terminating unused wiring bundle question

[Chuck H]

Anyone remember the old AAMCO commercial with the monkeys slapping at a transmission with plastic baseball bats? "Our mechanics are experts" (cue monkey shot). I looked for it on *outube, but couldn't find it.

[The Dude]

Here ya go, Chuck.

[Chuck H]

Right on! I didn't see it with my search for some reason. I put in "monkey" "AAMCO" "Commercial" and three different ones without monkeys came up first and I didn't scroll further than the first page. Wow, that was '82 !?! I would have been 14. Why do I remember that commercial?

[The Dude]

......Why do I remember that commercial?

Because monkeys, midgets, and fart jokes are always funny when you're 14.

[mikepukmel]

Oh man, I remember that commercial. What a riot. Funny even when we're slightly over 14 as well.

[mikepukmel]

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.

[Chuck H]

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.

[Helmholtz]

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.

[Chuck H]

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.

[Helmholtz]

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

[olddawg]

I’ve 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.

[mikepukmel]

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.

[mikepukmel]

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.

[mikepukmel]

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.

[mikepukmel]

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?