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Cap drain resistors

2/18/2000 12:17 AM
Cap drain resistors
Hello -  
What is the properer sized resistor to drain the high volt caps off the larger tube amps.  
2/18/2000 1:39 AM

Depends on the voltage. You want to size the resistor to bring it down quickly, yet not eat -and dissipate!- lots of power when the amp is running.  
What I do is make an estimation about how much power I'm willing to waste (maybe 1/2 to 1 watt) continuously. Then I take the voltage, say 450V, and calculate the resistor from  
P = Vsquared/R, or R=Vsquared/P  
so for 450V and 1/2w, you get R=405000 ohms, or about 470K.  
If you use a watt of continuous dissipation, you get R=202500 ohms, about 200K.  
The resistor WILL dissipate that much power so you must derate it. A resistor's power dissipation is usually based on a surface temperature of 100C to 200C - hot enough to brand the logo in your finger and shorten life. So pick a resistor that's at least twice, preferably three or four times the rated power from what it will actually see.  
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2/18/2000 3:11 AM

Are you talking about bleeder caps that are paralleled with the filter caps in the power supply, or are you talking about using a resistor to discharge the caps prior to servicing an amp(not permanently mounted but attached to an alligator clip)?  
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