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Old 23rd December 2007, 06:04 PM   #1
g(f(e)) is offline g(f(e))  United States
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Default Star ground help

I am building my first vacuum tube project and need a little help with the grounding scheme.

I would like to use a star ground system, unfortunately I do not totally understand how to implement it.

I have read about local ground nodes and then running one wire to the star. Is the local node to the chassis or is it isolated?

For my star connector, I have a 1" diameter, 5/16" thick machined copper, with 24 holes drilled in it, in a circular pattern to accept my various wires. Does this get isolated from the chassis and then tied to the green ground wire from the mains?

I have nylon washers for my RCA inputs and outputs, which seem to isolate them from the chassis, so I plan on using them.

Any help with grounding of a tube project ( in this case a Grounded Grid preamp ) would be appreciated.

Thanks

Gary
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Old 23rd December 2007, 06:19 PM   #2
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Hi,
everything needs to be isolated from the chassis for this grounding scheme to work. When done there should be only two connections to the chassis. The star ground and the earth ground.
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Old 23rd December 2007, 07:25 PM   #3
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Hi Gary,
The term "star ground" I believe is somewhat of a misnomer. It comes from the way some schematics are drawn on paper, and because some builders run their wires verbatim to that style of drawn schematic. Another practical term could be "single point" ground that is also widely accepted.

The whole concept is very simple. Make all power and signal grounds connect together at only one point to the metal chassis. The green mains earth wire connects there also. That point is generally somewhere near the center of the chassis.

This can be done by extending several wires from various circuit ground points to converge at the center "star" location. That location is connected to the chassis. Your copped disc will work here but may require a lot of soldering heat due to it's thickness.

Another way (the way I prefer) is to use a ground buss wire. A ground buss is a heavy bare copper wire suspended near and around the tube sockets. All local ground points of the circuit are soldered to this nearby buss wire. The buss wire is then, in turn, connected to the metal chassis at only one point. Usually near the center as in the star methode.

Also, the best ground point for the buss can be determined experimentally by connecting it to different points on the chassis while listening with a small speaked or phones for lowest hum level with no signal.

I hope this clears things up a little.

Victor
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Old 23rd December 2007, 07:27 PM   #4
g(f(e)) is offline g(f(e))  United States
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Thank you for the reply.


Can I connect my earth ground directly to the star also, or should there be two separate connections?
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Old 23rd December 2007, 07:35 PM   #5
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Yup, earth and star together as one. Or at least very close to it. (second paragraph above)
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Old 23rd December 2007, 08:24 PM   #6
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Quote:
Originally posted by HollowState

Another way (the way I prefer) is to use a ground buss wire. A ground buss is a heavy bare copper wire suspended near and around the tube sockets. All local ground points of the circuit are soldered to this nearby buss wire. The buss wire is then, in turn, connected to the metal chassis at only one point. Usually near the center as in the star methode. [/B]

ABSOLUTELY. I have been doing things that way for years and it usually works out best. As was previously mentioned, the misconception about a 'star-ground' is that you have a spider-web of wires running from each ground node to a common ground reference point, but as long as you reference the grounds of the amplifying stages to their proper respective PSU stage ground node--even along a 'bus' -- and then reference either each node (or a single node of the bus itself if you use that method) to a single common ground reference point, then the actual 'star' connection is just an equal-and-common reference for all the stages and you have achieved a 'star-ground' (albeit one that most people confuse with a bus ground at first glance). Using this type of scheme that HollowState pointed out, will also save you quite a bit of wire and effort, while keeping the build cleaner looking and simpler at the same time.
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Old 23rd December 2007, 08:43 PM   #7
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Victor,

You are right about the copper disc. After I posted I went out to the garage to try soldering a wire to it, and I could not get the solder to flow in the hole. So, I was going to ask if I could use a buss wire, but you answered that question for me already.

I cannot believe how after reading everything I could get my hands on for a few years now on tubes and tube theory, how hard it is to actually apply it all for the first time. I have wrestled with the chassis layout, component selection, and wiring. It is definitely a learning process.

Gary
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Old 23rd December 2007, 08:50 PM   #8
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Quote:
Originally posted by aletheian


will also save you quite a bit of wire and effort, while keeping the build cleaner looking and simpler at the same time.

Not only. Long multiple wires to ground suffer more exposure to fields. I had a buzz episode lately and nobody could guess until the cartoon style 'idea' light bulb turned on over my head and I shielded the returns. I am pro buss 100%. Less capacitive too. Sounds smoother to me also vs 'star'.
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Old 23rd December 2007, 08:57 PM   #9
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Quote:
Originally posted by HollowState
Hi Gary,[snip]Also, the best ground point for the buss can be determined experimentally by connecting it to different points on the chassis while listening with a small speaked or phones for lowest hum level with no signal.

I hope this clears things up a little.

Victor

This should give us some reason to pause at this.
Using a buss ground, you introduce unavoidably hum and noise from one stage into the other. Consider: any signal current, psu decoupling noise/hum/buzz current through the buss wire sets up a voltage that adds or subtracts to the signal reference for other stages.
You can try to fixit as described above, hoping that the noise and buzz at one point cancels or subtracts at the other stage(s), but only a start ground can totallly eliminate it.

Jan Didden
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Old 23rd December 2007, 09:17 PM   #10
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Quote:
I have wrestled with the chassis layout,
Yup, I've done my share of positioning parts around a chassis like moving chess pieces around on a board. The biggest challenge is making as few holes as possible in the chassis by mounting parts underneath with existing tube socket and transformer screws. Of course, counter sunk flat-head screws covered by the transformers on top is a help. But the more you do it, the easier it gets. Often I can now visualize most of it in my head.

Victor
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