fender wiring detail question

[mikepukmel]

Why did Fender do this on some amps: wrap a bundle of wire with another wire. its not the entire length of the bundle, and this appears to be spotty, some amps have it and some don't.

Is this to keep the bundle together, or also electrical, like a makeshift shield? If its a shield, then why didn't they wrap the whole length of the bundle? I've seen this also on some bundles going from the main board to the vol/tone controls.



If its shielding, can we take a piece of that shield mesh, push the cable bundle through that, then put shrink wrap tubing around the whole thing? I believe Catalin suggested something like this on another post, but can't find it.

[eschertron]

I have a CBS-era champ with a couple wires wrapped and raised the same question a few years ago. But in my amp, the wrap is fairly loose, with not much contact between the wires. Link HERE

First, establish what wires are in the bundle, and what wire is wrapped around it. The function of the wires, esp. the suitability of the 'wrapping' wire to be used as a shield, may point to a reason. [ASIDE] That's a lot of wire wrapped around that bundle! [/ASIDE] I think mostly the technique was used for tidiness, but also to keep wires from flopping into a position where unwanted effects could happen. IMHO shielding aspect runs a somewhat distant third, but that's a lot of pennies to spend on wire for the sake of someone's OCD

[Leo_Gnardo]

like a makeshift shield?

As good an answer as any.

In critical applications, like when I know the amp will be used for recording, or the user plays in a quiet room and is sensitive to hum, I'll add a copper foil shield between the hi voltage wire bundle and the back of the reverb, tone & volume pots. Copper foil glued to fish paper, copper ground-wired to any nearby convenient ground point, and foil/fish RTV cemented to chassis. Gets rid of another annoying hum source, fast & cheap.

[Justin Thomas]

Is that twisted wire attached to ground at one end only? That will answer any questions you have about its purpose. In older Fenders, the wiring from the tone and volume controls would be twisted. As wiring got sloppier as the years went on, noise and oscillations proliferated. This is one of the techniques they used, and it also doubled to keep wiring neat.

In effect, what you have there is a multi-conductor coaxial cable. And I would bet that at the time, a single strand of cheapy-wire was less than the price of cutting and stripping several different leads of a coax cable to the necessary lengths, etc. Great for PCB amps, not so much for eyelet boards. Or, using several pieces of shielded cable.

Justin

[nickb]

If it's working why change it? Adding a mesh sheild will add capacitance to ground and that could cause a loss of hf on signal wires.

[mikepukmel]

I have a CBS-era champ with a couple wires wrapped and raised the same question a few years ago. But in my amp, the wrap is fairly loose, with not much contact between the wires. Link HERE

First, establish what wires are in the bundle, and what wire is wrapped around it. The function of the wires, esp. the suitability of the 'wrapping' wire to be used as a shield, may point to a reason. [ASIDE] That's a lot of wire wrapped around that bundle! [/ASIDE] I think mostly the technique was used for tidiness, but also to keep wires from flopping into a position where unwanted effects could happen. IMHO shielding aspect runs a somewhat distant third, but that's a lot of pennies to spend on wire for the sake of someone's OCD

And what's more strange is if CBS fender did that, since they were shaving pennies here and there. Well, in my 'template' images, images I scraped off the web to show me how period amps were built, sometimes I don't see the bundles wrapped, sometimes they are. Several twisted bundles in the amp: 1) from the first preamp tube capacitors to the bass and treble controls, there are 3 wires, brown, blue, white, twisted together pretty hard. On some amps, I see a wire wrapped tightly around part of this bundle. 2) same for channel 2. 3) wires leading to tremolo and reverb pots. 4) and 5) two wires to input jack pairs. 6) bundle leading from the cap board, down through a grommet hole, and along the edge of the main eyelet board, toward the bias pot.

So far, Ive only seen the additional wrapping done on
- the bass/treble wire set, and
- the HV set leading from the cap board.

[mikepukmel]

As good an answer as any.

In critical applications, like when I know the amp will be used for recording, or the user plays in a quiet room and is sensitive to hum, I'll add a copper foil shield between the hi voltage wire bundle and the back of the reverb, tone & volume pots. Copper foil glued to fish paper, copper ground-wired to any nearby convenient ground point, and foil/fish RTV cemented to chassis. Gets rid of another annoying hum source, fast & cheap.

Thanks Leo, this confirms a suspicion I had about those 4 wires leading down from the cap board through the chassis: they must radiate like crazy. esp the wire leading from the first two capacitors, since this wire must have the most ripple, and is the highest voltage that leads to the output trans center tap and choke. It sounds like your mod should be put on all of these amps. Would it be better, less hum, if we ran the cable with 4 wires out of the cap can, and along the top of the chassis, and dropped it into a hole closer to where the wires connect to the eyelet board? There is quite a long run of this bundle about where the second set if input jacks are then all along behind channel 2 pots and the tremolo and reverb pots.

[mikepukmel]

Is that twisted wire attached to ground at one end only? That will answer any questions you have about its purpose. In older Fenders, the wiring from the tone and volume controls would be twisted. As wiring got sloppier as the years went on, noise and oscillations proliferated. This is one of the techniques they used, and it also doubled to keep wiring neat.

In effect, what you have there is a multi-conductor coaxial cable. And I would bet that at the time, a single strand of cheapy-wire was less than the price of cutting and stripping several different leads of a coax cable to the necessary lengths, etc. Great for PCB amps, not so much for eyelet boards. Or, using several pieces of shielded cable.

Justin

It appears so. One side definitely connects to a point on the board that the ground end of the cathode cap/resistor for I think the tremolo tube ges grounded. I can see the other end of the wire looks like it hangs loose, but can't tell.

[mikepukmel]

If it's working why change it? Adding a mesh sheild will add capacitance to ground and that could cause a loss of hf on signal wires.

Yeah, great point. I was just wondering what the function of that wrapped wire was. Mainly since I have this set of really good quality images from a period blackface Deluxe Amp, and it doesn't have either of the two cable bundles wrapped. I did not know if this was maybe a "must do" mod taht Fender added later, or if it might be a "patch up the terrible lead dress with a patch" type thing. The wiring quality, lead dress on the amp in the images I have is very good, though, compared to the one in the image above.

The only thing I do recall from all those old Fender and Marshall amps we had when we were kids way back when, was that a lot of them, MOST of them, had lots of hum. They were mostly pretty old and beat up at that point, and we were kids and would not know a bad capacitor or red plating tube if we saw one.

[Enzo]

Generally we shield things that are sensitive and would pick up noise, rather than shielding things that might radiate something. We are not too concerned about ripple being radiated, we are however very concerned that something sensitive like a grid wire - the wire to a grid from a volume controls say - We might find that only minimal shielding is enough to solve problems, so one grounded wire wrapped around the long cross chassis wires is enough to prevent some instability.

Obviously for a sensitive wire to be an issue, something must be radiating somewhere, and for that matter for radiation to matter, there must be something to pick it up. But for the most part, we shield mic cords and guitar cords, and we do not shield speaker cords.

[catalin gramada]

this is from one fixed few years ago. don.t remember if was a bassman or a twin. Spot the solder point to chassis

[Justin Thomas]

don.t remember if was a bassman or a twin...

Twin. No Bassman ever came with a tremolo roach/optocoupler.

Justin

[catalin gramada]

Twin. No Bassman ever came with a tremolo roach/optocoupler.

Justin

good eye. thx.

[Randall]

The way I look at this is, it is another thing CBS did that maybe wasn't really necessary. Right up there with the bias balance and feedback loops. If these designs worked just fine in BF's, then who cares? First, Fender did not do this, then they did, and then they did not. What does that tell you? It's just something someone decided to do during those production years, it doesn't meant it was needed or even helped anything.

[Leo_Gnardo]

Would it be better, less hum, if we ran the cable with 4 wires out of the cap can, and along the top of the chassis, and dropped it into a hole closer to where the wires connect to the eyelet board? There is quite a long run of this bundle about where the second set if input jacks are then all along behind channel 2 pots and the tremolo and reverb pots.

Looks like a good idea to me. Why Fender didn't do it - may have been thinking safety - keep the hi voltage behind a wall of steel and out of the reach of nibbling critters.