Post by ChrisK on May 28, 2009 13:31:09 GMT -5
tedfixx
Be sure to read Shock Hazards before starting anything related to this concept.
On GuitarNuts, John Atchley teaches the use of a 0.33 uF 400 VDC (600 VDC is better) blocking capacitor to capacitively "isolate" the operator from the amp chassis. This does not isolate things in an AC manner, but introduces capacitive reactance (impedance) between these points. It will block DC once the cap has charged up to the potential across it. Until that time, the charging current may flow through the operator.
If a vintage amp without a three prong mains cord is used, the chassis of that amp (I didn't say "ground") is common with the signal ground in the guitar. Many amps had a capacitor switchable between either power input conductor to reduce humming. This served to capacitively couple the amp chassis (I didn't say "ground" as there wasn't one) to either conductor. One could just as easily couple the chassis to the AC hot conductor as to the one purportedly connected to the neutral. When the chassis and the guitar (and hence the operator) is capacitively coupled to the AC hot, the operator gets a tingly feeling not necessarily related to the validity of his performance.
Use an outlet tester every time, use a GFCI cable every time, and use wireless every time.
When this capacitor fails, bad things happen to the operator. The blocking capacitor will reduce the blow to about 8,000 Ohms (a.k.a., in the U.S., to about only 25 mA AC peak through the operator). In some conditions and cases, this can still kill. Fortunately, this is AC and one's muscles likely won't lock up as with DC current.
Use an outlet tester every time, use a GFCI cable every time, and use wireless every time.
If the amp has a three prong mains cord, but the mains outlet is mis-wired (by one's brother-in-law, for example), the same bad things can happen to the operator.
Use an outlet tester every time, use a GFCI cable every time, and use wireless every time.
If something goes awry within the high voltage DC supply in a tube amp where the B+ potential can be up to 600 VDC, any conduction of this potential to/through the operator can easily be lethal.
Use an outlet tester every time, use a GFCI cable every time, and use wireless every time.
The first picture shows the effective grounding in a modern Stratocaster. Note that all three pot back shells are jumpered together. This costs money to do and Fender makes a lot of Stratocasters, but they feel that this is an important step. It is.
The shield foil on the pick guard is a shield, it is not a reliable conductor. One does not depend on a shield for use as a conductor. Unfortunately, in a guitar cord, the shield is also a signal conductor, but that's another sad tale.
Note that these jumpers are often called a "ground loop" because they are both "grounded" and "loopy". Ground loops can be a bad thing when they actually affect a signal.
The jumper marked "A" connects both tone pot shells together. These are also (hopefully) connected by the foil on the pick guard. On the lower tone pot, no circuit component is actually connected to the pot shell, so the jumpering of the tone pot shells together ("A") in parallel with the pick guard shield connection CANNOT be a meaningful ground loop since NO signal current flows between either. Period.
The jumper marked "B" connects both the volume pot and the neck tone pot shell together. These are also (hopefully) connected by the foil on the pick guard. The only signal current flowing between these two points is from the tone control capacitor signal return. While this point would properly be connected to the back shell of the volume pot (THE existing star ground point), the shortcut of connecting it to the tone pot back shell is used for historical reasons. Did I mention that these shells are also connected by a factory-installed wire jumper? These points are within 1 - 2 inches of each other. No appreciable current flows through the shield (it's a shield, remember). This small "loopy" circuit connected to "ground" is not a meaningful ground loop. It does not need to be removed.
Note that all "grounded" circuits and components are connected to and are part of the circuit known as the AMP GROUND SIGNAL RETURN OPERATOR TOUCHABLE COMPONENTS. If something happens in the amp or with the mains wiring/safety ground, the operator will be in direct electrical contact with this point.
If your guitar is wired this way, you absolutely should use an outlet tester every time, use a GFCI cable every time, and use wireless every time.
The second picture shows the separation of the AMP GROUND SIGNAL RETURN from the OPERATOR TOUCHABLE COMPONENTS. Note that all internal "grounded" circuits and components are connected to and are part of the circuit known as the AMP GROUND SIGNAL RETURN. However, all external OPERATOR TOUCHABLE COMPONENTS are connected to the AMP GROUND SIGNAL RETURN through the 0.33 uF 600 VDC capacitor (8,000 Ohms at 60 Hz). If something happens in the amp or with the mains wiring/safety ground, the operator will only have capacitively coupled electrical contact with this point.This is better. Not best, just better.
Even if your guitar is wired this safer way, you should still use an outlet tester every time, use a GFCI cable every time, and still use wireless every time.
Be sure to read Shock Hazards before starting anything related to this concept.
On GuitarNuts, John Atchley teaches the use of a 0.33 uF 400 VDC (600 VDC is better) blocking capacitor to capacitively "isolate" the operator from the amp chassis. This does not isolate things in an AC manner, but introduces capacitive reactance (impedance) between these points. It will block DC once the cap has charged up to the potential across it. Until that time, the charging current may flow through the operator.
If a vintage amp without a three prong mains cord is used, the chassis of that amp (I didn't say "ground") is common with the signal ground in the guitar. Many amps had a capacitor switchable between either power input conductor to reduce humming. This served to capacitively couple the amp chassis (I didn't say "ground" as there wasn't one) to either conductor. One could just as easily couple the chassis to the AC hot conductor as to the one purportedly connected to the neutral. When the chassis and the guitar (and hence the operator) is capacitively coupled to the AC hot, the operator gets a tingly feeling not necessarily related to the validity of his performance.
Use an outlet tester every time, use a GFCI cable every time, and use wireless every time.
When this capacitor fails, bad things happen to the operator. The blocking capacitor will reduce the blow to about 8,000 Ohms (a.k.a., in the U.S., to about only 25 mA AC peak through the operator). In some conditions and cases, this can still kill. Fortunately, this is AC and one's muscles likely won't lock up as with DC current.
Use an outlet tester every time, use a GFCI cable every time, and use wireless every time.
If the amp has a three prong mains cord, but the mains outlet is mis-wired (by one's brother-in-law, for example), the same bad things can happen to the operator.
Use an outlet tester every time, use a GFCI cable every time, and use wireless every time.
If something goes awry within the high voltage DC supply in a tube amp where the B+ potential can be up to 600 VDC, any conduction of this potential to/through the operator can easily be lethal.
Use an outlet tester every time, use a GFCI cable every time, and use wireless every time.
The first picture shows the effective grounding in a modern Stratocaster. Note that all three pot back shells are jumpered together. This costs money to do and Fender makes a lot of Stratocasters, but they feel that this is an important step. It is.
The shield foil on the pick guard is a shield, it is not a reliable conductor. One does not depend on a shield for use as a conductor. Unfortunately, in a guitar cord, the shield is also a signal conductor, but that's another sad tale.
Note that these jumpers are often called a "ground loop" because they are both "grounded" and "loopy". Ground loops can be a bad thing when they actually affect a signal.
The jumper marked "A" connects both tone pot shells together. These are also (hopefully) connected by the foil on the pick guard. On the lower tone pot, no circuit component is actually connected to the pot shell, so the jumpering of the tone pot shells together ("A") in parallel with the pick guard shield connection CANNOT be a meaningful ground loop since NO signal current flows between either. Period.
The jumper marked "B" connects both the volume pot and the neck tone pot shell together. These are also (hopefully) connected by the foil on the pick guard. The only signal current flowing between these two points is from the tone control capacitor signal return. While this point would properly be connected to the back shell of the volume pot (THE existing star ground point), the shortcut of connecting it to the tone pot back shell is used for historical reasons. Did I mention that these shells are also connected by a factory-installed wire jumper? These points are within 1 - 2 inches of each other. No appreciable current flows through the shield (it's a shield, remember). This small "loopy" circuit connected to "ground" is not a meaningful ground loop. It does not need to be removed.
Note that all "grounded" circuits and components are connected to and are part of the circuit known as the AMP GROUND SIGNAL RETURN OPERATOR TOUCHABLE COMPONENTS. If something happens in the amp or with the mains wiring/safety ground, the operator will be in direct electrical contact with this point.
If your guitar is wired this way, you absolutely should use an outlet tester every time, use a GFCI cable every time, and use wireless every time.
The second picture shows the separation of the AMP GROUND SIGNAL RETURN from the OPERATOR TOUCHABLE COMPONENTS. Note that all internal "grounded" circuits and components are connected to and are part of the circuit known as the AMP GROUND SIGNAL RETURN. However, all external OPERATOR TOUCHABLE COMPONENTS are connected to the AMP GROUND SIGNAL RETURN through the 0.33 uF 600 VDC capacitor (8,000 Ohms at 60 Hz). If something happens in the amp or with the mains wiring/safety ground, the operator will only have capacitively coupled electrical contact with this point.This is better. Not best, just better.
Even if your guitar is wired this safer way, you should still use an outlet tester every time, use a GFCI cable every time, and still use wireless every time.
