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Shielded Wire and Tone

2/17/2000 3:57 PM
Shielded Wire and Tone
So how much does using shielded wire really affect tone? I'm working on an old Harmony thats similar to a tweed Super and I'm modding the Harmony to get more of that tweed tone. The amp has quite a bit of shielded wire as grid and plate wires. It's a really wierd chassis with wooden 1X? sides and a wood block in the middle of the chassis separating the pre-amp section from the power section. I guess it was wise to use shielded wire since the heater wires and signal wires are running along the bottom together. Should I reroute everything and take some of that shielded wire out or just leave it and see?  
thanks for the advise!  
2/17/2000 5:44 PM

I think the sheilded wire, if done correctly, will affect noise rejection, but tone? .... I dunno...  
I've seen the "audiophoole" debates over wire, and even a few threads here, but the best example I've seen so far is this:  
Picture those two little cheesy wires that go from the speaker terminals to the voice coil on say, a classic blackface Deluxe. (those little ones inside the speaker basket). All that occurs upstream in the circuitry has to pass thru those. Are they "sound critical", or just chosen because they will withstand constant flexing? And does the choice of wire type in the circuit have any *perceptable* effect?  
Also, if there were a small audible effect of wire, would anyone who spent any time in front of a dimed Marshall (most of us?) pick it up anyways?  
... pardon the ramble..., hope that helps...  
... Whit  
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2/17/2000 6:52 PM
" I think the sheilded wire, if done correctly, will affect noise rejection, but tone? .... I dunno..."
well, there's one aspect that WILL affect the tone, and that is the real, measureable characteristic of capacitance. a shielded cable will ALWAYS have more paracitic capacitance than an unshielded one, by virtue of the fact that you've got a sheath sitting at ground potential less than a millimeter away. this will "slow down" the amp to some extent, limiting HF response.  
the unshielded wire also has some small shunt capacitance associated with it, but usually, it is much less of an issue by virtue of the fact that the distance between conductors is greater.  
of course, there are ways to compensate for the rolloffs introduced by shielded cable, but you can't really compensate for a noisy, buzzy amp! sometimes it is simply necessary for a decent SNR.  
although there is NO shielded cable anywhere in my preamp, i DID have to intentionally place a small (3nF) cap to ground to quench some nasty oscillations. perhaps if i used shielded cable this would have been unnecessary. as it was, the dc in the amp is well filtered, and noise was not a concern. the lack of shields did make it heaps easier to wire/rewire!  
ken gilbert  
2/17/2000 10:12 PM

Man, just when I thought I could sound partly intelligent, I'm reminded once again I still got a long way to go... but that's what this place is all about, eh?  
The (I guess) mild limiting of HF using shielded cable sounds like a usful feature to me. I used RG-174 extensively in my recent "Humble" clone and it's both quiet and not overly trebly. Just about ideal actually. Could the three or so collective feet of shielded cable have actually resulted in a welcome rolloff of HF? Is the effect significant enough to do that?  
... and thanks for adding a bit more to my understanding...  
... Whit  
2/17/2000 10:40 PM

If you dig deeply enough, you can find the capacitance-per-foot specification on shielded cables.  
The problem with capacitance is that it's a proximity effect - the closer the shield is to the signal conductors, the higher the capacitance and the smaller and more flexible the wire. Big, inflexible wire can give you lower capacitance - but pretty quick you're into plumbing if you want truly low capacitance.
2/18/2000 3:19 PM
Dave H.

Three feet of RG174 is about 100p. Connect it to the wiper of a 1M vol pot and set the pot to its electrical centre so the output resistance is ~500k and you get a -3dB roll off at 1/(2*PI*500k*100p)=3kHz  
2/18/2000 7:53 PM

Thanks for the info, I can calculate with this.  
And a 3db rolloff at 3k sounds perfect to me, so I must have lucked into something unexpected while attempting to minimize noise...  
... Whit
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