Sourcing shielded cable - and questions

Discussion in 'Shock Brother's DIY Amps' started by alathIN, Feb 25, 2020.

  1. alathIN

    alathIN Tele-Holic

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    I've been using the RG-174 coaxial shielded cable that Doug Hoffman sells on his web site. Seems like good quality stuff. The shielding is braided/woven - and I think these things are actually rated for whether the shielding provides 95% coverage or not (or maybe some other %?). Not sure what the intended purpose of this stuff is - audio/video? Computers? Antennas? But I do know it's rated 50 ohms which in the world this comes from is considered low impedance.

    Now I'm out and don't currently have anything else to order from Hoffman. I cobbled together some bits and pieces to get my current project up and running but very eager to replace the odds and ends with the good stuff.
    Amazon does not have anything that looks to be the same quality and I can't find a local source.
    Closest I can find locally: 1) Fry's Electronics says they have it sometimes but not now, try again next week. 2) There is a wire wholesaler where I can get a 1000' foot roll of it.

    Also - are there particular rules for what needs to be shielded?
    I know the signal to and from input/output jacks are usually shielded - and of course the ever-reliable "wherever the layout says it's supposed to be shielded."

    In my current project I have a long-ish run - the "wet" signal from the reverb circuit which runs past a couple of tube sockets and their associated heaters. Not right next to them, but in the neighborhood.
    On the layout this is shown as regular unshielded wire - but using their chassis and arrangement of components it's a much shorter path and not as much potentially noisy scenery along the way. A nagging voice is telling me to shield this one for sure, maybe others.

    Finally - asked in another thread - a local amp guru said you should always ground shielded cable at the "destination" end - where the signal travels to. Curious if anyone knows whether this is true and why it matters?

    Just curious what others have done, what you know works, and what are your "good luck" practices that may or may not help - or any areas where one shouldn't use shielded?
     
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  2. kbold

    kbold Tele-Afflicted Silver Supporter

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    You only ground the shielding at one end. That end is where the signal comes from (the signal source).
    Never really pondered the "why", but that's how I was taught.

    So ... AFAIK that "guru" is wrong.
     
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  3. Guitarteach

    Guitarteach Poster Extraordinaire

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    I was understanding the single end shield connection should be on the path of least resistance to ground... e.g. amp chassis, not the signal source. On a directionally shielded guitar cable to the amp, it would terminate at the amp end.

    That may be through the socket if metal on the internal amp wiring. So I suppose it can look like where the signal is coming ‘from’.

    I’ve only used it on the first high impedance runs from the input sockets to V1 on the amps Ive made.
     
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  4. sds1

    sds1 Tele-Afflicted

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    This is good quality stuff here at < $0.30 / foot shipped (not Prime but still good pricing compared to Hoffman-by-the-foot), it's from Jameco which is a fine supplier of electronics:

    https://www.amazon.com/dp/B00B8894OQ/?tag=tdpri-20

    I didn't know there was a preferred end of shielded cable to ground, but thinking aloud I reckon it would be the same discussion as with outer foil on capacitors. If so, then you'd prefer shield the side with lower impedance which would be the source. And not sure you'd notice any difference if you connected to the higher impedance side.
     
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  5. Old Deaf Roadie

    Old Deaf Roadie Tele-Afflicted Silver Supporter

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    50 ohm coax is typically used as antenna feeder cable. 75 ohm for video. I have never given thought to cable impedance in a guitar signal chain, and have no idea where brand name cables fall in the spectrum. As far as I am concerned, if it's quiet it is good. My one hard rule of thumb is to never used unmarked cable, as is typically found online for dirt cheap.
     
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  6. jimgchord

    jimgchord Tele-Afflicted

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  7. elpico

    elpico Tele-Holic

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    That is one of the only things you can make a rule about - ground the shield to the chassis, not the circuit. (Or at least the normal world of electronics can say this. If you're still using the chassis AS the circuit ground then your noise currents and your signal current are all in the same soup and it probably doesn't make much difference) Another hard rule is "make the pigtails as short as possible".

    Everything else is really "it depends". A shielded cable grounded at only one end is effective against low frequency electric fields, like 50/60Hz power noise, but it's less effective against RF pickup, particularly if the cable is long. If RF pickup is the problem you're trying to fix then grounding it at both ends may be the way to go. Or dc grounding it at one end and AC grounding it through a capacitor at the other. Co-ax grounded at one end also provides no protection from magnetic fields (but twisted pair does), so if you were fighting that kind of problem... well, like I said. It depends.

    Assuming you are going to ground it at only end then there's the question of which end? It depends o_O Usually you'd ground it at the source end, but if the source end is floating then it's better to ground it at the destination. An example of this might be running coax from a tube's coupling cap to a volume control, you probably want to ground the shield to chassis near the volume control not the coupling cap.
     
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  8. Zipslack

    Zipslack Tele-Meister

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    If you have old 70's-80's stereos laying around, they are often full of shielded wire that can be salvaged.
     
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  9. SacDAve

    SacDAve Poster Extraordinaire Silver Supporter

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  10. King Fan

    King Fan Friend of Leo's

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    As far as where to ground the shield, I always start my day by agreeing with anything @elpico says. :)

    The most common place folks run shielded is on the input leads -- I've always seen the pros (eg Paul Ruby) say to ground at the input jacks: "The key thing is that the input stage is a very sensitive node for noise.... The shield of the cable is only grounded at the jack end."

    I've also run shielded back from V1 to the first pot, and where signal goes near household AC, like when an MV replaces the ground switch on a 5e3, or a 'raw' pot does the same on the back of a PR. In these cases I've just grounded the shield to the preamp ground bus.

    As an added detail, Merlin mentions that *if* we grounded both ends of the shield we'd not only create noise in a long ground loop, but also noise by mutual induction (transformer action) between the signal wire and the ground-loop current in the shield.

    Although most of us know to ground just one end, it's really easy for a fine whisker of shield to stick out and ground the 'other' end -- or to contact the signal wire, which, instead of noise, can produce, um, silence... :D
     
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  11. sds1

    sds1 Tele-Afflicted

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    I guess it works out well for us then: the input jack, the pots that run to grids are all the preferred places to ground the shield. Finding your way to the ground bus from socket side of the chassis is messier.
     
  12. Bill Moore

    Bill Moore Tele-Afflicted Silver Supporter

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    I guess I was fortunate?, to be around when the mics we used could plug into an amps. Our first dedicated PA's also had 1/4" jacks, so all the mics I had in the 60's up to the mid 70's used a single conductor shielded cable. I have several "noisy" cables that seemed to end up on a shelf, which now are used for shielding in my amp builds.
     
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  13. TobyZ28

    TobyZ28 Tele-Meister

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    Slightly off topic... but has anyone ever used shielded cable to prevent the transmission of EM from higher power AC connections (obviously not on RG174)? :D
     
  14. Zipslack

    Zipslack Tele-Meister

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    Back in the day, I used to design power control systems and EMI/RFI filtering for military, medical, and aerospace. Twenty years gone since then....don't remember much.
     
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  15. Kevin Wolfe

    Kevin Wolfe TDPRI Member

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    Last edited: Feb 28, 2020
  16. tubejockey

    tubejockey Tele-Meister

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    A better approach than shielded cable to prevent transmission of EM is to use a twisted pair. Use one for the hot and the other for the return. These pairs can be shielded as well, but the twist does more to prevent radiation of EMI than the shield. The number of twists per foot should be determined by the frequency of the AC. When equal current is drawn in opposite directions, the EM flux cancels itself (remember the old "right-hand rule"). I used to build instrument panels for airplanes, we wired high voltage strobes this way to prevent setting off the storm-scope.
     
  17. jhundt

    jhundt Doctor of Teleocity

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    When I need some shielded wire for those short runs inside an amp, I grab an old RCA-stereo lead out of my box of old cables. These are the ubiquitous double cables with two RCA cinch plugs on each end - one red and one black, usually. You can find millions of them in the thrift store, for a penny a piece, probably. Cut the ends off, separate the two cables, and you have enough shielded wire to do quite a few amps.
     
  18. tele_savales

    tele_savales Tele-Holic

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    I am not a huge fan of the stiffness of RG-174. It just always seems to want to go in a different direction than where you're soldering it to. I got tons of shielded cable from the Silverface twin wreck I stripped and it's twice as thick and more supple. I've been using that here and there.
    I got 6 feet of 4-conductor shielded pickup wire for just a couple bucks on Amazon and it seems to work great.
    In my limited experience I kind of disagree w a twisted pair being superior- I put a type-3 MV control right in front of the grid leaks, literally 2 inches away, used a twisted pair, and it was the noisiest knob on the amp. Immediately cleaned up when I put shielded cable in. Helped on the reverb level also.
     
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