Take pride in how safe you can be, you can’t play guitar or build amps if you’re dead.
You may have heard various horror stories about things inside amps that can store a charge that can kill you. Well, it's true! However, those dangerous voltages can be easily drained in just a few minutes, so take your time and don’t rush.
It is absolutely vital that we drain the filter capacitors in the DC power supply before working on any guitar amplifier. These often hold a charge of several hundred volts, which is potentially deadly. Capacitors can hold their charge for a long time, even with the power off and the amp unplugged. It's very important that we learn to properly "drain" this residual voltage so that we may safely work on the inside of our amplifiers.
What You’ll Need Cacitor Discharge Tool
Safety glasses. Always use safety glasses. If you wire a capacitor in backwards there is a very good chance that it will explode in your face. Milwaukee Safety Glasses Clear Hard Coat
Digital multi-meter. Capable of reading 500 volts DC. Triplett Digital Multimeter
Capacitor discharge tool. A jumper wire consisting of alligator clips on the ends, with a resistor in series in the middle of the jumper.
Needle nose pliers. With very good insulation on the handles for holding the alligator clips of the capacitor discharge tool. Titan 5-Piece Precision Plier Set With Case.
Always make sure your tools are in good condition. If there’s any question, replace them. Cracked insulation is one of the first things to check, and check often.
Turn the power switch OFF, and place the standby switch in the ON position. This isn't the same as having your amp "on standby."
Always unplug the amp from the wall. Unplugging from the wall will not drain the filter capacitors completely, but again this is for our own safety. Unplugging is the equivalent to "locking out" before working on a downed machine.
Because filter caps are almost always of the electrolytic type, they're "polarized" and therefore have a positive and negative end. We can identify the positive or negative ends by looking for the "polarity indicator" printed on the cap's side.
Our first objective is to check for residual voltage with a multi-meter. In some cases most of the voltage may have already been drained. Many amplifiers will incorporate "bleeder" resistors, which will drain the capacitor charge automatically in a few minutes after the amp is turned off. Don't depend on this resistor to do the job. Never assume the caps have been drained to a safe level without checking with a reliable multi-meter. Here’s a couple of ways to measure the residual voltage contained in the filter capacitors. Note that these are also equally valid for draining the caps.
Across the capacitor.
From the positive side of the cap to the chassis. This is essentially the same as draining "across" the caps since the negative side is at the same electrical potential as the chassis.
From the chassis to a tube pin. This can be done at pin 3 of any power tube, or pins 1 or 6 of any preamp tube. By shorting the chassis to the correct pin the caps will be forced to drain through the plate load of that tube. This prevents sparking from high current.
In old electronics books they tell you to use a screw driver with an insulated handle to short out the caps. "Shorting" with a screwdriver will almost instantly drain the caps to zero volts, causing sparks from high current. This is not acceptable. If there are several hundred volts stored in the caps, be prepared for a nice sized spark. It will most likely leave a black mark on the chassis, and in extreme cases, partially weld the screwdriver to the chassis.
Now we’re ready to start testing and discharging.
Set your multi-meter to read DC voltage. Adjust the meter's range so that it's above 500 volts. Clip the black lead of your multi-meter to the chassis. Using only one hand, with the other hand far from the chassis (preferably in your pocket or behind your back), measure the DC voltage across one of the filter capacitors. You can adjust the meter's range down if you can’t get a clear reading. Any DC voltage readings less than 5 volts are harmless, so the amp can be safely worked on, but do a double check of all the caps just to make sure. If not we must drain off the residual voltage with a resistor until it is safe. Use a resistor value somewhere between 10K and 100K, with a power rating of 5W or so (a high power rating is not really necessary for heat dissipation reasons, but mainly because the higher power resistors are physically more robust and won't tend to break). The larger the resistor value, the longer it will take to discharge. If your capacitors do not have a dangerously high voltage, say around 30 volts, feel free to use a smaller 1K 1Watt resistor.
Now take your capacitor discharge tool and clip one side to the chassis. Always clip to the chassis ground first before connecting to the circuit. If you clip to the circuit first you will have a hot wire hanging out of your amp, which is very dangerous. Once you have one side clipped to the chassis, take your needle nose pliers with good insulation on the handles and hold the alligator clip that will go to the circuit with the needle nose pliers. Clip this to pin 1 or 6 of the first preamp tube. This seems to be the safest way of discharging the capacitors. If you’re working on the first preamp tube socket, then clip your discharge tool to pin 1 or 6 of the second preamp tube. Wait a few minutes (about 5 minutes, if the amp has bleeder resistors this time will be shorter) then, while the discharge tool is still in place recheck all the capacitors with your multi-meter and keep that other hand behind you. Keep checking until the voltage has reached a safe level. Note: This procedure must be done every time you turn the amp on then off.
Don’t forget to remove the discharge tool before powering on the amp. Make it a bright color like yellow and maybe put a piece of shiny tape on it to make it more visible. You can even put a fuse in series with the resistor on the discharge tool in case you forget to remove it.
Filter capacitors are wired in parallel, so draining one should drain them all, but never assume. Always recheck for residual voltage in every filter capacitor until there's absolutely no doubt in your mind that all are drained. Less than 5 volts is safe, but I prefer to drain mine to 1 volt for peace of mind. Don't be surprised if you find the capacitors slowly recharging themselves. This phenomenon is known as dielectric absorption and is quite normal in electrolytic capacitors. The filter caps shouldn’t recharge to a lethal level, but leave the discharge tool in place while working on the amp.
A few final notes:
Never work on an amp if you’re tired, fatigued, frustrated or intoxicated.
Never work while distracted.
Never work on an amp around small children. There are just too many dangers involved to take that chance.
Try to have a plan in case something does go wrong. You might consider asking someone in your household to take a CPR class.
Take your time and don’t rush.
Use common sense.
I know its a little late but good info
You may have heard various horror stories about things inside amps that can store a charge that can kill you. Well, it's true! However, those dangerous voltages can be easily drained in just a few minutes, so take your time and don’t rush.
It is absolutely vital that we drain the filter capacitors in the DC power supply before working on any guitar amplifier. These often hold a charge of several hundred volts, which is potentially deadly. Capacitors can hold their charge for a long time, even with the power off and the amp unplugged. It's very important that we learn to properly "drain" this residual voltage so that we may safely work on the inside of our amplifiers.
What You’ll Need Cacitor Discharge Tool
Safety glasses. Always use safety glasses. If you wire a capacitor in backwards there is a very good chance that it will explode in your face. Milwaukee Safety Glasses Clear Hard Coat
Digital multi-meter. Capable of reading 500 volts DC. Triplett Digital Multimeter
Capacitor discharge tool. A jumper wire consisting of alligator clips on the ends, with a resistor in series in the middle of the jumper.
Needle nose pliers. With very good insulation on the handles for holding the alligator clips of the capacitor discharge tool. Titan 5-Piece Precision Plier Set With Case.
Always make sure your tools are in good condition. If there’s any question, replace them. Cracked insulation is one of the first things to check, and check often.
Turn the power switch OFF, and place the standby switch in the ON position. This isn't the same as having your amp "on standby."
Always unplug the amp from the wall. Unplugging from the wall will not drain the filter capacitors completely, but again this is for our own safety. Unplugging is the equivalent to "locking out" before working on a downed machine.
Because filter caps are almost always of the electrolytic type, they're "polarized" and therefore have a positive and negative end. We can identify the positive or negative ends by looking for the "polarity indicator" printed on the cap's side.
Our first objective is to check for residual voltage with a multi-meter. In some cases most of the voltage may have already been drained. Many amplifiers will incorporate "bleeder" resistors, which will drain the capacitor charge automatically in a few minutes after the amp is turned off. Don't depend on this resistor to do the job. Never assume the caps have been drained to a safe level without checking with a reliable multi-meter. Here’s a couple of ways to measure the residual voltage contained in the filter capacitors. Note that these are also equally valid for draining the caps.
Across the capacitor.
From the positive side of the cap to the chassis. This is essentially the same as draining "across" the caps since the negative side is at the same electrical potential as the chassis.
From the chassis to a tube pin. This can be done at pin 3 of any power tube, or pins 1 or 6 of any preamp tube. By shorting the chassis to the correct pin the caps will be forced to drain through the plate load of that tube. This prevents sparking from high current.
In old electronics books they tell you to use a screw driver with an insulated handle to short out the caps. "Shorting" with a screwdriver will almost instantly drain the caps to zero volts, causing sparks from high current. This is not acceptable. If there are several hundred volts stored in the caps, be prepared for a nice sized spark. It will most likely leave a black mark on the chassis, and in extreme cases, partially weld the screwdriver to the chassis.
Now we’re ready to start testing and discharging.
Set your multi-meter to read DC voltage. Adjust the meter's range so that it's above 500 volts. Clip the black lead of your multi-meter to the chassis. Using only one hand, with the other hand far from the chassis (preferably in your pocket or behind your back), measure the DC voltage across one of the filter capacitors. You can adjust the meter's range down if you can’t get a clear reading. Any DC voltage readings less than 5 volts are harmless, so the amp can be safely worked on, but do a double check of all the caps just to make sure. If not we must drain off the residual voltage with a resistor until it is safe. Use a resistor value somewhere between 10K and 100K, with a power rating of 5W or so (a high power rating is not really necessary for heat dissipation reasons, but mainly because the higher power resistors are physically more robust and won't tend to break). The larger the resistor value, the longer it will take to discharge. If your capacitors do not have a dangerously high voltage, say around 30 volts, feel free to use a smaller 1K 1Watt resistor.
Now take your capacitor discharge tool and clip one side to the chassis. Always clip to the chassis ground first before connecting to the circuit. If you clip to the circuit first you will have a hot wire hanging out of your amp, which is very dangerous. Once you have one side clipped to the chassis, take your needle nose pliers with good insulation on the handles and hold the alligator clip that will go to the circuit with the needle nose pliers. Clip this to pin 1 or 6 of the first preamp tube. This seems to be the safest way of discharging the capacitors. If you’re working on the first preamp tube socket, then clip your discharge tool to pin 1 or 6 of the second preamp tube. Wait a few minutes (about 5 minutes, if the amp has bleeder resistors this time will be shorter) then, while the discharge tool is still in place recheck all the capacitors with your multi-meter and keep that other hand behind you. Keep checking until the voltage has reached a safe level. Note: This procedure must be done every time you turn the amp on then off.
Don’t forget to remove the discharge tool before powering on the amp. Make it a bright color like yellow and maybe put a piece of shiny tape on it to make it more visible. You can even put a fuse in series with the resistor on the discharge tool in case you forget to remove it.
Filter capacitors are wired in parallel, so draining one should drain them all, but never assume. Always recheck for residual voltage in every filter capacitor until there's absolutely no doubt in your mind that all are drained. Less than 5 volts is safe, but I prefer to drain mine to 1 volt for peace of mind. Don't be surprised if you find the capacitors slowly recharging themselves. This phenomenon is known as dielectric absorption and is quite normal in electrolytic capacitors. The filter caps shouldn’t recharge to a lethal level, but leave the discharge tool in place while working on the amp.
A few final notes:
Never work on an amp if you’re tired, fatigued, frustrated or intoxicated.
Never work while distracted.
Never work on an amp around small children. There are just too many dangers involved to take that chance.
Try to have a plan in case something does go wrong. You might consider asking someone in your household to take a CPR class.
Take your time and don’t rush.
Use common sense.
I know its a little late but good info
...runna pile-up on 6m SSB