From MetroAmp Wiki
The primary attribute that makes tube amplifiers dangerous to work on is that fact that filter capacitors in the power supply can hold a charge long after the amp is powered off, even unplugged from the wall.
To avoid electrocution, these filter caps must be "drained" every time the amp has been powered on. There are many ways to do this. Some are very safe while others are not so safe, i.e. bridging a screwdriver between the caps and the chassis. Not recommended and sure to cause fireworks!
We suggest using a simple alligator clip lead connected between the chassis (ground) and a 100k plate resistor in the first gain stage of most vintage amps.
Once clipped in place, this method will take 1-3 minutes on average to drain all the stored charge from filters caps. Which you should confirm with your meter BEFORE working inside the amp.
Also critically important is to remember to remove the clip before powering the amp on. Fortunately, with this method if you do forget to remove the clip, current is limited by the 100k resistor.