terminating unused wiring bundle question

[mikepukmel]

Hi All,
I have a project started months ago, trying to resurrect it before it disappears into the abyss.
JTM45 type thing, has (slightly overpriced) mercury RadioSpares OT. The unused bundle has many wires. I looked for ages to find out what other people do to terminate the unused bundle, but most are completed amps so I can't see the cable.
I twisted the bundle, but later learned the pairs should be twisted. Will fix that. I trimmed the ends of the unused wires neatly, then put heat shrink I then put a bigger diameter heat shrink over the bundle and tried to bend it so it fit under the main turret board.
The shrink wrapped bundle fits under the board, but it has too much tension in the bend, so it puts a lot of pressure on the bottom of the board. Im afraid if I bolt the board down, a little piece of lead or solder protruding from the bottom of the board will work its way down into the bundle and do something awful.
any suggestions greatly appreciated on how to get this bundle terminated well, and tucked under the board where it won't interact with any other signal and have a good mechanical bend so it doesn't stress anything else.

Thanks
MP

[Dave H]

TTIUWP

[mikepukmel]

TTIUWP

Ouch, sorry. (slapping forehead). Getting the camera out now. . .

[mikepukmel]

Board with the cockroach trapeze:




The bundle is pretty thick. Maybe should make separate bundles? Anyway, I can push the bundle down but it still makes contact with the bottom of the board. Was a royal pain (for me) to make the board sit, but maybe taller standoffs?



Board dry fit on the standoffs.

[Chuck H]

I might be concerned about bundling up the OT leads and hiding them under the board approaching the preamp. It's just begging to incite instability. What the hell are all those leads for and why do you wish to keep them? If it's a MM transformer or something then maybe I could see keeping the length to preserve the value of it. But I don't think bundling long output transformer leads under the board in that direction is a good idea.

[Helmholtz]

I might be concerned about bundling up the OT leads and hiding them under the board approaching the preamp. It's just begging to incite instability.

^^^^^That!

OT primary leads together with the power tube anodes are the strongest electric signal field radiators in an amp and should be kept away from the board or the preamp tubes. I can understand that someone wants to keep (some of) them for later experiments with different primary impedances but they should at least be kept on top of the shielding grounded chassis if it is not possible to hide them within the OT bells.

[Leo_Gnardo]

I might be concerned about bundling up the OT leads and hiding them under the board approaching the preamp. It's just begging to incite instability.

Agree with Helmholtz, and you. I can tell you about a couple of x-pensive boo-tiki amps I've had to "fix" by applying a layer of copper foil shielding below the circuit board, blocking the EMF from output transformer leads. All while thinking "hey these guys get paid THOUSANDS of bucks, why didn't they think of this???

[Pedro Vecino]

That supposes serious problems. My advice is keep them out of the chassis.

[Chuck H]

I like Helholtz's idea of stashing them in the endbells. There's usually some space in there. You can cut them only as short as you have to to get them to fit. That way you can fish them out later and (shrink tube an extension if you have to) to experiment with them later.

[mikepukmel]

I might be concerned about bundling up the OT leads and hiding them under the board approaching the preamp. It's just begging to incite instability. What the hell are all those leads for and why do you wish to keep them? If it's a MM transformer or something then maybe I could see keeping the length to preserve the value of it. But I don't think bundling long output transformer leads under the board in that direction is a good idea.

OK thanks Chuck. So, the primary has leads for a bunch of different impedances. 9k, 8.8k, 6k, ultra linear. And the output side has a really funky connection setup, all of the leads are used for every speaker impedance.

If I trim back the leads so they would reach the output tubes, maybe they would terminate about where the PI parts sit. Don't know where else to put them.

[mikepukmel]

^^^^^That!

OT primary leads together with the power tube anodes are the strongest electric signal field radiators in an amp and should be kept away from the board or the preamp tubes. I can understand that someone wants to keep (some of) them for later experiments with different primary impedances but they should at least be kept on top of the shielding grounded chassis if it is not possible to hide them within the OT bells.

Thanks, yeah, they all come out of the bells, so no way to pack them in there. Bells aren't big enough inside as well, I think. So, find a way to put them on top of the chassis?

[mikepukmel]

I like Helholtz's idea of stashing them in the endbells. There's usually some space in there. You can cut them only as short as you have to to get them to fit. That way you can fish them out later and (shrink tube an extension if you have to) to experiment with them later.

OK thanks again, I'll take the OT apart and see how much room between the transformer and bell body. Hope there's enough room. Looks like I made it way hard on myself for this build, thought it was going to be easy, breezy. No breezing so far. (Thank God for Jerry Murad and the harmonicats)

[mikepukmel]

Agree with Helmholtz, and you. I can tell you about a couple of x-pensive boo-tiki amps I've had to "fix" by applying a layer of copper foil shielding below the circuit board, blocking the EMF from output transformer leads. All while thinking "hey these guys get paid THOUSANDS of bucks, why didn't they think of this???

Hi Leo, that's in teresting. So, do you put copper foil over teh wiring, then some kind of insulation over that? I have a piece of fiberglass board that's thinner than the eyelet/turret board stuff. Can cut a piece of that to keep the copper from shorting something out.

In case there isn't room in the end bells.

[nosaj]

Hi Leo, that's in teresting. So, do you put copper foil over teh wiring, then some kind of insulation over that? I have a piece of fiberglass board that's thinner than the eyelet/turret board stuff. Can cut a piece of that to keep the copper from shorting something out.

In case there isn't room in the end bells.

Or you can use all the taps . I had a ceritone jtm45 and it had a plastic box attached to the chassis had 4,8,16ohm jacks. Would give the option of trying out different speakers. Especially if you start collecting organs, you'll get lots of speakers that way, variety of ohms.
nosaj
Or use this
https://www.tubesandmore.com/product...ement-marshall

[mikepukmel]

Or you can use all the taps . I had a ceritone jtm45 and it had a plastic box attached to the chassis had 4,8,16ohm jacks. Would give the option of trying out different speakers. Especially if you start collecting organs, you'll get lots of speakers that way, variety of ohms.
nosaj
Or use this
https://www.tubesandmore.com/product...ement-marshall

Thanks for the link. This ot is weird. I read an article by somebody someplace that said it was part of its 'mojo'. Also, had a post here on MEF about it as well. It doesn't wire up like normal transformers, which is prob. why marshall ditched them. Every impedance uses all of the wires, so I think some of the gurus here found a way to wire up 2 impedancs with a dpdt I think.



OK ok I bought it in a fit of crazy. If I had bought the standard JTM45 replacement transformer, the amp would be many solder joints closer to completion than it is now. live and ... live.