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Old 23rd April 2011, 07:58 PM   #1
Magz is offline Magz  United States
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Default Preamp Umbilical - Use Shield as Ground Lead?

Hello all,

I'm building an umbilical cable to connect my tube preamp power supply to the preamp chassis. I've done a lot of reading in this forum and others, and the general recommendation I have seen is to use a wire for the ground return between the two chassis, and a separate braided copper shield on the cable, connected to the chassis on the power supply end only.

However, in reading Morgan Jones' "Building Valve Amplifiers", I noticed that he said that the best way to ensure a low resistance connection between the two chassis was to use a heavy braided shield over the umbilical as both the shield and the ground connection. This implies that the shield is connected to both the power supply and the main preamp chassis. I know the Morgan Jones book is generally regarded as a "Bible" of sorts for valve circuits, so I'm a bit confused.

Is there a general rule of thumb on this subject? I'd love to hear from those in the know about what they prefer.
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Old 23rd April 2011, 09:04 PM   #2
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I'm very much on the side of MJ in this case. A braided shield, because of it's large cross section, will provide a nice low resistance ground connection betweem units. Grounding the shield at one end only refers much more to signal interconnects where the braid acts only as a shield and not a connecting conductor.

I'd also recommend using a quality (surplus) Amphenol, Bendix or Cannon "MS" type connector with a screw on collar. Cinch Jones locking connectors will also serve well. You know, the kind with rectangular flat pins. These will withstand the high voltages used.
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Old 23rd April 2011, 09:38 PM   #3
Magz is offline Magz  United States
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Thanks, Hollowstate,

What about an 8-pole Speakon? I know that is frowned upon because of the possibility of mistakenly connecting a speaker to the power supply, but I have nothing else with a Speakon connection and I can't imagine anyone thinking it would be a good idea to connect a speaker to a preamp power supply. I like the easy disconnect and locking features of the Speakon - how bad is that idea?
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Old 23rd April 2011, 09:57 PM   #4
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Quote:
Originally Posted by Magz View Post
What about an 8-pole Speakon?
I'm unfamiliar with those connectors because I'm old school. So I looked them up to see what they were regarding voltage and current levels. If the specifications are correct, they should be suitable.
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Old 23rd April 2011, 11:42 PM   #5
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Thanks. I'm planning to use some litz wire, braided shield and Techflex braid I have left from another project, so I really only need to buy the connectors. I love projects made from leftovers.

Any dissenting opinions about using the braid for the ground connection?
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Old 24th April 2011, 05:07 PM   #6
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Quote:
Originally Posted by Magz View Post
Thanks. I'm planning to use some litz wire, braided shield and Techflex braid I have left from another project, so I really only need to buy the connectors. I love projects made from leftovers.

Any dissenting opinions about using the braid for the ground connection?
Using the shield of the umbilical for signal reference would be like using the chassis. It would prevent one from using a proper star ground scheme, where the signal reference connects to the chassis at a single point.

IMO and IME, the best way to do this is to route each power supply return (B+, heater, etc.) through the umbilical on a separate conductor and put the single point ground in the amplifier chassis. The power supply returns should connect to star ground AFTER the rectifier current loop, i.e. you don't want the filter cap charging current pulses on the return to star ground.

Litz wire for DC???
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Old 24th April 2011, 09:02 PM   #7
Magz is offline Magz  United States
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Litz wire is being used because I have it already, it's nice and flexible, of the proper gauge, and teflon insulated.

So now I have conflicting recommendations again. Perhaps I just need to try both and see?
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Old 24th April 2011, 09:07 PM   #8
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speakon cable connects necessitates multiple umbilicals/ multiple umbilicals = problems if u grounded every shield...
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Old 24th April 2011, 09:19 PM   #9
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micael koster is absoltely correct. A grounding system that mixes signal and power will be compromised
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Old 24th April 2011, 09:31 PM   #10
Magz is offline Magz  United States
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Originally Posted by quikie22 View Post
speakon cable connects necessitates multiple umbilicals/ multiple umbilicals = problems if u grounded every shield...
I have 7 wires and if I use an 8-pole speakon, I can make one umbilical to cover it all.

According to Morgan Jones, the ideal scheme would be to use all the shields together as both shields and earth conductors in such a situation. He even advocates using two or three layers of screening to ensure there are no gaps in the screens while also ensuring a low earth resistance.
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