Tube Amps / Music Electronics
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|previous: Anthony Stauffer Well you all have certainly scared ... -- 12/8/1998 3:18 PM||View Thread|
|12/9/1998 5:15 AM|
|Dave H||Re: How many folks do this crazy thing?|
Half an hour may not be long enough if there are no bleeder resistors. Caps can retain enough charge to give you a serious shock days after switch off. I always fit at least one bleeder resistor in all my amps sometimes more so that there is always a resistive discharge path for the power supply caps even when the standby switch is open and all fuses are removed.
To select the bleeder value I assume that the only discharge path for the caps is through the bleeder. You could be working on an amp with the tubes removed, standby open or fuses blown so the bleeder may be the only discharge path. A capacitor discharging through a resistor will be down to 1/10 voltage about 2 x RC time constants after switch off. I add up all the B+ caps in the amp and chose a bleeder to make 2 x RC < 30sec so that any caps charged up to 500V are down to less than 50V in 30sec.
e.g. If the caps add up to 100u then the bleeder is R = 30/(2xC) = 150k. Make sure the power and voltage ratings of the bleeder are high enough. Two 75k 2W MF resistors in series would be OK.
I wait 30sec after unplugging the amp before I go in and double check the B+ voltage with a DVM before I touch anything. You don’t have to worry about the cathode bypass caps. They are only charged up to a few volts and are connected across a low value resistor so they discharge rapidly after switching off.
|Anthony Stauffer Dave,|