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stray capitance in p to p wiring

5/4/2006 4:27 PM
chris vought
stray capitance in p to p wiring
newbie here so go easy on me if the answer is obvious. i understand(at least think i do) what stray capitance is and how it can occur in an amp. but couldn't i just as easily use shreilded wire for all connections in an amp??? ive not yet built an amp. i work on my guitars, built a distotion pedal and am planning to do a microphone mod befor i tackle a fender princeton clone that i have plans for.any advice appreciated.
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5/4/2006 5:21 PM

Don't confuse stray capacitance and stray coupling. Two wires side by side can couple inductively as well as capacitively. That's how transformers work. (Yeah, I know, think RF transformers)  
You could use shielded wire everywhere, but why not learn about proper layout and lead dress instead? Fender managed to build their Princetons without it. If you are going to clone one, you should be able to as well.  
I think if you read through a bunch of threads about construction and problems here, you will find a recurring theme of keeping leads short, avoid excess wire, and try to keep inputs and outputs away from each other. So for example, if your channel volume control is an inch from an output tube, perhaps a shield between them would be needed. Better practice would be to not put it there in the first place.
5/4/2006 5:27 PM
Enzo is correct - you can couple signal inductively as well as capacitively.  
But here's another thing to watch for.  
As you bring conductors closer together, the capacitance between them rises, right? The shield in a shielded wire is closer than any other wire could ever get, and sure enough, the capacitance between conductor and shield is measurably higher. So putting shielded wire on your signal is like hanging lots of little capacitors to ground all over it. Too much of this and you start losing treble. The amp starts sounding dull.  
This problem is worst with high impedance signal lines - like most of the ones in a tube amp.  
The best thing for signal integrity is, as Enzo pointed out - short, direct signal wires that are as far from other conductors as you can **reasonably** put them.
5/4/2006 7:36 PM
chris vought

hmmmm and i thought tubes were the only "mojo" going on in the tube amp. thx for your feedback.
5/5/2006 9:43 AM
The fact that tubes have both high input AND output impedances leads to many things being different between a tube circuit and lower impedance solid state circuits.  
Lower impedance circuits let you get away with much more in terms of poor layout and poor wiring practice than a typical tube amp does.  
Still, the best layout and wiring for ALL circuits is the same: short, direct signal path, input isolated from input both electrically and physically, good ground and power supply wiring practice.
5/4/2006 6:23 PM
dai h.

what does a shielded wire do? It typically (unless there are shields with mu-metal shields) provides electrostatic shielding (due to the type of material typically used--that is--copper or alu foil, perhaps both), therefore it's useful in blocking out highs and things like "buzz" (the higher harmonics of hum) but not real effective at blocking out hum (which are lower frequencies like 50/60Hz, 100/120Hz. The shielding provides some isolation from the surrounding environment so you could use it to keep stuff out or keep stuff in. There is not really a need to use it for all the connections in an amp because the signal strengths and sensitivities are different. For example, it might makes sense to shield a wire carrying the signal to the grid at the input, since typically the impedance is high making it more susceptible to outside influence (which could be a problem if that outside influence is buzz or perhaps a more powerful signal which could interact and lead to oscillation). It would not make sense, conversely to shield the wire to the speaker output to protect it since that signal is low impedance and more impervious to outside influence. It's sort of like trying to pee in the wind versus using a firehose with lots of pressure behind it. The firehose won't need much help maintaining the direction or be as easily influenced by outside forces. Shielding can also be done with sheets of metal blocking off one part of the circuit from another. You might see this in electronics like VCRs an amp like the THD Univalve. The chassis itself is a shield and you can get some shielding effect by running wires down against the chassis.  
re: the capacitance from a shielded cable, it matters less the lesser the impedance for that signal being carried on it, and matters more the higher the impedance it is. If an output is high impedance, more capacitance makes it harder to drive and you lose more highs, plus the effect is exacerbated if there is a pot in the path (when the pot is turned down, making the signal even higher impedance). Loss of the highs being good or bad depends on the situation and what someone is trying to do. Trying to use shielded wire for all connections inside an amp might be like forcing everyone to wear a helmet whether they rode a bicycle or motorcycle or not (unnecessary overkill).

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