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Lead dress - easy question

6/2/2000 8:58 PM
Fred G.
Lead dress - easy question
Question :  
I am incorporating a star grounding scheme into a Kalamazoo Model I. In routing the preamp ground wires, I have the choice of running them directly across the filament wires ( much shorter) or routing them around the preamp tube along the outside edge of the chassis ( at least a couple more inches of ground wire, but avoids the filament wires). Which is the better approach ?  
Fred G.
6/3/2000 5:47 AM
Fred,I like to keep all wires routed away from the filament supply,a few more inches of ground wire is not a big deal.
6/3/2000 2:34 PM

Why not slip a braid shield over the filament wires, or replace them with shielded, twisted pair?  
My 'druthers are for a slightly longer ground wire, but crossing the filaments at a right angle isn't all that bad, especially if the filament wires are lying against the chassis - where they should always be!
6/3/2000 7:03 PM
Steve Ahola

...especially if the filament wires are lying against the chassis - where they should always be!  
    Would you care to elaborate on this? Like should the B+ wires be close to the chassis as well? And wouldn't unshielded signal leads benefit from being close to the chassis, too? (assuming that your amp has a star ground- if not the audio leads could pick up noise from the chassis- right?)  
Steve Ahola
6/3/2000 11:33 PM

Anything that you want to be kept quiet should be kept near the chassis. This includes B+, ground wires, and filament wiring, both AC and DC. This gives capacitve coupling to a ground point, but more importantly is as far away from the signal lines as it's reasonable to get. In a steel chassis, the chassis actually sucks some of the stray M fields into the steel, so it doesn't broadcast so badly.  
Placement of signal wires can be determined by the impedance of the source feeding the piece of wire. the lower the impedance the source, the less placement matters, because a truly low impedance source can supply enough current to force the wire's voltage to be correct even in the face of interference. The higher the impedance, the more you have to worry about capacitive coupling of interfering signals. The only good way to keep truly high impedance lines quiet is to isolate them with either distance or shielding. That's why coax works - it Faraday screens the interior wire. The price you pay for that is that the capacitance to the shield also sucks some treble. It's calculable.  
Isolating with distance is also good. 1/2" is good for about 10db isolation from a parallel wire, even better for non-parallel wires. The returns start to diminish for separations more than 1" or so. Distance is low capacitance, of course 8-) so this doesn't suck treble. A high impedance line lying against the chassis sees treble losses similar to coax.  
You're correct that a non-quiet chassis can capacitively induce stuff in high impedance signal lines, but generally the voltage is pretty low on a chassis. It's not the same as paralleling an input wire with a plate wire carrying 50Vac of signal.
6/3/2000 10:25 PM
Stephen Conner

Ground wires should always be as short as possible. They won't pick up hum by being near the heater wires.  
Steve C.  
6/5/2000 10:49 PM

I agree with that Stephan, I also found the "star grounding" method to be very effective, another big Thanks to this forum! Gajan.  
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