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Thread: Shielded cable

  1. #1
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    Shielded cable

    Does anyone on here have any recommendations for the two lengths of shielded cable I need to run in my amp build the first goes from the input jack to pin 2 of the 12ax7 (hot wire only) the shield is grounded on an earth tag of the 12ax7 valve socket bolt
    The second run goes from pin 7 of the 12ax7 to the middle lug of the volume pot (hot wire only) the shield is grounded to the back of the volume pot

    My main question is what is the best cable for this I have some rg174 shielded cable but I've also got an old fender guitar cable which had the plug pulled off one end and after examining the cable it looks perfect for the job could this be used? admittedly it is a lot chunkier than the rg174 but as it was a guitar signal cable to start with surely it would be perfect ?
    Thanks
    Scott

  2. #2
    Supporting Member eschertron's Avatar
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    "Best" is relative. For me, the best cable was the few feet of microphone cable salvaged from the end of an install. Not flex-rated, rubber-jacketed mic cable, but the thin stuff to be pulled through conduit. Foil shield, OK for inside a chassis. And the price was right

    Some grounding schemes suggest multi-conductors so that the ground reference of the input stage is on its own conductor inside the shield.
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    Ha ha thanks maybe the only concern is heat tolerance and the guitar cable has the braided cloth outer covering so might be a fire issue 🤔

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    I'd think ANY kind of guitar cable is WAY too fat for inside an amp chassis. Not to mention, a lot of those braided covers have metallic thread in them somewhere. Bad idea if so. Very inflexible, too.

    An idea of what you built and what it looks like inside might coax (pun intended?) some different ideas. And did you try it without shielded cable first? Lots of amps never used the stuff...

    Justin
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    Senior Member Enzo's Avatar
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    I occasionally would tear down an old cassette deck or even a VCR, and I saved the small shielded cables that ran from the record and playback heads to the main boards. I found it real convenient to use in these cases. And I didn't have to buy a roll of anything.
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    I do the same. I find really thin, high-quality screened cable is difficult to find. Some of the OEM cable is much better than that sold by most (UK) suppliers. Sometimes I need to use screened cable in FX pedals and it needs to have good screening as well as being slim. A lot of the reels of commercial stuff is usually cheap crap that will start to break the centre conductors after only three or four flexes.

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    Quote Originally Posted by Jarvini View Post
    Does anyone on here have any recommendations for the two lengths of shielded cable I need to run in my amp build the first goes from the input jack to pin 2 of the 12ax7 (hot wire only) the shield is grounded on an earth tag of the 12ax7 valve socket bolt
    The second run goes from pin 7 of the 12ax7 to the middle lug of the volume pot (hot wire only) the shield is grounded to the back of the volume pot

    My main question is what is the best cable for this I have some rg174 shielded cable but I've also got an old fender guitar cable which had the plug pulled off one end and after examining the cable it looks perfect for the job could this be used? admittedly it is a lot chunkier than the rg174 but as it was a guitar signal cable to start with surely it would be perfect ?
    Thanks
    Scott
    You have to consider the capacitance effect of shielded cable especially if long runs are encountered. Good shielded cable will use a braided net to cover the inner signal wire, if you look at some old amplifiers this braided shield was in some instances soldered to the chassis along it's path from point to point. Made for a very neat appearance.
    Also be aware that many of the over-the-counter audio leads sold today do a very poor job of shielding. The inner cable is surrounded by a lose wires which act more like the ground conductor rather than as a proper shield.
    Good luck with your project. Mickey
    Mick Bailey likes this.

  8. #8
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    Quote Originally Posted by Jarvini View Post
    Does anyone on here have any recommendations for the two lengths of shielded cable I need to run in my amp build the first goes from the input jack to pin 2 of the 12ax7 (hot wire only) the shield is grounded on an earth tag of the 12ax7 valve socket bolt
    The second run goes from pin 7 of the 12ax7 to the middle lug of the volume pot (hot wire only) the shield is grounded to the back of the volume pot

    My main question is what is the best cable for this I have some rg174 shielded cable but I've also got an old fender guitar cable which had the plug pulled off one end and after examining the cable it looks perfect for the job could this be used? admittedly it is a lot chunkier than the rg174 but as it was a guitar signal cable to start with surely it would be perfect ?
    Thanks
    Scott
    Hi Scott, I use 26 GA shielded RG-174 coaxial cable:

    https://www.jameco.com/z/RG174-U-100...et_644818.html

    No problems with it so far, very nice braided shielded on it.

  9. #9
    Senior Member SoulFetish's Avatar
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    I would try grabbing some shielded cable from apexjr and see if you like it:
    Wire
    or
    http://stores.ebay.com/apexjr?_rdc=1
    he sells it for cheap, usually has plenty, and I've had great experiences purchasing from him in the past.
    I, having said that, think mick brings up some good points about shield coverage, reliability, and capacitance being important things to consider in the cable you decide to use. Some of the RG174 I've used is extremely thin and too flexible, so I made my own shielded cable for my last build out of 20AWG/1000V solid core wire. That's a bit obsessive, I fully admit it. I didn't have any long wire runs, so I wasn't overly concerned about slight increase in capacitance. Plus, since I'm not using for HF transmission, I wasn't concerned about any differences in relative permeability between PVC and (polyethylene?).
    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

  10. #10
    Senior Member Enzo's Avatar
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    While I like to use the salvaged tiny coax from tape decks and VCRs, I bought a roll of lavaliere mic cable (lapel mic) and really liked it. It is small, very flexible, shielded well enough for anything I used it in. The shield was over a pair, so I usually only needed one conductor, but there was the second in cases where it was useful.
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  11. #11
    Supporting Member Chuck H's Avatar
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    I've tried buying mono shielded lead a couple of times and was never happy with the stuff when I actually had it in my hand. I still have the two rolls and I use them in a pinch, but the last time I needed some for a project I had to show off I went to Radio Shack and bought a cheap little microphone with a 3.5mm jack. Small diameter, reasonable capacitance and a nice, silicone jacket. I think it cost me about five bucks for five feet of cable. Worth it for the project. You can actually get stuff like that cheaper now. Though it will probably be a PVC jacket. NBD

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  12. #12
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    Quote Originally Posted by Chuck H View Post
    ...went to Radio Shack and bought a cheap little microphone with a 3.5mm jack. Small diameter, reasonable capacitance and a nice, silicone jacket. I think it cost me about five bucks for five feet of cable. Worth it for the project. You can actually get stuff like that cheaper now. Though it will probably be a PVC jacket.
    Maybe I'm just unlucky.

    When I've chopped the wire off cheap mics, I've usually found that the "shielded" cable was actually two conductor cable.

    Once, I thought I'd found a great deal on RG174 from China. The listing was for a CB mic cable. I forget if the <$3.00 cable was 20 or 30 feet long, but the text specifically said RG174. The cable assembly was beautiful, sporting two gold-plated, crimped BNC connectors- which I lopped off to find coax cable with an ALUMINUM braided shield. The coax probably worked fine with crimped connectors, but it didn't meet RG174 spec. And I sure couldn't solder the shield.

    Sigh,
    -rb

  13. #13
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    If it is a 2 small conductor tight twist pair?
    Solder the two together on both ends which makes it a one conductor.
    I do that with the shielded 28ga pair, pickup cable, I have, on occassion.
    With them soldered together, still small, then ground the shield on one end, you're good to go.
    I'm sure that won't meet the code here, but I find it to work great!
    T
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    Terry

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    One of the 7 organs I've gutted was a Kimball solid state organ. It set me up with shielded cable in a variety of colors. I doubt I'll ever have to buy shielded cable again. There are tons of solid state organs out there that people need hauled off. Help someone out and help yourself.

    Though it was solid state, the chassis turned out to be perfect for one of my amp builds and the Utah speakers in the organ weren't half bad either.

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