Tube Amps / Music Electronics
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|2/18/2000 12:17 AM|
||Cap drain resistors|
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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