
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Im looking to string a couple 100uF 400V filter caps to get 50uF @ 800V. Going by merlins book it suggests 50/C for the voltage sharing resistors to balance things out. That would mean a couple 470K resistors in parallel with each cap. My question is what wattage resistor should be used in this application. If I assume 400V across each resistor that works out to 0.34W in a worst case full 800VDC situation. I'd likely use at least a 1W if that checks out but would a 1/2W suffice?
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I would not be scrimpy here.
I built a bench-type power supply from junk parts and used for the filter caps 2 in series like you are planning. Those were used, reformed caps. Can-type, the upper one in an insulating sleeve. As it turned out, I also used 470K balancing resistors for no good reason other than they were there. Things started out OK, but one of those caps blew on maybe the 3rd or 4th time I turned my project on. Before that happened, I measured volts across the caps and one had 90% of the volts and the other had 10%...or something like that.
My non-scientific impressions: The usual value for these R's is 220K. (If you are saying "a couple 470K resistors in parallel with each cap, meaning a 235K in parallel with each cap, that would be fine.)
It would still be my preference to have a flameproof 2-3 watt R in this application, because as those caps age, they could conceivable drift in different directions and the "perfectly balanced" state could become unbalanced.
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I'd likely use at least a 1W if that checks out but would a 1/2W suffice?
1/2W is too light IMO. Fender was fond of CC 220K @ 1W. I would prefer 3W metal oxide because the size/cost is about the same as a 1W CC.
A good rule of thumb when calculating the proper wattage for a resistor...
1. Calculate the actual power dissipation as you did.
2. Double that value for safety margin.
3. Choose the next higher wattage rating that is available to you.
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Check the voltage rating of the resistor options.
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> If I assume 400V across each resistor
If it WAS 400V, then you would not need balance resistors.
I'd like to figure as-bad-as 1/3rd 2/3rd. 266V and 533V. Ooops, the 450V cap would blow-up, though not instantly. Still, 533V in 470K is 0.6W. Half-Watt may hang-in about as long as the cap.
If the caps are way over-spec and identical, only gentle balance is needed. If they are within 10% of voltage rating (you are within 11.2%), and/or not *known* to be just-alike, then fairly substantial balance may be needed.
As trobbins says, many smaller resistors are rated 300V max. Go over, they work a while, but are drifty and may drift open.
I'd lean more to _four_ 100K 1W in series across the 800V, tap at center (effective 200K 2W each side). As 100K 1W/2W is or should be a handy part in guitar-amp work, this may be convenient.