Fig 1--Filter Caps Discharger Tool
Working safely on tube amps requires discharging the Filter Capacitors.
Filter Capacitors will hold a charge for a long period of time even after the amplifier is off. I'm sure you've all heard all the warnings about tube amps, these are why.
There are several ways to discharge filter caps--some safe and some not. I'll cover a couple of ways to discharge caps here.
Preferred way 1: Using a resistor between the postitive end of the cap to ground. It's a very simple tool to make with a pair of insulated aligator clips, some wire and a resistor. I have heard several different sizes of resistors to use. Personally I use a 2 watt 10k resistor as called out in Torres book-Fig 1.
Wire it up as shown, make sure the aligator clips have some good insulation on them. As for the wire, I prefer a 600v rating. Which should safely cover most applications. Use a 2 watt 10k metal film or wirewound resistor. That should last longer and be safer in the long run. Make sure to cover any expose wire with electrical tape.
When using this discharger tool, you should hook one of the clips to ground first. Then attach the other clip to the positive terminal of the capacitor. Since the caps are all connected, this should discharge all the capacitors. You'll notice on some designs there is a capacitor on the other side of the Standby switch. Personally I leave the standby switch in the on position to make sure everything drains.
When working on the amp, leave it hooked up. Capacitors have a memory and sometime regenerate some charge--safety counts here.
Another Way 2: When shutting down your amp, if you have a standby. Keep playing the guitar and hit the POWER switch. The sound will slowly fade out and then quit. The filter caps should be discharged, use your DVM to be sure.
Another Way 3: The dead short--using a screwdriver with a thick insulated handle. Place the end on the chassis and lean across the positive terminals of the caps. Watch the sparks--great huh? This is NOT reccomended--the danger to you and the filter caps isn't worth a $1.50 discharger tool. Filter caps aren't meant to be rapidly discharged like that, you can dramatically shorten the life of the caps by using this method.
Most of the older amplifier designs have some resistor to ground in line with the capacitors. Reference a Fender schematic and you'll notice some 220K in paralell with the two initial capacitors that are in series. Marhsall also has some 56K bleeder resistors in their earlier designs. These will also effectivly bleed off the voltage, remember to keep that standby open on those Fenders. But as a extra precaution, I always use my discharger tool as a visual reference that the caps have been discharged.