Wrapping versus shielded cable?

[Normster]

Is there a "capacitance" advantage to wrapping long signal runs versus using shielded cable? I know Fender used to do this to shield HT voltage from the tone controls and I've seen the same technique used in Komet amps. Anyone ever tried it? I used twisted pairs for the OD run in my Bassman and it seems to work OK, but I was thinking of bundling all 4 wires and wrapping with 26AWG (grounded at one end, of course).

[dogears]

I do this sometimes too. You may get less capacitance by using twisted pair. I would not bundle the hot leads. You get capacitance between them as well! Bottom line is to only use shielded where neccesary. If you can get away with straigth wire, then do it. Fwiw, look at some of the grail Dumble pics you have. He did not use shielded on the master.......

If you look at the Emmy Pro pics that were at the gear page, those other guys seemed to prefer running hot leads in a tight ground bundle. I have also seen this in some Fuchs mods on short runs.

Is there a "capacitance" advantage to wrapping long signal runs versus using shielded cable? I know Fender used to do this to shield HT voltage from the tone controls and I've seen the same technique used in Komet amps. Anyone ever tried it? I used twisted pairs for the OD run in my Bassman and it seems to work OK, but I was thinking of bundling all 4 wires and wrapping with 26AWG (grounded at one end, of course).

[Normster]

I'll probably forget about it for now. In the Dumble, there are no hot wires to bundle, but the Bassman does have a long run of HT wires that run at the end of the preamp board from the cap can. No noise so far so I think I'll leave well enough alone. I was primarily thinking of using the technique for the OD drive and level wires, but I think laying them on the chassis provides adequate shielding.

[dogears]

Me bad, I meant hot as in the wiper and input to the pot. If you run them in a bundle you will leak signal between them......

I'll probably forget about it for now. In the Dumble, there are no hot wires to bundle, but the Bassman does have a long run of HT wires that run at the end of the preamp board from the cap can. No noise so far so I think I'll leave well enough alone. I was primarily thinking of using the technique for the OD drive and level wires, but I think laying them on the chassis provides adequate shielding.

[tonelab2]

I hope others will share their experience with shielded cable. Have you tested the wrapped unshielded cable run for capacitance per foot?
I purchased several metres of teflon coated shielded cable (thin, nice looking stuff) a couple of years ago and recently decided to start using it.
Thought I would check it on a capacitor meter before I started and it measured 87pf per foot. I measured a foot length of cheap video(vcr)/TV cable and it came in at 6pf per foot. I'm interested in Norm's idea of wrapped unshielded cable and the measured capacitance of it.

[glasman]

I hope others will share their experience with shielded cable. Have you tested the wrapped unshielded cable run for capacitance per foot?
I purchased several metres of teflon coated shielded cable (thin, nice looking stuff) a couple of years ago and recently decided to start using it.
Thought I would check it on a capacitor meter before I started and it measured 87pf per foot. I measured a foot length of cheap video(vcr)/TV cable and it came in at 6pf per foot. I'm interested in Norm's idea of wrapped unshielded cable and the measured capacitance of it.

If you look at a few of the pictures out there of some of the "grail" Dumbles you will see Tandy coax used on the signal lines. That stuff (worked for RS while in college) was about 95% cheap braid with a copper conductor. Probably the 75 ohm would be the best choice. It is a little large, but when searching for tone, who cares!



Gary

[dobbhill]

Kimber makes a 3-conductor braid (easily copied) of high quality teflon wire that I have used in the past- low capacitance. I got it from Michael Percy and also Welborne Labs. Signal + and - and then a ground (one end only, of course).
D

[llemtt]

-> tonelab

87pF per foot is common spec for audio quality shielded cable, it will round your highs in a significant way

the best video "signal" cable I found is 67pF per meter which is roughly 20pF per foot, if you go for video "antenna" cables maybe you can go lower but probably is unnecessary

common digital meters are not very accurate at such low capacitances and the test environment for these measures is very critical, it's better to measure at least 10-20mt of cable or refer to cable specs if availables

-> Gary

do you know what cables HAD uses in his "grail" amps? (I mean the white one 3-5mm thick with big core and thin insulation)

Teo

[glasman]

-> Gary

do you know what cables HAD uses in his "grail" amps? (I mean the white one 3-5mm thick with big core and thin insulation)

Teo

The only information that I have is that it is a MIL spec teflon jacket with 22gauge center conductor. I do not know the rated capacitance but would guess based on the diameter of the cable and dialectric that it will be in the 15 to 20pf / foot range. I also think it only braid wrapped, no foil.

I am currently looking for a source. Probably have to buy 1000', but thats cool.

The cable I currently use is rated at 16pf/ FT but actually measures a little above 20pf/FT.

Gary

[tonelab2]

Thanks guys for sharing,

My bench type tester although is not a lab tester it is fairly good
at very low capacitance, I've used it on many low pf caps and it seems
accurate enough for audio stuff ( within 10% I expect). I wouldn't compare my results with proper text book/lab situation testing but rather just a quick look at cable I had laying around.
The 6pf cable ( old stuff from the '80s to early '90s) on closer inspection
has a cheap, not very dense, loosly wound braided shield which may account for the low reading. I presume that the better the shielding ( thicker braid,foil, ect) the higher the pf capacitance but I don't really know. I guess the physics have to play a part somewhere.
I've seen old radio guys using the wrap method for curing HF ossilation when repairing test gear (tube testers ect) and recall seeing it used in an old Mesa Boogie mk3 I once owned many years ago. Never thought about it since untill this post came up.

[Chris G]

I hope others will share their experience with shielded cable. Have you tested the wrapped unshielded cable run for capacitance per foot?
I purchased several metres of teflon coated shielded cable (thin, nice looking stuff) a couple of years ago and recently decided to start using it.
Thought I would check it on a capacitor meter before I started and it measured 87pf per foot. I measured a foot length of cheap video(vcr)/TV cable and it came in at 6pf per foot. I'm interested in Norm's idea of wrapped unshielded cable and the measured capacitance of it.

If you look at a few of the pictures out there of some of the "grail" Dumbles you will see Tandy coax used on the signal lines. That stuff (worked for RS while in college) was about 95% cheap braid with a copper conductor. Probably the 75 ohm would be the best choice. It is a little large, but when searching for tone, who cares!





Gary

Hi Gary,
Do you use the sheild or braid in that 75 ohm cable to ground? or do you just use the center conductor?

I have a ton of this stuff here at work(75 ohm cable) used for satilite tv hook ups.

Seems pretty thick and also stiff at shoter lengthes.
Thanks
Chris

[BobW]

The purpose of wrapping a few turns of wire around a bundle and then connecting to a suitable ground source is for adding a few pFs of shunt capacitance into a bundle. This method is usually used to reduce instability or tame oscillation due to either component or local feedback. This creates a low pass filter, and although there is only a small difference in capacitance coupling, obviously the wires on the outside of the bundle will have the greater capacitance coupling compared to the inner wires, since they are closer to the wrapped outer wires. Fender's decision to use this method typically on their silverface amps was because they ignored layout techniques that could have avoided local oscillation. Since some of the silverface amp designs at that time had also increased in output power, it probably was more economical to tame the amp using this method rather than rewire the bundles.

You can increase capacitance in two general ways:
1)decrease the distance from each capacitor plate, or in this case, decrease the distance from the grounded wire to each of the bundled wires. wrap the ground wire tighter. Closer coupling means a larger capacitance.
2)increase the plate areas of the capacitor, or in this case, add more turns. This effectively increases the plate area of the capacitor.

Knowing
A)the distance from the wrapped wire to each wire in the bundle,
B)the dielectric constant of the dielectric materials of both wires, and
C)the surface area or circular mils of the wires, it would then be possible to calculate the capacitance on a per turn basis.

[hellhound]

I have used George L´s guitar cable. I have never measured it or anything but it works for me. It has a reputation of being (overly) bright cable.

[ayan]

-> Gary

do you know what cables HAD uses in his "grail" amps? (I mean the white one 3-5mm thick with big core and thin insulation)

Teo

The only information that I have is that it is a MIL spec teflon jacket with 22gauge center conductor. I do not know the rated capacitance but would guess based on the diameter of the cable and dialectric that it will be in the 15 to 20pf / foot range. I also think it only braid wrapped, no foil.

I am currently looking for a source. Probably have to buy 1000', but thats cool.

The cable I currently use is rated at 16pf/ FT but actually measures a little above 20pf/FT.

Gary

The wire, AFAIK, is probably 20 AWG MIL-SPEC teflon jacket, braid (no foil) shielded. I got mine at Apex Electroncis (not Jr.), where Dumble may have gotten his too, and I know that no amp can sound genuine unless it uses this particular type.

Apex is in a not-so-great part of L.A. county and out of the way (for me)... but, it's great to go there every few years and shop, if you have some time (you gotta look hard to find some things) and don't mind having to take a shower and change clothes when you get back home (yep, it's dusty and dirty). I don't know whether they sell over the phone, but here is their info just in case anyone is interested. I get all my teflon cable out there and pay much less for it than I do for regular cable at Fry's Electronics:

Apex Electronics
8909 San Fernando Road
Sun Valley, CA. 91352
Phone: (818) 767-7202 or (323) 875-1308
Fax: (818) 767-1341
http://www.apexelectronic.com/

Gil

[glasman]

The wire, AFAIK, is probably 20 AWG MIL-SPEC teflon jacket, braid (no foil) shielded. I got mine at Apex Electroncis (not Jr.), where Dumble may have gotten his too, and I know that no amp can sound genuine unless it uses this particular type.

Maybe Heliax is in order for the ultimate tone

Apex is in a not-so-great part of L.A. county and out of the way (for me)... but, it's great to go there every few years and shop, if you have some time (you gotta look hard to find some things) and don't mind having to take a shower and change clothes when you get back home (yep, it's dusty and dirty). I don't know whether they sell over the phone, but here is their info just in case anyone is interested. I get all my teflon cable out there and pay much less for it than I do for regular cable at Fry's Electronics:

Apex Electronics
8909 San Fernando Road
Sun Valley, CA. 91352
Phone: (818) 767-7202 or (323) 875-1308
Fax: (818) 767-1341
http://www.apexelectronic.com/

Gil

Hey, that is not too far from where I will be in Burbank. Sounds like a road trip while my boys are soaking up Fangoria.

Hey Gil, up for a trip to the "dark side". My gas!!

Gary