Warning! Discharge those Capacitors!

Working inside a tube amplifier can be dangerous if you don’t know the basic safety practices for this kind of work. If you aren’t prepared to take the time to learn and apply the right precautions to keep yourself safe, don’t work on your own amp. You can seriously injure yourself or get yourself killed! This is not intended to be a complete guide to safety in tube equipment, just to hit the high points as refresher for those of you who have some experience. The best way to learn the requirements and practices for safety in tube equipment is to find someone who will teach you one on one.

BASIC REQUIREMENTS

• UNPLUG IT FIRST: Pretty self explanatory. Do not, ever, ever, leave the equipment plugged in and start work on it unless you specifically intend to make some live-voltage measurement. Leaving it plugged in guarantees that you will have hazardous voltages inside the chassis where you are about to work. This is like setting a trap for yourself.
• LET IT DRAIN: If the amp has been turned on recently, the caps will still have some high voltage left in them after the switch is turned off. Let it sit for five minutes after you turn it off.
• SUCK IT DRY: When you open up an amp, you need to drain off any residual high voltage. A handy way to do this is to connect a shorting jumper between the plate of a preamp tube and ground. This jumper will drain any high voltage to ground through the 50k to 100K plate resistor on the tube. To do this successfully, you will need to know which pins are the plate pins. Look it up for the amp you’re going to be working on. You’ll need to know this for the work anyway. Leave the jumper in place while you do your work (high voltage electrolytic caps can “re-grow” voltage like a battery sometimes. Really.) Remember to remove it when you finish your work.
• TEST IT: Take your multimeter and ground the (-) lead. Probe the high voltage caps and be sure the voltage across them is down, preferably to less than 10V.
• BUTTON IT BACK UP FIRST: Take the shorting jumper out. Put the chassis back in the cabinet, making sure all of your tools, stray bits of solder, wire, etc. are out of it. You don’t have to actually put all the screws and so forth back in if you believe more work might be needed, but make sure that the chassis is sitting stably in the cabinet and won’t fall out. At the end of a listening test, either continue buttoning up if you’re done, or go back to UNPLUG IT FIRST.
• For more information, follow this link: www.repairfaq.org/sam/captest.htm

Source: https://mercurymagnetics.com/pages/_misc/FAQ.htm#DISCHARGE_THOSE_CAPACITORS!!!