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Late to the thread. Safety ground wire to chassis is quite special, as noted. Some of this will be repetitive, as earlier posts got it right. I put an illustration of this in my Thomas Vox repair books, but I'll describe it here.
1. The safety ground wire should go immediately to the chassis as its first connection. It should be longer than the AC line and neutral wires, so it would break last if the AC cord was forcefully pulled out.
2. The safety ground wire should be terminated in a crimped-on ring terminal. It should not be a soldered-on terminal, and should be a full ring.
3. The safety ground wire should be terminated on a bolt or welded stud on the chassis that is not used for any other purpose. The bolts holding down the power transformer were once considered OK, but no longer are. This is why you see this in older amps.
4. Any paint or corrosion on the chassis around the mounting screw should be sanded down to bright metal; no paint, covering, or corrosion.
5. There is a specific stack of washers and nuts on the grounding stud. This is, from chassis metal outwards, external-tooth star washer; the ring terminal on the ground wire; external tooth star washer; nut. The star washers are to give a bite into the metal chassis and the ring terminal. They provide a lot of microscopic "high pressure gas-tight contact points" that effectively do not corrode.
If you're trying to get formal safety lab approval for this, the stud location has to be marked with an earth-ground symbol, but that probably won't matter to self-builders.
If you're using an IEC entry connector, the breaks-last stuff is done by the connector. Same rules apply for terminating the safety wire ring terminal and running the wire to the safety stud on the chassis.
The safety wire termination should not be used as the star ground point for the rest of the circuitry. There really ought to be one and only one connection from signal and DC ground to the chassis, and not here. This would ideally be one wire from the signal star ground point, or one input jack to the chassis. The speaker output jack really should be isolated from chassis, and a wire run back to the OT from the isolated jack.
I say "really ought to be" because clearly other schemes have gotten good-enough results, even very good results by using the chassis itself as a return ground for inputs and outputs, but that approach is open to a fair amount of tinkering as to where exactly to connect to chassis ground. Isolated jacks and return wires can eliminate the tinkering. If you use isolated input jacks, you need to provide low impedance RF shunting to the chassis for the incoming cable shield, in the form of a low-value ceramic cap from the shield terminal to the chassis, as close to the input jack as possible. the chassis then does its shielding function by shunting the RF to safety ground.
1. The safety ground wire should go immediately to the chassis as its first connection. It should be longer than the AC line and neutral wires, so it would break last if the AC cord was forcefully pulled out.
2. The safety ground wire should be terminated in a crimped-on ring terminal. It should not be a soldered-on terminal, and should be a full ring.
3. The safety ground wire should be terminated on a bolt or welded stud on the chassis that is not used for any other purpose. The bolts holding down the power transformer were once considered OK, but no longer are. This is why you see this in older amps.
4. Any paint or corrosion on the chassis around the mounting screw should be sanded down to bright metal; no paint, covering, or corrosion.
5. There is a specific stack of washers and nuts on the grounding stud. This is, from chassis metal outwards, external-tooth star washer; the ring terminal on the ground wire; external tooth star washer; nut. The star washers are to give a bite into the metal chassis and the ring terminal. They provide a lot of microscopic "high pressure gas-tight contact points" that effectively do not corrode.
If you're trying to get formal safety lab approval for this, the stud location has to be marked with an earth-ground symbol, but that probably won't matter to self-builders.
If you're using an IEC entry connector, the breaks-last stuff is done by the connector. Same rules apply for terminating the safety wire ring terminal and running the wire to the safety stud on the chassis.
The safety wire termination should not be used as the star ground point for the rest of the circuitry. There really ought to be one and only one connection from signal and DC ground to the chassis, and not here. This would ideally be one wire from the signal star ground point, or one input jack to the chassis. The speaker output jack really should be isolated from chassis, and a wire run back to the OT from the isolated jack.
I say "really ought to be" because clearly other schemes have gotten good-enough results, even very good results by using the chassis itself as a return ground for inputs and outputs, but that approach is open to a fair amount of tinkering as to where exactly to connect to chassis ground. Isolated jacks and return wires can eliminate the tinkering. If you use isolated input jacks, you need to provide low impedance RF shunting to the chassis for the incoming cable shield, in the form of a low-value ceramic cap from the shield terminal to the chassis, as close to the input jack as possible. the chassis then does its shielding function by shunting the RF to safety ground.
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