fender wiring detail question

[mikepukmel]

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.

OK this is big (for me) thanks. If the HV wires coming from the cap can are radiating, the pots would be more susceptible, since the signals come from the preamp section. So better to put a shield like Leo suggested between the big HV cable and the back of the pots. But for CBS era Fender, a piece of wire was cheap and easy to add. Some of the chassis images Ive seen, the HV cable was not twisted tight at all, and the 'looped' wire wrapped around the cable was very sloppy as well.

So, if you could put all the pots right near the preamp section, say in two rows, one over the top of the other, and have no reverb, tremolo, bass, treble, vol wire run near the area where the HV cable comes through, you'd have a quieter amp. For hum anyway. Maybe? Would not be as pretty as the Fender chassis, but would be quieter?

[mikepukmel]

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

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Oh yeah, seen a whole bunch like this. I bet this is a Twin from the late 70's. Really messy looking wiring. The HV cable is not twisted tight. And the coil wrapped around it is not very neat as well. Someone standing over the people doing the wiring at CBS: "Faster, faster, faster, ...."

Maybe it was a lot harder to put a good twist on the wire cables because of the thick plastic insulation, just wasn't very pliable?

[mikepukmel]

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.

OK yes, but ... CBS changed the type of wire used, and also production ramped way up after CBS took over. So, maybe what was not necessary when Leo F ran the business, became necessary after they changed the wire, ramped up production and let the quality/lead dress standards go into the toilet.

[mikepukmel]

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.

Thanks Leo, more like a "what if" question than anything. Since I'm a home tinkerer, and this amp will be used at fairly low volume, very close to my head, anything to reduce hum and hiss is welcome (will use your suggestion to put a copper foil ground behind the pots that are near the evil HV bundle). But of course, don't want family and friends at risk either.

I'm planning a future build to move the filter caps between the mail eyelet board and the PT. since they are so much smaller than the big paper electrolytics. So, the HV wires would be very short, except for the feed to preamp tubes, would not have to run along behind pots or input jacks..

[Leo_Gnardo]

I'm planning a future build to move the filter caps between the mail eyelet board and the PT. since they are so much smaller than the big paper electrolytics. So, the HV wires would be very short, except for the feed to preamp tubes, would not have to run along behind pots or input jacks.

Another good plan. Many modern electrolytics are spec'd for 105C so heat's less of a problem than it was in the good ol' days. Still it's best to keep filter caps cool if possible. Mounted atop the PT - like Bogner did in mods I've seen - deafinitly not recommended. In that gap between PT and circus board - should be OK.

[nickb]

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.

Fender did do it, belatedly, on the Deluxe Reverb RI that I know of. Maybe others too. The bundle exits through a hole in the dog house making it really awkward to get that cover on and off. I wish they'd used a "U" shape and then you could just lift it off the wire bundle.

[Mick Bailey]

That wrapping technique isn't confined to Fender - Sansui used it in the 60s too. With those amps if you remove it the amp becomes overly-sensitive to hand capacitance and will sometimes oscillate if you place your hand on the cabinet.

[catalin gramada]

Maybe it was a lot harder to put a good twist on the wire cables because of the thick plastic insulation, just wasn't very pliable?

No need to be. I love PVC insulation just for one reason: If you heat a little bit with a heat gun and keep it twisted till cooling down it will remain as so, very nice permanent twisting