Long Audio runs inside the amp - any reason NOT to use shielded cable?

[wizard333]

Any place you should NOT use shielded cable for long audio runs? I've always wondered about that when I look inside a vintage Fenders and see audio signals shooting half way across the board with no shielding. Is there a reason other than cost not to use shielded wire here?

[Steve Conner]

Mainly capacitance, most shielded cable runs about 30pF per foot if I remember right. If your circuit can tolerate that, you can use it to your heart's content.

Leo Fender was probably thinking of cost. Not just the cost of shielded wire, but the labour cost of preparing the ends.

[Old Tele man]

...and, be sure to ground only ONE END of the shielding, otherwise you'll risk creating "ground loops."

[pdf64]

The type of lap shielded cable that tends to get used inside equipment is around 100pF/foot. With the long to and fro runs in a twin reverb, the high end roll off would likely be significant, as the circuit impedances tend to be high.

[wizard333]

100pf per foot seems really crappy, given good cable (klotz, George L) runs about 21pf/foot. I wonder what the pf/foot is for the mogami shielded cable. Anyone know?

[wizard333]

Looks like the Mogami interconnect wire has a capacitance of 35pf/foot. Not the best but a lot better than 100pf/foot.

SPECIFICATIONS
Part No. W2330
No. of Conductor 1
Conductor Details 16/0.08 A
Conductor Size(mm2) 0.08mm2 (#28 AWG)
Insulation Ov. Dia.(mm) 1.5Ø (0.059")
Insulation Material LNXLPE (Low Noise Cross - Linked polyethylene)
Insulation Color Clear
Served Shield Approx. 36/0.12A
Jacket Ov. Dia.(mm) 3.0Ø (0.118")
Jacket Material Flexible PVC
Jacket Color Black
Roll Sizes 100m (328Ft)
200m (656Ft)
Weight per 200m Roll 2.5kg
ELECTRICAL & MECHANICAL CHARACTERISTICS
Part No. W2330
DC Resistance at 20°C Inner Cond. 0.23O/m(0.070O/Ft)
DC Resistance at 20°C Shield 0.05O/m(0.015O/Ft)
Capacitance at 1kHz, 20°C
(Partial C. Value)
See below figure*
K0 115pF/m(35pF/Ft)
Inductance between conductors at 1kHz. 20°C 0.3µH/m(0.092µH/Ft)
Electrostatic Noise** 50mV Max.
Electromagnetic Noise** 0.05mV Max.
Microphonics at 50KW Load** 300mV Max.
Voltage Breakdown Must withstand at DC 500V/15sec.
Insulation Resistance 100000 MO × m Min. at DC 125V, 20°C
Flex Life** 15,500 cycles
Tensile Strength 255N
Emigration Non-emigrant to ABS
Applicable Temperature -20°C~ +70°C(-4°F~ +158°F)
* Patial Capacitance
** Using standard testing methods of Mogami Wire & Cable Corp.

[tedmich]

to see if shielding helps, I've had good luck sticking hook up wire down to the chassis with narrow (1/4") 3M copper conductive adhesive tape, or pinching it around free wire and tying an end to ground. For coax some has lower capacitance, RG59-11 is ~16pF/ft

[wizard333]

Hmm never thought of Coax. Is it easy to work with? I'd think it would be quite stiff.

[Dtube]

Hi all. Long time lurker - first time poster. I use Belden RG-174 (8216) which is about 30pF/ft. Its about 3/16" in diameter and very pliable. Gotta use reading glasses though to neatly trim the braid back and heat-shrink it.
-D