help with shielded input wiring hell

[mikepukmel]

I got this Mogami shielded wire, for the amp inputs. It has really thin conductor, like about 5 human hairs, can't remember the gauge, 26 or thinner maybe.

So, I strip and wrap the wire on, say, the input jack loops and apply a little bit of solder. So far so good. But when I fiddle with something in the amp, changing some part, whatever, it only takes a couple of REALLY small movement on the wire before the conductor snaps off, right at the place where the solder wicks down the conductor. It happened when I took the jacks out a few times, then 3 or 4 times for other repairs/mods. Just happened again.

What type/gauge shielded wire do you all use? Is it OK to use thicker wire, like guitar cable, that has thicker conductor? I found some shielded wire with I think it was 20ga. I am not quite ready to toss the amp out the window, but the aggravation level for this input wiring breaking is quite high at this point.

[galaxiex]

I have a spool of that Mogami wire.

I make the inner conductor (with insulation) a bit long, then anchor the thing with the shield relatively short.
This makes the inner form a small loop.
The shield then takes the strain.

On the end with the un-terminated shield, I find a way to anchor the cable somehow,
so there is as little movement as possible on that fragile inner conductor.

Some locations require some creative way to lay the wire so I can anchor it, and have a small loop on the fragile connection.

HTH

Edit;
The alternative is some RG-174 co-ax.
The inner conductor has some steel in it.
Can be hard to solder, but the wire is much stronger.

Edit;

Ooops, my bad, it's the outer shield that is steel,
but the inner strands are much thicker and stronger than the Mogami.

[eschertron]

I've run into wire like that. So how much of this wire do you have? Too much to throw it all out?

Making a pigtail as described above is a classic solution, or realize that this wire gives you one chance to get it right, and any tweak that involves moving it will involve replacing it.

This doesn't help, but I've had good luck with a mic cable rated for wireway, i.e., it doesn't have a heavy rubber jacket, just the PVC. Branded by Horizon, 22 gauge with 7 conductors. And yes, the strands are stiff enough to not flex repeatedly at one point. I think the cable itself is designed to not be very flexible, because of the intended use in wireways.

[Helmholtz]

Edit;
The alternative is some RG-174 co-ax.
The inner conductor has some steel in it.
Can be hard to solder, but the wire is much stronger.

Edit;

Ooops, my bad, it's the outer shield that is steel,
but the inner strands are much thicker and stronger than the Mogami.

The RG-174 I have used had copper plated steel strands in the inner conductor - not in the shield.

[galaxiex]

The RG-174 I have used had copper plated steel strands in the inner conductor - not in the shield.

Hmmm, that could be...
I do know that the rg-174 I have, the shield is difficult to solder, and very stiff.
Come to think of it, the inner core is quite stiff as well, but copper colored.

[Helmholtz]

Hmmm, that could be...
I do know that the rg-174 I have, the shield is difficult to solder, and very stiff.
Come to think of it, the inner core is quite stiff as well, but copper colored.

You can find out using a strong magnet.

[Leo_Gnardo]

The RG-174 I have used had copper plated steel strands in the inner conductor - not in the shield.

I have some of that stuff, left over from an ion implanter installation I helped with many moons ago. Very handy.

A few years ago I got a call from up Schenectady way. An old GE engineer wanted his amps worked on. He asked me if I'd be interested in some special RG-174 that was made with all copper, no steel. Meant for use inside MRI machines. Well of course! What a treat that would be - but he never did show up, drat. But now we know that stuff exists, it's just where do you get it?

[mikepukmel]

Thanks everyone! Well, I only had a few feet of the really thin, thin stuff. The Mogami 2330 is 29 awg inner conductor, just looked it up. Eeek. No wonder my klutzy self could not work with it. Thanks for the tips, but I think I don't have the skills for this stuff.

Their 2524 cable basically looks like standard guitar cable. The inner conductor is 20ga. I think I can't klutz that up. Capacitance is higher, 45pf /foot vs 36pf/foot, but its probably less than 1/2 a foot from the jacks to the tube socket pins. I don't think the extra 5pf will kill anything, from what little I know and since there will be an inch or so stripped, capacitance is probably lower than 5pf or so, total. (Rationalizing to myself why I should not improve my skills rather than ditching and running to auto jumper cables for the task) And there's plenty of room under the fiberglass turret board in this amp.

[mikepukmel]

For Fender style amps, the input jacks are not isolated, so the ground right to the front panel on the chassis. The conductors go to the jack tips, of course, but should the shield pig tail just go to the ring lug on the jacks, so the shield grounds to the front plate as well? I read in multiple places, I think here on MEF, that the shield should get grounded at the input jacks and not near the tube socket side. (Don't know where my vintage fenders were grounded, but I do remember hearing "Tie A Yellow Ribbon" song from an AM radio station on that amp. I think it was WEOK).

[galaxiex]

You can find out using a strong magnet.

Yes, the inner conductor is indeed, very slightly magnetic.
Also the outer shield.

I used a very strong rare earth magnet to check.

[Leo_Gnardo]

For Fender style amps, the input jacks are not isolated, so the ground right to the front panel on the chassis. The conductors go to the jack tips, of course, but should the shield pig tail just go to the ring lug on the jacks, so the shield grounds to the front plate as well? I read in multiple places, I think here on MEF, that the shield should get grounded at the input jacks and not near the tube socket side. (Don't know where my vintage fenders were grounded, but I do remember hearing "Tie A Yellow Ribbon" song from an AM radio station on that amp. I think it was WEOK).

Looks like the right approach. And WEOK, yeah they would be playing an old chestnut like Tie a Yellow Ribbon, music for the over 90 crowd. Turn up your hearing aids kids!

Years ago I bought a roll of some white Belden coax, very thin & flexible. But soldering that center conductor, jeeze you just look at that and it breaks, phooey! You can see why I was lusting after no-steel RG-174. Even with the steel, it's OK in guitar amps. It sure would raise hob in an MRI what with the super intense magnetic fields, I can imagine harnesses whipping around like crazed snakes on a Red Bull jag.

[mikepukmel]

Update: well the stuff I had bought (thicker stuff) was actually 2 conductor. it has I think 22 ga innards, so was easy for a klutzy type to wire it up to the inputs. Turned out better than I thought it would, appearance wise anyway. That left the wiring to the volume controls from the tube pins. I bit the bullet, wired up the thin stuff that was there, REALLY carefully stripped the wire twisted it and just put a teeny bit of solder, and its on there now. Amp seems to still work and sound OK so I didn't do anything really stupid (at least for this repair). Thank GOD these amps are not as sensitive as MRI machines!

Made permanent some previous ground wire re-routing, soldered and screwed a lug to the nice steel chassis. neatened up the grounds on the pot side of the preamp.

All that's left to do now is debug a ringing in the reverb. When I turn the pot up beyond a point that I would never use it anyway (like 6 or7), a ringing right in the audio band starts up and SLOWLY gets louder and louder until I turn the pot back down.

[mikepukmel]

Got the input wiring all redone, thanks for the help. It's close, but I think there's still a slight hum. I narrowed it down to this gain stage.

[galaxiex]

Got the input wiring all redone, thanks for the help. It's close, but I think there's still a slight hum. I narrowed it down to this gain stage.

[ATTACH=CONFIG]51023[/ATTACH]

I'm sure you could buff that out....

[g1]

That's funny. Let's see it in all it's glory.