Depends on the Voltage - if a tubed amplifier you should be EXTREMELY careful - voltages inside are lethal - always work with one hand in your pocket.
But as described above use a bleeder resistor - 1Mohm definitely if a tubed Amplifier , a SS with much lower voltages you can use one of considerably lower resistance - I for an example use a 25W 50 Ohm resistor for bleeding the rails fast when working on my amplifiers, rails are between 35VDC and 65VDC 9 (The internally installed bleeders are 22K)
Best of luck
Peter
But as described above use a bleeder resistor - 1Mohm definitely if a tubed Amplifier , a SS with much lower voltages you can use one of considerably lower resistance - I for an example use a 25W 50 Ohm resistor for bleeding the rails fast when working on my amplifiers, rails are between 35VDC and 65VDC 9 (The internally installed bleeders are 22K)
Best of luck
Peter