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Thread: terminating unused wiring bundle question

  1. #36
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    Hi All,

    Fast forward a month and change. Got wires unsoldered (couple that I did have wired), and the transformer out and apart. I don't believe I ruined anything so far (which is nice). The wire insulation isn't the good stuff, (not the really high temp PTFE mil spec stuff) its the cheaper wire REALLY thick melty, melty insulation. Barely enough room for 2 extra wires inside those end bells, but have 6. <frown>

    So, I have some of that braided shield stuff I took off some old shielded wire I took apart.

    Nevetslab, you suggested copper tape, will this shield braid stuff work as well? If I put it over the bundle, then shrink wrap over that, and ground one end would that do it, or do I need to use copper?

    These wires sticking out into space with nothing connected on one end must have different radiating properties than wires connected in circuit? Also, will it help if I twist the pairs before putting the braid (or copper if must be)?

    Geez, you really gotta know stuff to build amps, or at least, amps that probably won't oscillate themselves to death!

    Thanks everyone. Looking forward to getting the longest running JTM45 type thing closer to annoying the family and neighbors.

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  2. #37
    Bent Member Chuck H's Avatar
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    Honestly, you shouldn't need to shield the extra OT leads. No one else ever did (Fender, Marshall, etc.) Just don't bundle them under the preamp board!!! Direct them AWAY from the preamp. Maybe put in a terminal strip and just solder them to it unterminated. Just direct them as instantly and acutely AWAY from the preamp as you can. No matter where else you put them.

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    Never saw (long) unused OT primary wires inside the chassis of a Marshall or Fender amp. All OT primary leads (except CT) emit strong electric signal fields that may couple with a non-inverting input (e.g. via a coupling capacitor) and eventually produce HF oscillation at some amp setting.

    I would keep unused primary leads outside/above the chassis.

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  4. #39
    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Helmholtz View Post
    I would keep unused primary leads outside/above the chassis.
    With exposed ends and some sort of childrens toy or a piece of candy placed very near them

    Maybe if you constructed some sort of "doghouse" to keep them in. Otherwise I'm opposed to having any HV outside the chassis for safety reasons. You just can't predict what someone will do and Mike may not be the only person ever after to own this amp.

    If it were mine (and it's not) I would seriously consider just committing the transformers to the project and just snip off and shrink tube any unused leads so that they could be left under the end bells and have only the leads the amp will be using ever entering the chassis. Given the choices of:

    1) A transformer dedicated to a specific project that can only be re purposed for a similar project forever.

    2) A transformer that CAN be re purposed for a different project, but is currently in an unstable amp.

    Marshall DID terminate unused PT leads by soldering them to unused turrets at the far edge of the board in some amps. But it was a well considered placement and implementation IMO.

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    "...less ear-friendly but handy for jazz." Leo_Gnardo

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    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

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    With exposed ends and some sort of childrens toy or a piece of candy placed very near them
    Of course I assumed careful/reliable insulation and fixation of the wires. I would probably just wrap them around the OT and fix with cable ties or similar (certainly not conforming to safety standards but I assume the amp is not going to be sold in this preliminary state).
    Another option would be a metal tube fixed and grounded to the chassis with clamps. This as well as a "doghouse" would take care of safety and shielding.

    I have actually seen pictures where someone managed to wind the unused wires of an RS type OT on top of the coil, which hides them inside the endbells. Maybe only some of the wires and somewhat shortened. As said, the problem are mainly the unused primary wires.

    If you to place and fix them inside the chassis, the wire lenghts inside the chassis should be kept as short as possible (and of course away from preamp tubes and circuit). Field radiation increases with (unshielded) surface area.

    Marshall DID terminate unused PT leads by soldering them to unused turrets at the far edge of the board in some amps.
    I have seen unused PT wires (insulated with shrink tubing) in Fender amps as well.
    But I explicitely spoke of OT primary wires. PT wires don't cause feedback and oscillation

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    Last edited by Helmholtz; 12-22-2018 at 05:37 PM.
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  6. #41
    Old Timer olddawg's Avatar
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    Ive seen manufactures simply put wire nuts on them (Black or grey ones are a little less obtrusive) and secure them on with a little hot glue or silicone.

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  7. #42
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    Quote Originally Posted by Chuck H View Post
    Honestly, you shouldn't need to shield the extra OT leads. No one else ever did (Fender, Marshall, etc.) Just don't bundle them under the preamp board!!! Direct them AWAY from the preamp. Maybe put in a terminal strip and just solder them to it unterminated. Just direct them as instantly and acutely AWAY from the preamp as you can. No matter where else you put them.
    Thanks Chuck. I assumed taht I should leave the leads long, but there is just enough room to wire them to a terminal board.

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  8. #43
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    Quote Originally Posted by Helmholtz View Post
    Never saw (long) unused OT primary wires inside the chassis of a Marshall or Fender amp. All OT primary leads (except CT) emit strong electric signal fields that may couple with a non-inverting input (e.g. via a coupling capacitor) and eventually produce HF oscillation at some amp setting.

    I would keep unused primary leads outside/above the chassis.
    Thanks Helmholtz. The only good images Ive seen, the original original Radio Spares transformer has a board with contacts on it, so no unused wiring anyplace. I tried routing a bunch of ways, if I leave the leads factory length, the only place is under the board, then copper wrap, shrink wrap over that. ... or Chuck's recommendation: shorten, and solder to term strip.

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  9. #44
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    Quote Originally Posted by Chuck H View Post
    With exposed ends and some sort of childrens toy or a piece of candy placed very near them

    Maybe if you constructed some sort of "doghouse" to keep them in. Otherwise I'm opposed to having any HV outside the chassis for safety reasons. You just can't predict what someone will do and Mike may not be the only person ever after to own this amp.

    If it were mine (and it's not) I would seriously consider just committing the transformers to the project and just snip off and shrink tube any unused leads so that they could be left under the end bells and have only the leads the amp will be using ever entering the chassis. Given the choices of:

    1) A transformer dedicated to a specific project that can only be re purposed for a similar project forever.

    2) A transformer that CAN be re purposed for a different project, but is currently in an unstable amp.

    Marshall DID terminate unused PT leads by soldering them to unused turrets at the far edge of the board in some amps. But it was a well considered placement and implementation IMO.
    Oh man. heh. I had to read that 4 times till I got the joke. OK, ok no HV wiring hanging outside, but I was looking at some metal cans like the Fender cap cap, but they're all too damn big.

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  10. #45
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    Chuck, thanks, man yeah, re

    "If it were mine (and it's not) I would seriously consider just committing the transformers to the project and just snip off and shrink tube any unused leads so that they could be left under the end bells and have only the leads the amp will be using ever entering the chassis. Given the choices of:

    1) A transformer dedicated to a specific project that can only be re purposed for a similar project forever.

    2) A transformer that CAN be re purposed for a different project, but is currently in an unstable amp."

    spot on. OK personality issue, on my part I suppose. Which is why Im terrible at wiring. Always worried the wire will end up "this much" too short. so wires are too long and stuff all over that should be neater.

    I was not completely sure that: the OT primary leads for KT66's were correct. This transformer has several, was worried Id have to change them, and the wires would not be long enough.

    Since most kit mfr's use a standard OT for this amp, I can't find much talk on which impedance I should use. 9k, 8k, 6.6k. ultra linear.

    I read and re-read all the posts that talk about reflected impedance and all that, and got lost. Some gear page article said they changed from 6.6k then "upgraded to 8k". No idea what that means. More k's better?

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  11. #46
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    I read and re-read all the posts that talk about reflected impedance and all that, and got lost. Some gear page article said they changed from 6.6k then "upgraded to 8k". No idea what that means. More k's better?
    There was a lenghty forum discussion some months ago on this.
    Early JTM 45s with RS OTs mainly used 6.6k. Later ones with Drake OTs only used 8k. 6.6k is closer to the KT66 manufacturer's recommendation of 5.0k. 6.6k gives more output power than 8k.
    8k may clip smoother, sound more compresssed. Try both options and tell us.
    As the KT66 and 6L6GC charts are very close, impedances down to 4k can be used, increasing output to around 45W with a plate supply voltage of 400V at full power.

    I have seen reports claiming that some early Bluesbreaker combos with RS OT used the 8k primary and hardwired 16 Ohm secondary with the 8 Ohm speaker load (two early 16 Ohm/20W Celestions in parallel). This wiring results in a primary load of 4k, increasing output by around 15W. I wonder how long the weak speakers survived!

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    Last edited by Helmholtz; 12-23-2018 at 05:52 PM.
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  12. #47
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    Thanks Helmholtz! Will re-read. And, I'll wire up the 6.6k, but keep the 8k handy and try that as well. Ohhh, can't wait to get sound out of this thing!

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  13. #48
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    Ran out to MicroCenter in Cambridge yesterday and got a bunch more heat shrink tubing. For the unused wiring, I put a piece of shield I salvaged from old serial cable, (the wiring has neat trimmed ends and heat shrink over that), then ran a long piece of heat shrink over that. Decided not to heat the heat shrink, so it can be removed if necessary.

    The green wires are 6.6k, will wire up the amp first with 6.6k. Yellow are 8k. I left the 8k wiring out of the bundle since Id like to try Helmholtz' suggestion and see which sounds better, at some point.

    hard to see in the photo, but the shielding is sticking out 1/2" or so from the end of the heat shrink, so I can attach it to something.

    Click image for larger version. 

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    Nothing routed under the preamp board. I can do a neater job and route the yellow (8k tap) wires close into the corner of the chassis and put a cable clamp on the whole thing.

    This way, when my great grandchildren get this amp and decide to go ultra-linear, all they need to do is slide the heat shrink and shielding off, switch the wires, and they're good to go.

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  14. #49
    Bent Member Chuck H's Avatar
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    "I" think that'll work.

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    Awesome, thanks Chuck.

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    hard to see in the photo, but the shielding is sticking out 1/2" or so from the end of the heat shrink, so I can attach it to something.

    Please do ground the shielding. An non-grounded electrical shield is no shield. It just acts a floating capacitor with a large radiating surface.

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    Quote Originally Posted by Helmholtz View Post
    Please do ground the shielding. An non-grounded electrical shield is no shield. It just acts a floating capacitor with a large radiating surface.
    Thanks for the reminder Helmholtz, will do.

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