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Effect of placing twisted wire in a Stainless tube

  1. Jun 17, 2016 #1
    Consider 12 inches of 22 g twisted insulated copper wire with a 1 amp AC current at 6.2 volts.
    The idea is to reduce the magnetic field effect on near by components.

    Would placing this wire in a stainless steel tube reduce the magnetic field?

    Can this be proven with a Gaussmeter? Note: I see Gaussmeters for sale in the one to two hundred dollar range but have no experience using them. Link http://www.always-review.com/searcher/gaussmeter


  2. jcsd
  3. Jun 17, 2016 #2


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    I'd say save your money and forget the Gauss meter.
    Twisted pair cables are
    bold by me

    If you want to try this, why not just use shielded twisted pair wire?
    Last edited by a moderator: May 8, 2017
  4. Jun 17, 2016 #3


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    You shouldn't be getting much net B-field from a twisted pair cable. What frequencies are involved? What are you trying to protect? What is the lay length of the TP (the distance between twists)?
  5. Jun 17, 2016 #4
    Hi guys,

    For this issue I will confine the conversation to the typical 60hz AC 6.2 VAC circuit coming from a multi tap transformer used to power the heater elements in a typical tube guitar amp. They sometimes produce 60hz hum. The typical solution is to twist the wire. There is a limit to how many twist per inch that is possible. The cause of the hum is also very directly related to the grounding method.

    I am exploring what better methods of containing magnetic fields in general exist and what tools exist to detect those fields in a practical way.

    There are companies that sell many forms of material for this purpose. The SS tube example was just a way to communicate the general idea.

    The twist I generally see are about 3/16 of an inch for heater feeds.

    The thinking for buying a Guassmeter was to have some way of getting a visual on where the fields exist and how strong they were. There are pretty strong magnetic fields generated by power transformers and output transformers for example. The field strength gets less with distance and orientation but I am getting ahead of myself...lol



    EDIT: Trying to protect pre amp components
  6. Jun 17, 2016 #5


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    Once you twist the wire, any hum will not be coming from the TP's magnetic field. It could be coming from other sources with other coupling mechanisms...
  7. Jun 17, 2016 #6
    Many but not all "modern" guitar amps are pretty quiet. Older ones are notorious for noise issues. Twisted pairs are common but shielded twisted pairs are infrequently used although I am not sure why that is true. Cost and complexity could be one answer. Also I am speaking of point to point wiring and not addressing PCB designs. Point to point has come back into use in many "reissue" style units in the last few years.

    Tube guitar amps in general are have high THD and limited bandwidth of perhaps 50hz to 5000hz. They contain both high voltages and current. They are not high fidelity amps to begin with and are designed to produce certain types of distortion as a wanted component of there design. They often are used at pretty high volume levels which cover up a lot of sins.

    My reason for studying these issues is to better understand the cause and effect relationships that exist that cause not only hum but unwanted oscillations. These effect are generally most problematic near the pre amp section. My second reason is a increased demand for me to repair amp kits others have built and made a mess of and do not work. To that end I need to be able to know if the design was bad or the installation was bad or both.

    I think I know the answer to the question about the SS tube. I assume the answer is yes it will diminish the magnetic field. The fact it is SS and not steel should change thing by some small amount I assume. I also assume that the shield on a cable is more or less a type of Faraday cage and that it would only be grounded on one end.

    In general there is an endless amount of incorrect information in books and for sure on the internet as it pertains to design of guitar amps and there sub systems. I am trying to learn enough to sift through the information and not be lead astray by nonsense.


  8. Jun 17, 2016 #7

    jim hardy

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    400 series stainless is fairly ferromnagnetic so will provide some magnetic shielding
    but it's maybe a tenth so permeable as good mu-metal ,
    300 series is nonmagnetic.

    Are you trying to reduce hum in an amplifier that comes from heaters by shielding the heater wires?
    I was taught simply twist them and route away from signal lines & preamp components. I kept them in the corners of the chassis close to the metal (usually steel) .
    Centertap of heater winding was usually grounded to assure zero common mode voltage .

    Some fine old amplifiers used the preamp tube heaters as cathode resistors for the output pentodes
    that did a couple things
    it removed AC from the wires carrying heater current in vicinity of preamp components killing one source of hum
    it killed another source of hum by replacing preamp heater power with DC >>> DC can't couple across heater-cathode capacitance .
    It assures no signal reaches output tubes until they've had time to warm up and establish plate current .

    old jim
  9. Jun 17, 2016 #8

    jim hardy

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    Test everything against your basics. You'll enjoy seeing the boundaryof your understanding expand.

    Have fun !

    old jim
  10. Jun 17, 2016 #9
    Thanks Jim

    Jim ask "Are you trying to reduce hum in an amplifier that comes from heaters by shielding the heater wires? "

    The answer is yes in general. The heater wiring is twisted and placed like you indicated. The chassis is more often aluminum now days but lots of steel chassis exist.

    There are many ways to control all this including biasing the heater circuit.

    Mainly I am trying to form a good mental picture of the many possible solutions.

    In the larger picture, I need to well understand the wiring issues along with the more complex issue of grounding. With the green AC mains ground wire being more or less a requirement we are at once at odds with other grounding requirements. We need at least one other ground and so we are going in most cases to form a ground loop. How bad...depends on many things. Don't assume ground is ground because it is not in all to many cases. Not using a star ground system and placing several things directly grounded to the chassis in different places will pretty much assure problems. Little attention is normally given to optimize designs to eliminate sensitivity to ground noise.

    There is no excuse in today's world for having to put up with noise in guitar amps. If I design a guitar amp I can pretty much assure you it will not make noise. The issue is all the amps I work on are designed by others and I need to understand why each amp is making noise and sound methods to fix it. I have no doubt I can learn those skills for point to point. PCBs......that is another ball game...lol

    Thanks for all the instructions!!


    BTW....Any time you see me write "there" assume I mean " their" and we will both be right about 50% of the time...lol...Jeezz, I read my post and wonder where the blazes did that boy go to school...lol
  11. Jun 17, 2016 #10


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    A note from my experience playing in a little band years ago; before third wire ground in plugs. And you probably already know, but ...
    Amps with two terminal unpolarized plugs can be produce lots of hum depending on which direction they are plugged in.
    And can also cause good ground loops from the lips to a microphone. :oldsurprised:
  12. Jun 18, 2016 #11
    Hey , Plano

    Apart from bad wiring and not twisted wires etc, I think the other most serious factor in terms of noise is the very tube heating filament being driven by AC.Atleast I have seen real difference between changing various tubes of the same type , some were older some newer, ones made more hum the others less.When you come to think about it the supply wire runs usually with a large distance of signal wires yet the very filament is within the tube and for smaller tubes the difference between filament and plates is what? mm in size.
    Maybe you can try running your tube filaments from DC ,In modern times that's not a problem , we have rectifiers and linear regulators all in small TO 220 packages.
    Ads a bit complexity but then you don't need to worry about your heating wire twisting or placement and also for the tube filament.

    If you remember when I made my thread about the old tube amps I got , I had very little hum and they use no grounding via power cable etc , then I changed the driver tubes to new ones and also the output tubes to new ones , adjusted the idle current etc and now there is virtually no hum , so in my case much was to blame on the tubes themselves ,at one point i thought about making the heating DC myself.
  13. Jun 18, 2016 #12

    jim hardy

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    Check the tube data sheets
    since the heater runs red hot it softens and can sag
    for that reason some tubes should only be run vertical ,
    meaning if you lay the amp on its side you risk sagging the heater, bringing it closer to cathode

    some tell you to keep the tube oriented to prevent sag ... for example keep pins 2&7 in a vertical plane

    i only recall seeing that caution on big tubes .
  14. Jun 18, 2016 #13


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    If you use thermionic tubes with an AC filament supply you can expect to get hum from the filament asymmetry within the cathode. Half a century ago there were circuits, I think they were called “hum-buckers”, that removed hum from amplifiers. One was a wire-wound pot of about 1000 ohms across a floating filament winding on the transformer. The pot wiper was grounded. The pot was adjusted for minimum hum. More complex hum-buckers used a capacitor to produce a phase shift or quadrature cancellation.

    An alternative would be to commit blasphemy by providing a DC 6.3V filament supply from a switching converter.
  15. Jun 18, 2016 #14
    Hi guys,

    Yes, some of the the "un-grounded" two wire amps even had a switch on them so you did not have to unplug it.

    For sure AC creates the possibility of 60 cycle and 120 cycle hum. Some amps now days use DC for the first pre amp tube. As for tubes in general, they can cause all manner of issues mainly do to using tubes that do not perform to standards. I don't have tube issues any longer due to having found a vendor who provides me with well tested tubes based on what I want.

    There are only a few tubes in "common use" today that require vertical positioning. There seems to be a race with designers too see who can disregard the data sheets the most and get away with it. I have seen 12AX7s running at 350V

    I don't think designers or players would have much resistance to switching power supplies except that when they first got common they were very noisy. I think it is more an issue of cost and complexity. The repair of switching power supplies is not easy and for many guitar techs not doable. The AC heater voltage is there so a bridge and a cap or two and you have the DC voltage...simple

    What all these above comments demonstrate is the multitude of possible causes for "noise" just based on wiring and grounding and the use of AC heater circuits and not even addressing other causes.

    I am sure we would not even be in total agreement of what to call the noise. Hissing, static, Humming both 60 and 120 hz, crackling, Rattling, Motorboating, Unnatural harmonics, popping, sputtering, buzzing, fizzing. All those terms are out of Gerald Webers's book "Tube Amp Talk". Gerald is a super well skilled repair guy and if he says to "do this" you can bet it is correct. His method of repair is based on years of hand on experience and sometimes is lacking in technical explanation.

    It is less problematic to find the cause of noise in a brand and model of amp I have worked on a lot because I know what it normally sounds like when running correctly and know the noise is not a a function of bad design. Well....that is if someone has not made changes. You would be amassed at what some owners do to their amps...lol

    Worse case is some amp made by some boutique amp maker with no schematic and no information about the amp. At that point if one does not have the skill to analyze the design for issues, solving the problems becomes just guess work. Even educated guess work can take hours and be frustrating as hell.

    To that end I intend to become better educated.


  16. Jun 18, 2016 #15
    On a related subject there is a guy, Kevin O'Connor from Thunder Bay, Canada who has written many books about amp building. One called "The Ultimate Tone Volume 3" describes what some consider the best method of grounding. Keven is well known on the DIY forum and others. Some agree with him and some don't.

    I have in mind to buy his book. I only hesitate due to the high cost of $68 for a photo copied spiral bound book. Seems a bit pricey. Here is a link: http://www.londonpower.com/audio-design-books/TUT3

    Anyone here read this book?


  17. Jun 18, 2016 #16


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    In my experience, the worst hum usually was due to ground currents because:
    • In the "good old" tube days, the steel/aluminium chassis was almost always used as "ground".
    • Between the mains transformer and the smoothing capacitor, a relatively high ripple current would run in the chassis
    • This current would not necessarily run in a straight line, it would spread out in the chassis.
    • If you the connected the "ground" part of the input signal to the chassis, a noise voltage caused by the ripple current would be added to the signal.
    So remember:
    • The "ground" part of an input is a signal, and should be connected directly to the point where the input tube cathode is connected to "power ground"
    • Always assume that the chassis is electrically noisy (if you have ever tried to get an electronic device to pass FCC part 15 you know what I mean)
    Shielding your heater cables will usually have little effect. Twist them and route them close to the chassis and away from low-level signals.
  18. Jun 18, 2016 #17
    I hope you will find this related to your problem somehow , I just finished building 4 channels of a high power Ab class amp, powered from a rather crude smps.I have a thread here on EE forums about the smps transformer core running hot, but that's another story.The thing that amazed me the most is that at first when powering only the first two channels I got some smps high frequency harmonics and noise through to the speakers, I almost made peace with that as I understood it's a crude self made thing and most importantly the noise was coming through the DC output wires as a sort of ripple leftover from the square switching waveform.
    My wires , the ones coming from the psu to the boards etc are a rather big mess with wires going everywhere.
    What i did is I simply sat down and thought what can I do , and I rerouted the input signal grounds and added some filtering resistors and guess what I hear no more noise in my output.
    I'm not saying it's a perfect design now , no for heaven's sake , but I just proved myself that even with some messy wiring and far from low noise psu I can get decent sound simply listening to some advice and doing a bit of soldering the right wires in the right places.
    So i guess it's not how the wires run in general but from where to where they run that's most important.\

    I think the posters above are right the best you can do with heater wires is twist them locate them away from signal wires and let them be.Most of the hum comes from the filament itself and ground loops , I have had a fairs share of headache with ground hum.

    As for the tube amps signal ground to chassis problem , well not always , I have two tube amps both have a metal chasis with tubes located inside and for both the signal ground is added to the chassis with the ground running also in the signal wire itself as the screen.at first I got a bit of HUm when I got the amps and I thought who puts ground like that but then I changed the old tubes and most importantly the filter caps, adjusted idle currents and output tube balance and now i'm silent.
    Grounding chassis is not always a problem , it just matters how it's done.Those amps are from older times and they don't have the earthing third wire for the chassis , but I can touch the chassis or flip the power socket and nothing changes.

    Maybe a stupid advice but when your amp is running, maybe try to tap it with your fist a little to make the filaments shake and listen whether the hum gets louder ?
    I did this with my older tubes and they got louder for a moment , with newer ones this effect is much less noticeable.

    P.S. why you say 120hz hum ? why would you get 120hz in a heater wire which is coming out of a 60 hz mains transformer?
  19. Jun 18, 2016 #18


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    120Hz could be due to a lack of filtering on the HT resulting in modulation by the plate or screen ripple voltage. Check for dry reservoir capacitors and for a shorted turn in the HT filter choke. Possibly a sick dual diode rectifier tube.
    If you rectified the filament supply and had no voltage regulator then you will get 120Hz rather than 60Hz hum. With 60 Hz filament hum you should not have harmonics. With 120 Hz there may be harmonics of 120 Hz.
  20. Jun 18, 2016 #19
    Hi guys,

    In all this discussion, I think the original question got a bit lost or was never stated correctly to begin with. The question was of a purely theoretical nature. I am not having any issue with a specific amp. I am studying both shielding issues and grounding issues in general from a practical perspective. I am exploring what ways exist to use shielding and what different shielding exist and how one type may differ from the next. The original question had little to do with grounding directly but as the two thing can both produce "noise" it is natural to get into the subject of grounding.

    My need to understand these issues come from having to repair amps that are noisy. My other need is to be able to do better re-design work on new amps I build. Currently I am only building "kit" amps and re-designing some things for better results.

    In the near future I have in mind to design and build a new amp from scratch. As I have no intention of producing this amp for sale, and I am not under the typical constraints of the market place I can build it with little consideration as to cost. Also, as I have no intention of using the amp outside my house, it does not really matter how large or small it is or if it looks like a typical amp.

    I value the feedback you guys have provided as it either provides new ideas I have not thought of or confirms what I think I know. So, none of the conversation is wasted, at least not for me.

    This new amp I have in mind will contain both tube and transistor components along with a integrated computer system to run amp modeling software but that is a whole different thread...lol



    BTW My 120 hz hum statement was a mistake.
  21. Jun 19, 2016 #20

    jim hardy

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    Take a look at 7199 it's designed for low hum audio

    that 15 watt hifi amp in back of RCA tube manual RC30 works really well, i built one in high school
    found that tube manual online as a pdf

    old jm
  22. Jun 19, 2016 #21
    Hi Jim,

    I assume the grounding was connected to the chassis. How many separate ground points existed? I also assume a two wire AC mains with no low impedance path to earth ground. Is this correct? I also have some questions about the hum balance circuit but we can leave that for later.

  23. Jun 19, 2016 #22

    I would also like to discuss this schematic. Should I keep it here or put it in a new thread?


  24. Jun 19, 2016 #23

    Photo of the chassis of the above schematic. There are a few minor differences from the schematic.

  25. Jun 19, 2016 #24


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    What about it?
  26. Jun 19, 2016 #25
    Better photo of the above


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