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Thread: Extending primary leads on OT?

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    Extending primary leads on OT?

    I have an Eden E300T 300 watt tube head that had a blown OT. I got a decent deal on an NOS SVT CL transformer (no replacements for original that I could find). One problem I have is that the lead for the OT primary CT is not long enough, by about 5 or 6 inches. Is there a way to safely extend this lead or do I now have two expensive boat anchors?

    Here is a picture of the SVT OT, the primary CT is the red/white wire:
    Click image for larger version. 

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    Thanks in advance,
    Greg

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    Sure, you can solder extensions to the wires. The primaries carry 120vAC, so the wire insulation and shrink tubing needs to capably handle that much voltage (not a tall order at all).

    Wait...you said "primary CT" which doesn't make sense to me. Only the secondaries have CTs.

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    Quote Originally Posted by xtian View Post
    Sure, you can solder extensions to the wires. The primaries carry 120vAC, so the wire insulation and shrink tubing needs to capably handle that much voltage (not a tall order at all).

    Wait...you said "primary CT" which doesn't make sense to me. Only the secondaries have CTs.
    It's an OT.

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    Quote Originally Posted by xtian View Post
    Sure, you can solder extensions to the wires. The primaries carry 120vAC, so the wire insulation and shrink tubing needs to capably handle that much voltage (not a tall order at all).

    Wait...you said "primary CT" which doesn't make sense to me. Only the secondaries have CTs.
    This is an output transformer. By CT I mean the lead that feeds the B+ voltage to the primary side of the OT. On this amp this is about 650V.

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    It is a WIRE, of course you can extend it. It is just a wire. SPlice a longer wire to it and cover the splice with some heat shrink tubing. If you want to be cool about it, use the same color wire.

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    Quote Originally Posted by Enzo View Post
    It is a WIRE, of course you can extend it. It is just a wire. SPlice a longer wire to it and cover the splice with some heat shrink tubing. If you want to be cool about it, use the same color wire.
    Mostly didn't know if I needed to use HV wire and connectors, and if I should solder splice to the existing wire (and if a certain type of splice is best) or leave the existing connector and use make a male/female pigtail, etc. Looking around my bins most of the wire seems to only be rated 300V, have some that says 300/500, but am not up on wire ratings.

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    Last edited by glebert; 04-09-2020 at 09:56 PM.

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    Quote Originally Posted by glebert View Post
    Mostly didn't know if I needed to use HV wire and connectors, and if I should solder splice to the existing wire (and if a certain type of splice is best) or leave the existing connector and use make a male/female pigtail, etc. Looking around my bins most of the wire seems to only be rated 300V, have some that says 300/500, but am not up on wire ratings.
    You could always add heat shrink all the way over your additional 300V rated hook wire used to extend the C/T lead. Alpha FIT 221 heat shrink carries a voltage rating of 600V, so that would certainly take care of the voltage rating.

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    I don't really care for the idea of using the existing connector, but you could use a crimp type butt connector, then heatshrink over. Or solder splice w/heatshrink as mentioned.

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    I can't imagine anything more reliable than a standard Western Union splice, soldered and tubed.

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    Quote Originally Posted by Enzo View Post
    I can't imagine anything more reliable than a standard Western Union splice, soldered and tubed.
    Yea your not using it like a rope should be good enough for a lifetime.
    https://en.wikipedia.org/wiki/Western_Union_splice
    nosaj

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    I don't even take it THAT far. It's not a tether or any sort of physical support or stress point. I usually just spread the stranding on each end I want to splice and then sort of insert them into each other, if you get my meaning. Then I squeeze down the spread ends and give a little twist to tighten it up. Solder and add shrink tube... It makes for a skinnier, smoother looking splice if appearances matter. Otherwise the Western Union splice is certainly more reliable.

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    Quote Originally Posted by nosaj View Post
    Yea your not using it like a rope should be good enough for a lifetime.
    https://en.wikipedia.org/wiki/Western_Union_splice
    nosaj
    Well, I learned something new today. I always called it "twisting two wires together"



    edit: but upon reflection, I can well imagine back in the telegraph days there needed to be a standard for the splices, and plenty of 'em - I doubt there weren't too many 1000-mile-long rolls of telegraph wire being spun 150 years ago.

    edit: I see the WU splice is meant to be a structural member, the wire solid core and stiff!

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    Last edited by eschertron; 04-10-2020 at 07:33 PM.
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    For what it is worth, to me twisting two wires together is a pigtail splice.

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    This is all really helpful to me. My splice game has always been weak because I would do it so infrequently and was never shown different ways to splice. I definitely used the term pigtail improperly above because to me the pigtail was the section of wire that was added, not the wire joint itself.

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    Quote Originally Posted by glebert View Post
    This is all really helpful to me. My splice game has always been weak because I would do it so infrequently and was never shown different ways to splice. I definitely used the term pigtail improperly above because to me the pigtail was the section of wire that was added, not the wire joint itself.
    I would think, semantically, that if the added wire is the 'pigtail', then the splice used to join the pigtail to the rest of the wire would be the 'pigtail splice'.

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    Well I learned something too.
    Pig-tail splice is also known as rat-tail splice, like this: https://en.wikipedia.org/wiki/Rat-tail_splice

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    Trying to "describe" a splice in my post above seemed clumsy. I found a couple of examples on line and grabbed the photo's from one. Can't seem to find a name for this splice but I think the term "inline splice" was used a couple of times.

    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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    An inline splice just means it is more or less just an extension of the wire in linear shape. Which your is. I would call a Western Union splice inline as well. As opposed to a pigtail where two wires are placed parallel and twisted together.

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    Update on this and two more silly questions. Extended the wire, cased it in heat shrink, and did a test with only two power tubes and a 16 ohm speaker and it worked good. Did have to reverse the speaker leads to prevent feedback oscillation.

    Question 1: Is this transformer supposed to bolt down flush to the chassis, ie remove the nuts that are shown in the above photo and use those to secure it, or should those nuts sit on top of the chassis. Note: turns out the transformer is not really from a SVT CL, which stood up vertical on brackets, but for an SVT VR or maybe SVT II non-Pro, where they go down into the chassis (this actually is better for me btw). I would think sitting down flush would be better for strength and thermal, I just always feel weird unscrewing nuts covered in varnish.

    Question 2: This transformer is a bit bigger than the previous one, so sits a little closer to the 6550 power tubes, with maybe 3/4" clearance to the closest tube. Should I put a heat shield in that gap, or is it OK? I'm planning on increasing the size of the fan.

    TIA,
    Greg

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    Question 2: This transformer is a bit bigger than the previous one, so sits a little closer to the 6550 power tubes, with maybe 3/4" clearance to the closest tube. Should I put a heat shield in that gap, or is it OK? I'm planning on increasing the size of the fan.
    It's an OT which doesn't get very hot by itself. And because of its mass, the tube's heat radiation won't heat it up significantly. So no need for a heat shield.

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    No, do not remove the existing nuts that hold it together.

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