Fender Mustang III modeling amp question....Enzo?

[daz]

I figure Enzo may know this being a fender repair tech and a hell of a tech period. The mustang III combo has your normal positive and negative wires going to the speaker's + and - terminals, but it also has a wire that goes to the speaker frame. Why is that wire necassary, what does it do? I have run mine to a external cab by hooking the + and - to a jack and using that to connect to the external speaker and not using that wire that goes to the frame. And it worked fine. Only did it to see how a EV sounded. Anyways, fender says that wire is necassary and this is a quote...

"The speakers installed in the Mustang Series combo amplifiers are
designed to be voiced specifically for each model. We recommend
replacing defective speakers with the exact same speaker, which may
be obtained from your Fender Dealer.

If a speaker is changed to a different type there are specifications to
keep in mind. The impedance (ohms) and power handling (wattage)
must match exactly between the original and replacement speakers.
In Mustang amplifiers there is also a third wire that is attached to the
frame of the speaker that must be attached to the frame of the
replacement as well – if this is not done properly, the performance
of the power amp may be affected.

There are speakers on the market that have the negative terminal
common with the frame of the speaker – these speakers CANNOT
be used in a Mustang amplifier, as this will result in damage to the
power amp.

Any deviance from stated specifications or improper
installation of the speaker (especially pertaining to the
attachment and proper grounding of the third speaker wire)
will void the amplifier’s warranty."

Can anyone tell me if this is some kind of misleading explanation for a reason they don't want you to know or what?

[Enzo]

Did your mom ever lie to you to get you to do something? "If you don't eat your vegetables, your hair will fall out."

Grounding the frame is part of noise reduction and possibly certain electrical codes. But the amp would work without it.

[daz]

Did your mom ever lie to you to get you to do something? "If you don't eat your vegetables, your hair will fall out."

Grounding the frame is part of noise reduction and possibly certain electrical codes. But the amp would work without it.

Well, yeah, thats exactly what my first thought was and always is with things like this. They never tell you the whole truth for any number of reaons, but i still was curious as to what it's for. Not sure what you mean by "electrical codes" but noise i can see i suppose. What about speakers with the neg, lug connected to the frame? They say that will damage it. ya think? And if so i would assume not as long as you leave the frame wire off. Just curious is all.

[52 Bill]

Is it possible with this amp that the - wire is not internally grounded and shouldn't be? I haven't seen a schematic, but some Fenders have the speaker negative isolated from the chassis ground.

So use any speaker you want, just don't ground the negative wire.

[Malcolm Irving]

Safety codes ask for metal parts which could be touched by the user to be grounded. That is so that if a fault developed where the metal part could become 'live' (if it was not grounded) it would just blow the fuse instead. With a speaker, I think there would have to be at least two (or maybe three?) simultaneous faults for the speaker frame to become live, so it seems extremely unlikely.
I'm not sure, but in very dry conditions it might be possible for the speaker frame to build up a static charge and grounding would prevent this from happening.
Also, grounded metal areas act as a screen against electrical noise and interference. Again probably not a big issue with a speaker frame.

[Enzo]

When a code says any exposed metal part, it doesn't include a disclaimer as to how likely a short might be.

And codes may be from other places. We have no RoHS here in the USA, so lead solder is OK here, while lead-free solder must be used in Europe. So it means all amps are now made with lead free solder. That way they don't have two separate assembly lines for amps going to the two continents. SO that grounded speaker frame COULD be because of rules somewhere else.

I think we recently discussed the grounding of speaker frames. Some speakers - look at the terminal block - even strap the negative terminal to the frame. Yes there are also plenty of amps wher neither side of the speaker is ground. So don't ground the frame of a frame ground speaker in those amps.