Capacitance of Wire

[Diablo]

I modded a tube amp to make one of the coupling caps switchable. This required some long leads of shielded wire so I wouldn't have to butcher the amp chassis by drilling another hole for the switch. I now have about 48 inches of shielded wire connecting the coupling caps to the preamp. Is there a handy rule to estimate the amount of capacitance I added by the extra length of wire? I'm wondering if my experiment is doomed because the length of wire is dominating over my choice of caps.

[pdf64]

It's very dependant on cable used, best to measure what you've got if possible, or check specs with the supplier. Use 50pF/foot as a very rough average as a last resort.
You mention this is for coupling caps, whereas the cable capacitence will be to ground, so it will primarily affect the high frequencies, not bass.

[Diablo]

It's very dependant on cable used, best to measure what you've got if possible, or check specs with the supplier. Use 50pF/foot as a very rough average as a last resort.
You mention this is for coupling caps, whereas the cable capacitence will be to ground, so it will primarily affect the high frequencies, not bass.

Thanks for the info. Unfortunately, I don't have any more of the cable to test the capacitance. I used it all up during the build. My ears confirm your guesstimate of the capacitance increase due to additional wiring. The amp seems to have lost some of the top end sizzle frequency. It uses a 500pF coupling cap as standard, and perhaps the wires add another 200-400pF. It's not a bad thing, since this amp (Matchless DC30 clone) is so very bright anyway.

[pdf64]

Is this twin core screen cable? If not it won't add to the coupling capacitence.

[Diablo]

The cable has a single multi-strand copper conductor surrounded by a multi strand spiral wire sheath. I have the sheath connected to a ground at one end only to shield the conductor from picking up any stray signal.

[Enzo]

DO you know the brand of wire? And the type number? Wire makers publush data on their wires, you could look it up.

[Diablo]

DO you know the brand of wire? And the type number? Wire makers publush data on their wires, you could look it up.

The wire says 75-2 coaxial cable, and I think I got it from Weber, who likely sourced it somewhere in China. No manufacturer's name.

[Enzo]

So ASK Weber. Chances are it is something specific, not just some wire they found at a street vendor in Shanghai.