sds1
Friend of Leo's
Which part is confusing? You could build the circuit inside a wooden box it would still work the same, minus the shielding and hazard protection provided by an earthed metal chassis.
Why terminate circuit ground(s) to chassis, rather than directly to mains ground point?
- Thread starter chas.wahl
- Start date
On three prong amps, I've always had the safety ground separate. I've only ever have seen it that way.
I've tried it a bunch of different ways, and when doing a build about 15 years ago I decided to try the various grounding schemes to see which method was the most noise free for that build.
It's easy to do this little experiment every time you do a build, and might be the best approach because different circuits induce noise in the signal in their own unique way. I can say with confidence that one way will be quieter than the other ways you try.
sds1
Friend of Leo's
Not sure if you knew the Valve Wizard article that was cited is online?
Section 15.9 is where this info is, and the provided graphic helps make some sense of it:
But to be honest I don't fully understand it either. The graphic as drawn makes sense, but why is the preamp current returning to the input side of the amp? That is not how we build amps, the power supply is always on the output side of the amp. The power amp diagram is more consistent with how we build amps. Very confusing to me.
Kev-wilson
Tele-Meister
As a beginner at amp building I read the grounding discussions with interest, for what it's worth I've followed RobRobs schematics and used the ground bus for the pre amp, a separate chassis connection for the power amp ground and a separate safety earth to it's own screw at the back of the chassis near the ie socket and was very happy not to hear any hum. The safety earth wire is soldered to the ie terminal with a soldered lug for the screw as I didn't want any crimps inside the chassis other than ends for the excess wires from the PT & OT.
There's no hum/hiss or unwanted noise whatsoever (so far) so am a fan of the 2 grounds & the earth.
There's no hum/hiss or unwanted noise whatsoever (so far) so am a fan of the 2 grounds & the earth.
Without reading any of this... My guess would be that it uses less wire.
And maybe you want to ground the circuit/chassis to an equipment rack which might have a local dedicated grounding point, instead of grounding it to the building's mains.
Or maybe you want the amp circuit to reference the same ground as a distant grounded source, and not the local power ground.
My point being is that maybe it's more flexible??
And maybe you want to ground the circuit/chassis to an equipment rack which might have a local dedicated grounding point, instead of grounding it to the building's mains.
Or maybe you want the amp circuit to reference the same ground as a distant grounded source, and not the local power ground.
My point being is that maybe it's more flexible??
Paul G.
Friend of Leo's
Didn't go all the way through, and won't go into theory but -- BY LAW -- the safety ground must not be connected to any other parts of the circuit. Whenever possible, I solder the safety ground directly to the chassis, no bolt, no lug.
gabasa
Tele-Afflicted
Perhaps the red arrow is only meant to show that a loop exists with a direction of flow. The ends of the arrow may not necessarily represent source and sink locations within the preamp.
Lowerleftcoast
Poster Extraordinaire
In this case we are discussing ground loops, and AC Mains current that is magnetically induced. This is not DC current from rectified DC in either the preamp device or the power amp device.
Yes, most of the guitar amps we build have the preamp and power amp wired together. The diagram is showing something like a separate audio mixer connected to a power amp. As mentioned earlier, one could imagine this being an AC powered reverb unit before a guitar amp.
Right, Merlin is drawing a *separate* preamp there. Confused me for a long time. 
sds1
Friend of Leo's
Roger that, we're talking earth loops here so up and thru the primary side of the power transformer only (right?), but what do you reckon the capacitors are there to indicate?
I do realize these are separate devices.
Doesn't the drawing imply the preamp device's power supply is physically located on the input side of the amp, whereas the power amp device's power supply is located on it's output side?
Lowerleftcoast
Poster Extraordinaire
I can only guess it is showing part of the circuit... not much different than the shielding tutorial in Fig. 3.15 of the same chapter.
I interpret the red and green current paths as *signal current* like in Fig. 3.15. Although it is ?troubling? to see the direction of the arrows.
One could come to that conclusion but... a device like the Fender 6G15 Reverb unit would have the same filter node at the input and the output. It is hard to say where the power supply would/should be drawn.
This kind of statement is made all the time. It's not accurate. Post the law that you're referring to.
Peace, my brothers. 
Gotta agree, I'd always heard that just soldering ground to chassis wasn't in modern codes. Here's the relevant language from the NEC as of 2020 (if you're late for work, skip down to the last item).
250.8 Connection of Grounding and Bonding Equipment.
(A) Permitted Methods. Equipment grounding conductors, grounding electrode conductors, and bonding jumpers shall be connected by one or more of the following means:
(1) Listed pressure connectors
(2) Terminal bars
(3) Pressure connectors listed as grounding and bonding equipment
(4) Exothermic welding process
(5) Machine screw-type fasteners that engage not less than two threads or are secured with a nut
(6) Thread-forming machine screws that engage not less than two threads in the enclosure
(7) Connections that are part of a listed assembly
(8) Other listed means
(B) Methods Not Permitted. Connection devices or fittings that depend solely on solder shall not be used.
Gotta agree, I'd always heard that just soldering ground to chassis wasn't in modern codes. Here's the relevant language from the NEC as of 2020 (if you're late for work, skip down to the last item).250.8 Connection of Grounding and Bonding Equipment.
(A) Permitted Methods. Equipment grounding conductors, grounding electrode conductors, and bonding jumpers shall be connected by one or more of the following means:
(1) Listed pressure connectors
(2) Terminal bars
(3) Pressure connectors listed as grounding and bonding equipment
(4) Exothermic welding process
(5) Machine screw-type fasteners that engage not less than two threads or are secured with a nut
(6) Thread-forming machine screws that engage not less than two threads in the enclosure
(7) Connections that are part of a listed assembly
(8) Other listed means
(B) Methods Not Permitted. Connection devices or fittings that depend solely on solder shall not be used.
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I honestly don't understand how is is possible to make return current circuit this confucing!!! There is almost no discussion of the feed circuit and it is much more complicated because there are voltage dropper resistors and sometimes a tube rectifier and sometimes a choke 
sds1
Friend of Leo's
You know, after crashing so many build threads (builds that would have turned out perfectly fine regardless) about how returning current on the chassis is sooo baaadd... we're finally talking about it!!!I honestly don't understand how is is possible to make return current circuit this confucing!!! There is almost no discussion of the feed circuit and it is much more complicated because there are voltage dropper resistors and sometimes a tube rectifier and sometimes a choke
And now you are mocking folks for not picking up the concepts right away?
This seems antithetical to your mission.
This thread was started Monday. It's now Wednesday. How long have you been studying electronics? Have some patience with folks why don't ya?
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Whatizitman
Poster Extraordinaire
Just came to say that I love you all for starting and posting to this thread. I have nothing to add and everything to gain from it. Please, please, please don't let any differences of opinions or occasional snark let y'all get sidetracked from it. I for one need all this type of info and instruction!! I can't imagine I'm the only non-engineer type trying to make sense out of grounding. I'm stuck on my current project until I can make some headway with all this. Carry On My Wayward Sons (and daughters, fathers, mothers, grandfathers, grandmothers, etc....).
Well, for me the ground reference {<< edit} aspect is less clear, because it's at the end of the circuit, and seemingly passive, rather than being what I can understand as being the "active" part of the circuit, with the PT providing the motive force.I honestly don't understand how is is possible to make return current circuit this confucing!!! There is almost no discussion of the feed circuit and it is much more complicated because there are voltage dropper resistors and sometimes a tube rectifier and sometimes a choke
If you like, I could start another thread about how to calculate/predict voltage drop when both a choke and a dropping resistor are used (or resistors in both B+ and ground buses, as in Merlin Blencowe's suggested "balanced" reservoir filter, q.v. Grounding fig. 15.7) ;-)
Also, if irritated, you could simply stop reading the thread.
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^^^this!
58 posts but over a thousand views. I'm thinking there are lots of B-teamers like me tuned in.
ALL 58 posts are instructive.
Best thread ever!
Kev-wilson
Tele-Meister
I'd certainly not complain being a little short of understanding it all
, and as others have mentioned, it's a good thread!Similar threads
