grounding
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davebolden44
- Posts: 217
- Joined: Sun Feb 02, 2014 6:13 pm
grounding
I received my bassman kit in the mail today and I started installing some of the lighter hardware in the chassis. my question for today is about grounding, can I just use some copper wire I have laying around as a grounding bus? what is the appropriate way to do this right the first time? if the bus is soldered to the back of the pots does that count as the ground connection or does it require a positive connection to the chassis. I need to read more about ground loops and such but im sure I can get a quick answer here.
Re: grounding
I'm fairly opinionated about grounding. But it's not the same opinion as most amp builders. 
There are an infinite number of ways to wire up the grounds in an amp. There are a vast number of ways to get it really wrong, and have nasty grounding issues.
There are a certain number of ways to ground it with little, less or minimal hum and grounding problems.
There are a few ways of getting nearly no hum.
There is one way to be certain it will have minimum hum without experimentation or any thinking and tinkering about which ground connects where.
The one way is with pure star grounding. Not even I do that. I do a variant, local star collections of ground for related parts of the circuit, then a star ground to the One True Ground.
Most of the few ways of getting nearly no hum involve well-thought out ground busses of various thicknesses, from bare copper wire up to enormous bands of copper. The early guitar amps were usually grounded with experimented/tinkered grounding systems to get low-enough hum and easy production.
So my advice is to either go collections-of-local-stars or to copy the amplifier you're replicating.
There are some big hum contributors that are subtle. Item 1 is to take the negative side of the rectifiers (whether that's the PT center tap or the - end of a bridge) ONLY to the negative terminal of the first filter cap. Not to chassis, not to a point on a wire 1" from the negative terminal. Otherwise, the rectifier charging pulses will cause hum on the signal ground that can't be removed except by moving the wire where is should go. If you use SS diodes for rectifiers, snub them, or use fast/soft recovery diodes.
There are others, but you have to get past those first.
There are an infinite number of ways to wire up the grounds in an amp. There are a vast number of ways to get it really wrong, and have nasty grounding issues.
There are a certain number of ways to ground it with little, less or minimal hum and grounding problems.
There are a few ways of getting nearly no hum.
There is one way to be certain it will have minimum hum without experimentation or any thinking and tinkering about which ground connects where.
The one way is with pure star grounding. Not even I do that. I do a variant, local star collections of ground for related parts of the circuit, then a star ground to the One True Ground.
Most of the few ways of getting nearly no hum involve well-thought out ground busses of various thicknesses, from bare copper wire up to enormous bands of copper. The early guitar amps were usually grounded with experimented/tinkered grounding systems to get low-enough hum and easy production.
So my advice is to either go collections-of-local-stars or to copy the amplifier you're replicating.
There are some big hum contributors that are subtle. Item 1 is to take the negative side of the rectifiers (whether that's the PT center tap or the - end of a bridge) ONLY to the negative terminal of the first filter cap. Not to chassis, not to a point on a wire 1" from the negative terminal. Otherwise, the rectifier charging pulses will cause hum on the signal ground that can't be removed except by moving the wire where is should go. If you use SS diodes for rectifiers, snub them, or use fast/soft recovery diodes.
There are others, but you have to get past those first.
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Stevem
- Posts: 5144
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: grounding
The only type of bussing you should do would be per preamp gain stage as in there cathode and grid ground along with tone section and or volume pot grounds.
The more current you may have flowing thru a bussed ground, the more you need to stay away from connecting preamp stage grounds to it!
The more current you may have flowing thru a bussed ground, the more you need to stay away from connecting preamp stage grounds to it!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: grounding
Thinking of all grounds simply as "ground" is a gross oversimplification, possible perpetuated by the schematic way of thinking. There are multiple types of ground, and making a quiet amp requires knowing the differences.
There are
- reference grounds; a source of 0.0000000... V which tells the circuit what "zero" really is
- signal send/return grounds; a conductor that sends/returns the small currents sent to another section of the circuit
- shield grounds; a metal cage that fences out the electronics noise outside the delicate signal stuff inside it
- sewer grounds; conductors that return the large currents and other 'used electricity' back to the power supply.
The only thing that these "grounds" share is that they're all connected to the One True Ground to hold them at as close to 0V as possible.
Where you get into trouble is combining them. If you run your reference ground path through a sewer ground wire, the currents on the sewer ground cause a voltage through the resistance of the wire, and the circuit that is getting the reference ground now has a moving reference, which also contains the sum of the sewer currents wiggling it around.
Running everything through the shield ground may work, may not, depends on how low the resistance of the shield is and whether that resistance is small enough to get the current-induced voltages below noticeability. What makes this bad is when the input jack ground is also tied to the shield and so the whole gain of the circuit amplifies whatever is being impressed on the shield.
Likewise, when AC power line neutral/ground currents flow through a signal return/shield wire, large offset voltages from the power line currents get impressed on the input that's amplifying the signal from the cable.
To do grounding correctly, you have to (1) know what **currents** are flowing in the "ground" conductor and (2) ensure that these currents do not cause offsets, feedback, and hum when amplified.
There are
- reference grounds; a source of 0.0000000... V which tells the circuit what "zero" really is
- signal send/return grounds; a conductor that sends/returns the small currents sent to another section of the circuit
- shield grounds; a metal cage that fences out the electronics noise outside the delicate signal stuff inside it
- sewer grounds; conductors that return the large currents and other 'used electricity' back to the power supply.
The only thing that these "grounds" share is that they're all connected to the One True Ground to hold them at as close to 0V as possible.
Where you get into trouble is combining them. If you run your reference ground path through a sewer ground wire, the currents on the sewer ground cause a voltage through the resistance of the wire, and the circuit that is getting the reference ground now has a moving reference, which also contains the sum of the sewer currents wiggling it around.
Running everything through the shield ground may work, may not, depends on how low the resistance of the shield is and whether that resistance is small enough to get the current-induced voltages below noticeability. What makes this bad is when the input jack ground is also tied to the shield and so the whole gain of the circuit amplifies whatever is being impressed on the shield.
Likewise, when AC power line neutral/ground currents flow through a signal return/shield wire, large offset voltages from the power line currents get impressed on the input that's amplifying the signal from the cable.
To do grounding correctly, you have to (1) know what **currents** are flowing in the "ground" conductor and (2) ensure that these currents do not cause offsets, feedback, and hum when amplified.
Re: grounding
R.G., glad to see you here! When I started out I learned a lot from your posts on the other place. Thank you.