Shielded Wire Questions - 2204 Build.

Discussion in 'The Workbench' started by Rick Lee, Aug 8, 2016.

  1. Rick Lee

    Rick Lee Active Member Gold Supporting Member

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    Please take a look at the pink and blue wires at the volume pot and hi input jack here.
    http://site.triodestore.com/JCM8002204Stock.pdf

    Exactly what kind of wire do I need here and does the braided/shield part connect to anything? The Metro kit build instructions make no mention of this, and so the wire I ordered is just the pre-tinned stuff, not braided/shielded. I have some shielded 4-wire stock, but it's really tiny and I use it for guitar stuff. I'm assuming it's too small a gauge for amp voltage. What to buy and, well, do I really need it or will the regular, pre-tinned stuff get me there?
     
  2. Gunner64

    Gunner64 Well-Known Member Gold Supporting Member

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    In the notes at the bottom of the page it says the shielded wires are grounded at one end only. I grounded mine on the jack side. I would use the shielded wire here. A decent quality phono cable can be cut and used if you have one..just watch the heat as not to melt the center insulator..
     
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  3. ampmadscientist

    ampmadscientist Well-Known Member

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    Shielded wire from input jack to grid resistor is grounded at the jack side. From grid resistor to tube socket, the shield is not grounded, it's connected to the plate of the preamp stage on one end only, the other end of the shield is shrink wrapped... Also known as hot shield.
    The hot shield kills HF oscillations (squealing) and microphonic tendencies of the first preamp stage...
    But the shielded wire is forming a capacitor between plate and grid. (you could also solder a cap on the socket between plate and grid)
    This is in fact the factory method. The wire should be rated 600 volts or higher, I like to use very flexible wire. The longer the hot shield wire is, the more negative feedback will be introduced.
    Therefore the type of wire (pf per foot) will be a consideration.
     
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  4. Gunner64

    Gunner64 Well-Known Member Gold Supporting Member

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    Good advice, and im not arguing. Just for the record i used the same layout from triode. I didnt do the hot shield method and dont have any issues with hf oscillation or microphonics. Its actually the quietest amp i own. Your results may vary..but mine is very quiet, even with the preamp maxxed..i just followed the layout as written.
     
  5. ampmadscientist

    ampmadscientist Well-Known Member

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    If you use high gain preamp tubes or heat up the bias to get rid of Xover distortion....there is usually ringing in the preamp.
    (cold bias works like a noise gate)
    But with cold bias or lower gain tubes, its probably less of an issue.
    (the factory adjusts the bias hot to get rid of X-over distortion) and that's the (hot shield) factory method.

    The factory does not use a bias probe...they use a scope.
    They verify that X-over distortion does not occur until after the power amp clips. Hence the bias is hotter than a bias probe would leave it.
     
  6. Rick Lee

    Rick Lee Active Member Gold Supporting Member

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    I have a bunch of shielded Mogami cable stock, but it's probably too thick. Guess I have to go back to Fry's again. Any chance hardware stores carry this kind of cable?
     
  7. gldtp99

    gldtp99 Active Member

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    Many builders use RG-174 cable----- it isn't the easiest cable to work with (inner insulator melts easily) and there are other similar types with Teflon inner insulation (google Apex Jr., they usually stock several types)---- I've learned to work with RG-174 and I use it all the time: https://www.tubesandmore.com/search/node/s-w112
     
  8. gldtp99

    gldtp99 Active Member

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    I'm taking one of my amp customers through a 2204 type build (slowly--- he wants to learn how to "build amps" but our schedules don't match up very well)------ I make him do all the actual assembly/soldering, under my direction, and I've shown him how to use RG-174 cable with proper diameters of shrink tubing, how to lightly tin stranded wire, and how to attach grid resistors (and cover/reinforce with shrink tubing) to the tube socket end of a custom made RG-174 cable assembly.
    He has picked up my tricks about using RG-174 pretty quickly---- I wish he would do the backround tube amp theory study I have suggested so that he could understand how a tube amp actually works----- instead of just waiting for me to tell him what to solder next.................... gldtp99
     
  9. Guitar-Rocker

    Guitar-Rocker Well-Known Member

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    Gldtop is right on the money. I use RG-174 too.
     
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  10. Gunner64

    Gunner64 Well-Known Member Gold Supporting Member

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    Not to derail the thread, but you guys who build amps for a living..do you use the hot shield method ams describes in a stock 2204 build? I did not in mine, bias with the 1ohm method at 70% and have not had any oscillation or microphonics issues. Amp is not modded for xtra gain, and has RCA 12ax7s in the first 2 positions..just curious as to if It would be benificial to do so..
     
    Last edited: Aug 9, 2016
  11. Rick Lee

    Rick Lee Active Member Gold Supporting Member

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    Ok, I've used this kind of shielded cable before when making my own pedalboard cables. Well, it was Mogami and a little thicker. Two questions on this stuff.

    1) What is the trick for peeling back the braided shield without tearing it all up?
    2) What's the problem with melting the inner shielding if you have to strip it anyway to solder the inner core?

    Gldtp99, I've done a ton of soldering and heat shrink tubing, though soldering is one thing I never get better at, no matter how much I do it. Still, I have never figured out the trick for working with shielded cable. The teflon shielded stuff is easy, but not the braided shielding.
     
  12. gldtp99

    gldtp99 Active Member

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    1) I use a Mechanic's Pick (similar to a dental pick but available at most tool suppliers)--- Mine came as a set from Ace Hardware long ago--- I use the small hooked one to carefully undo the shield braid so that it can be twisted into a separate external connector. I usually insulate this running twisted braid with shrink tubing.

    2) The problem is with melting the inner insulation that is still inside the finished cable----If this part of the insulation melts (either completely or is just degraded so that it fails at a later date) then the center conductor and the braided shield (usually grounded but sometimes connected to high voltage) are shorted together and the cable assembly won't work at all (signal shorted to ground) or you have a condition where high voltage is introduced into the signal path, probably to the grid of the following Triode Gain Stage.
     
  13. Guitar-Rocker

    Guitar-Rocker Well-Known Member

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    I do not use the hot shield method for fear that someday, someone could get hurt if it ever leaked through. Just don't want to take that chance, no matter how small the odds may be on it. And one never knows what a user might decide to try to get "even more tone" from an amp.
     
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  14. gldtp99

    gldtp99 Active Member

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    I don't build amps for a living, just as an obsession----- but I don't like to use the Hot Shield, the concept makes me uneasy from a general safety standpoint.
    But I have used hot shields on some occasions when it was the only practical way to install a shielded wire in a particular area in a particular amp.
    My '81 Marshall JMP 2204 has a factory Hot Shield----- the amp is in stock configuration and I have no plans to change anything.
    I think AMS is a very intelligent tech and has a lot of experience----- I may not agree with everything he posts but I do seriously consider his input.................... gldtp99
     
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  15. ampmadscientist

    ampmadscientist Well-Known Member

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    Marshall invented that hot shield at the factory and some of these amps have it built in from stock.
    They also did 7 pf 1000 V cap from plate to grid of first preamp input stage...
    and so did Fender, on some models.
    These 2 methods do the same basic thing---kill the hiss of high gain stage or help to stop the microphonic ringing at V1.

    But I like these methods because they have less effect on the crunch of the amp...it preserves a lot of high frequencies compared to other methods. Other methods make the amp sound muddy....
    So I like the hot shield best, for sound quality.

    Considering...any part in the amp "could" short at any time....but usually does not.
     
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  16. Rick Lee

    Rick Lee Active Member Gold Supporting Member

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    What is the hot shield method and what is the alternative? How hard can this be? I just want to install the two shielded wires shown in that diagram? There's not even a note at the bottom of the diagram saying it's extra tricky. What to do here?
     
  17. Gunner64

    Gunner64 Well-Known Member Gold Supporting Member

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    Mine is wired as drawn..i dont have any issues with squealing or microphonics. If it was mine I would wire it as drawn, then if you have issues with squeal ect. Do the hot shield. And I agree with gldtp..ampmad knows his stuff..and Im not arguing. The only reason im saying anything is I built with the same layout and transformers and had zero issues..
     
  18. ampmadscientist

    ampmadscientist Well-Known Member

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    The old method is grounding the shield. Just like any other amp....
    Look on the Marshall schematics, the correct schematic shows the hot shield.

    No, it's not tricky, just takes practice.
     
  19. jgab

    jgab Active Member

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    I have two Canadian 2204 (86' and 82') and both of them don't have shielding on the grids of the V1. The also don't have a 100pf cap either.

    I always wondered why some 2204 stock amps have shielded and some don't?

    I read that a shielded grid on V1 can reduce your top end, or treble. In saying that, I think it would be best to try without shield and see if you have noise issues. If you have noise issues, then try the shield thingy. If not, then don't worry about it...
     
    Last edited: Aug 14, 2016
  20. HAMPAMP TUBE AMP SERVICES

    HAMPAMP TUBE AMP SERVICES Member

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    I found this video that explains how I work with shielded wire, glad I found it because it saves me a lot of typing.........<iframe width="560" height="315" src="" frameborder="0" allowfullscreen></iframe>
     
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